Document:

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                                                                     EXHIBIT 4.2
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                          FIRST SUPPLEMENTAL INDENTURE

                                     BETWEEN

                              QUANTA SERVICES, INC.

                                       AND

                    CHASE BANK OF TEXAS, NATIONAL ASSOCIATION

                                   AS TRUSTEE

                                   ----------

                                   DATED AS OF

                                  JULY 25, 2000

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                                TABLE OF CONTENTS

<TABLE>

<S>                   <C>                                                                                         <C>
                                                            ARTICLE ONE

                                                           THE 2007 NOTES

SECTION 101          Designation of 2007 Notes; Establishment of Form.............................................2
SECTION 102          Global Securities; Certificated Securities; Paying Agent and
                     Conversion Agent.............................................................................2
SECTION 103          Transfer and Exchange........................................................................3
SECTION 104          Amount.......................................................................................4
SECTION 105          Accrual of Interest..........................................................................4
SECTION 106          Interest Payment and Record Dates............................................................4
SECTION 107          Denominations................................................................................5
SECTION 108          Place of Payment.............................................................................5
SECTION 109          Redemption...................................................................................5
SECTION 110          Conversion...................................................................................5
SECTION 111          Stated Maturity..............................................................................5
SECTION 112          Repurchase...................................................................................5
SECTION 113          Defeasance of 2007 Notes.....................................................................6
SECTION 114          Other Terms of 2007 Notes....................................................................6

                                                            ARTICLE TWO

                                                AMENDMENTS TO THE ORIGINAL INDENTURE

SECTION 201          Amendments Applicable Only to 2007 Notes.....................................................6
SECTION 202          Definitions..................................................................................6
SECTION 203          Amendment of Article Eleven of the Original Indenture........................................9
SECTION 204          Amendment of Article Seventeen of the Original Indenture.....................................9
SECTION 205          Addition of Article Eighteen to the Original Indenture Applicable Only
                     to 2007 Notes...............................................................................12

                                                           ARTICLE THREE

                                                      MISCELLANEOUS PROVISIONS

SECTION 301          Integral Part...............................................................................16
SECTION 302          General Definitions.........................................................................16
SECTION 303          Adoption, Ratification and Confirmation.....................................................17
SECTION 304          Counterparts................................................................................17
SECTION 305          Governing Law...............................................................................17
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<S>                  <C>                                                                                         <C>
SECTION 306          Conflict With Any Provision of Trust Indenture Act..........................................17
SECTION 307          Effect of Headings and Table of Contents....................................................17
SECTION 308          Seperability Clause.........................................................................17
SECTION 309          Successors and Assigns......................................................................17
SECTION 310          Benefit of First Supplemental Indenture.....................................................18
SECTION 311          Acceptance by Trustee.......................................................................18

ANNEX A              Form of 2007 Notes.........................................................................A-1
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                          FIRST SUPPLEMENTAL INDENTURE

         THIS First Supplemental Indenture, dated as of July 25, 2000, between
Quanta Services, Inc., a Delaware corporation (the "Company"), and Chase Bank of
Texas, National Association, a national banking association (the "Trustee"),

                                   WITNESSETH:

         WHEREAS, the Company has heretofore executed and delivered to the
Trustee a Subordinated Indenture, dated as of July 25, 2000 (the "Original
Indenture"), providing for the issuance from time to time of one or more series
of the Company's unsecured debentures, notes or other evidences of indebtedness
(collectively the "Securities");

         WHEREAS, Sections 201 and 301 of the Original Indenture provide that
the Company may enter into supplemental indentures to establish the form and the
terms, respectively, of a series of Securities to be issued pursuant to the
Original Indenture;

         WHEREAS, Section 901(7) of the Original Indenture provides that the
Company and the Trustee may from time to time enter into one or more indentures
supplemental thereto to establish the form or terms of Securities of a new
series, without the consent of any Holders;

         WHEREAS, Section 901(2) of the Original Indenture permits the execution
of supplemental indentures without the consent of any Holders to add to the
covenants of the Company for the benefit of all or any series of Securities;

         WHEREAS, the Company desires (i) to issue 4% Convertible Subordinated
Notes due 2007 (the "2007 Notes"), a new series of Security, the issuance of
which has been authorized by the Board of Directors of the Company, and (ii) to
establish the form and terms thereof as well as to add to the covenants of the
Company solely for the benefit of the 2007 Notes;

         WHEREAS, the Company, pursuant to the foregoing authority, proposes in
and by this First Supplemental Indenture to supplement and amend in certain
respects the Original Indenture insofar as it will apply only to the 2007 Notes
(and not to any other series); and

         WHEREAS, all things necessary have been done to make the 2007 Notes,
when duly issued and executed by the Company and authenticated and delivered
hereunder the valid obligations of the Company, and to make this First
Supplemental Indenture a valid agreement of the Company, in accordance with
their and its terms.

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         NOW, THEREFORE:

         In consideration of the premises provided for herein, the Company and
the Trustee mutually covenant and agree for the equal and proportionate benefit
of all Holders of the 2007 Notes as follows:

                                   ARTICLE ONE

                                 THE 2007 NOTES

SECTION 101         Designation of 2007 Notes; Establishment of Form.

         There shall be a series of Securities designated "4% Convertible
Subordinated Notes due 2007" of the Company, and the form thereof shall be
substantially as set forth in Annex A hereto, which is incorporated into and
shall be deemed a part of this First Supplemental Indenture, in each case with
such appropriate insertions, omissions, substitutions and other variations as
are required or permitted by the Indenture, and may have such letters, numbers
or other marks of identification and such legends or endorsements placed thereon
as may be required to comply with the rules of any securities exchange or
Depositary therefor or as may, consistently therewith, be determined by the
officers of the Company executing such 2007 Notes, as evidenced by their
execution of the 2007 Notes.

SECTION 102         Global Securities; Certificated Securities; Paying Agent and
                    Conversion Agent.

                  (a) Issue of Global Securities. All of the 2007 Notes shall be
issued initially in the form of one or more Global Securities, which shall be
deposited on behalf of the purchasers of the 2007 Notes represented thereby with
the Trustee, at its Corporate Trust Office, as custodian for the Depositary, The
Depository Trust Company, and registered in the name of its nominee, Cede & Co.,
duly executed by the Company and authenticated by the Trustee as provided in the
Indenture. The aggregate principal amount of a Global Security may from time to
time be increased or decreased by adjustments made on the records of the
Securities Custodian as hereinafter provided, subject in each case to compliance
with the Applicable Procedures.

                  (b) Global Securities in General. Each Global Security shall
represent such of the Outstanding 2007 Notes as shall be specified therein and
each shall provide that it shall represent the aggregate amount of Outstanding
2007 Notes from time to time endorsed thereon and that the aggregate amount of
Outstanding 2007 Notes represented thereby may from time to time be reduced or
increased, as appropriate, to reflect exchanges, redemptions, purchases or
conversions of such 2007 Notes. Any endorsement of a Global Security to reflect
the amount of any increase or decrease in the principal amount of Outstanding
2007 Notes represented thereby shall be made by the Securities Custodian in
accordance with the standing instructions and procedures existing between the
Depositary and the Securities Custodian.

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                  Members of, or participants in, the Depositary ("Agent
Members") shall have no rights under the Indenture with respect to any Global
Security held on their behalf by the Depositary or under the Global Security,
and the Depositary (including, for this purpose, its nominee) may be treated by
the Company, the Trustee and any agent of the Company or the Trustee as the
absolute owner and Holder of such Global Security for all purposes whatsoever.
Notwithstanding the foregoing, nothing herein shall (A) prevent the Company, the
Trustee or any agent of the Company or the Trustee from giving effect to any
written certification, proxy or other authorization furnished by the Depositary
or (B) impair, as between the Depositary and its Agent Members, the operation of
customary practices governing the exercise of the rights of a Holder of any 2007
Note.

         The Company initially appoints The Depository Trust Company to act as
Depositary with respect to the Global Securities.

                  (c) Certificated Securities. Certificated Securities shall be
issued only under the limited circumstances provided in Section 305 of the
Indenture and Section 103(a)(1) hereof.

                  (d) Paying and Conversion Agents. The Company initially
appoints (i) the Trustee's agent in the State of New York, The Chase Manhattan
Bank, to act as Paying Agent and (ii) the Trustee to act as Conversion Agent
with respect to the 2007 Notes.

SECTION 103        Transfer and Exchange.

                  (a) Transfer and Exchange of Global Securities.

                     (1) Certificated Securities shall be issued in exchange for
         interests in the Global Securities only if (x) the Depositary notifies
         the Company that it is unwilling or unable to continue as Depositary
         for the Global Securities or if it at any time ceases to be a "clearing
         agency" registered under the Exchange Act if so required by applicable
         law or regulation and a successor Depositary is not appointed by the
         Company within 90 days, or (y) an Event of Default has occurred and is
         continuing and the Security Registrar has received a written request
         from the Depositary to issue Certificated Securities. In either case,
         the Company shall execute, and the Trustee shall, upon receipt of a
         Company Order (which the Company agrees to deliver promptly),
         authenticate and deliver Certificated Securities in an aggregate
         principal amount equal to the principal amount of such Global
         Securities in exchange therefor. Certificated Securities issued in
         exchange for beneficial interests in Global Securities shall be
         registered in such names and shall be in such authorized denominations
         as the Depositary, pursuant to instructions from its direct or indirect
         participants or otherwise, shall instruct the Trustee. The Trustee
         shall deliver or cause to be delivered such Certificated Securities to
         the Persons in whose names such Securities are so registered. Such
         exchange shall be effected in accordance with the Applicable
         Procedures. Nothing herein shall require

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         the Trustee to communicate directly with beneficial owners of interests
         in any 2007 Note.

                     (2) Notwithstanding any other provisions of this First
         Supplemental Indenture other than the provisions set forth in Section
         103(a)(1) hereof, a Global Security may not be exchanged or transferred
         except as a whole by the Depositary to a nominee of the Depositary or
         by a nominee of the Depositary to the Depositary or another nominee of
         the Depositary or by the Depositary or any such nominee to a successor
         Depositary or a nominee of such successor Depositary.

                  (b) Transfer and Exchange of Certificated Securities. Any
Certificated Securities may be transferred and exchanged for other Certificated
Securities as provided in Section 305 of the Indenture.

SECTION 104        Amount.

                  (a) The Trustee shall authenticate and deliver 2007 Notes for
original issue in an aggregate principal amount of up to $172,500,000 upon
Company Order for the authentication and delivery of 2007 Notes, in accordance
with Section 303 of the Indenture.

                  (b) The Company may not issue new 2007 Notes to replace 2007
Notes that it has paid or delivered to the Trustee for cancellation or that any
Holder has converted pursuant to Article Seventeen.

SECTION 105        Accrual of Interest.

         Interest shall accrue with respect to the 2007 Notes at the rate per
annum specified in the form of the 2007 Notes contained in Annex A hereto,
commencing on the Issue Date of the 2007 Notes.

SECTION 106        Interest Payment and Record Dates.

         Interest shall be payable on the 2007 Notes on July 1 and December 31
of each year, beginning December 31, 2000, to Holders at the close of business
on the preceding June 15 or December 15, as the case may be, except interest
payable upon redemption or repurchase of the 2007 Notes by the Company shall be
paid to the Persons to whom principal is payable, unless the Redemption Date or
Repurchase Date is an Interest Payment Date. If the Redemption Date or
Repurchase Date is also an Interest Payment Date, interest shall be paid to the
Holder on the preceding Regular Record Date.

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SECTION 107        Denominations.

         The 2007 Notes shall be in fully registered form without coupons in
denominations of $1,000 of principal amount or any integral multiple thereof.

SECTION 108        Place of Payment.

         The Place of Payment for the 2007 Notes is at the office or agency of
the Trustee in New York, New York; provided, however, that payment of interest
may be made at the option of the Company by check mailed to the address of the
Person entitled thereto as such address shall appear in the Security Register
(provided that any Holder of Notes with an aggregate principal amount in excess
of $2,000,000 shall, at the written election of such Holder, be paid by wire
transfer or immediately available funds).

SECTION 109        Redemption.

                  (a) There shall be no sinking fund for the retirement of the
2007 Notes.

                  (b) The Company, at its option, may redeem the 2007 Notes in
accordance with the provisions of and at the Redemption Prices set forth in the
form of the 2007 Notes contained in Annex A hereto and in accordance with the
provisions of the Indenture, including, without limitation, Article Eleven, as
amended by Section 203 of this First Supplemental Indenture.

SECTION 110        Conversion.

         The 2007 Notes shall be convertible in accordance with the provisions
and at the Conversion Price set forth in the form of the 2007 Notes contained in
Annex A hereto and in accordance with the provisions of the Indenture,
including, without limitation, Article Seventeen of the Original Indenture, as
amended by Section 204 of this First Supplemental Indenture.

SECTION 111        Stated Maturity.

         The date on which the principal of the 2007 Notes matures and is
payable shall be July 1, 2007.

SECTION 112        Repurchase.

         The 2007 Notes shall be repurchased by the Company in accordance with
the provisions and at the Repurchase Price set forth in the form of the 2007
Notes contained in Annex A hereto and in accordance with the provisions of the
Indenture, including, without limitation, Article Eighteen as added to the
Original Indenture by Section 205 hereof.

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SECTION 113        Defeasance of 2007 Notes.

         The 2007 Notes may be defeased by the Company in accordance with the
provisions of Article Fifteen of the Indenture.

SECTION 114        Other Terms of 2007 Notes.

         Without limiting the foregoing provisions of this Article One, the
terms of the 2007 Notes shall be as set forth in the form of the 2007 Notes
contained in Annex A hereto and as provided in the Indenture.

                                  ARTICLE TWO

                      AMENDMENTS TO THE ORIGINAL INDENTURE

SECTION 201        Amendments Applicable Only to 2007 Notes.

         The amendments contained herein shall apply to the 2007 Notes only and
not to any other series of Security issued under the Indenture, and any
covenants provided herein are expressly being included solely for the benefit of
the 2007 Notes and not for the benefit of any other series of Security issued
under the Indenture. These amendments shall be effective for so long as there
remain any 2007 Notes Outstanding.

SECTION 202        Definitions.

         Section 101 of the Original Indenture is hereby amended, subject to
Section 201 hereof and with respect to the 2007 Notes only, by inserting or
restating, as the case may be, in their appropriate alphabetical position, the
following definitions:

         "Accepted Purchased Shares" has the meaning specified in Section
1704(e)(2) of this Indenture.

         "Agent Members" has the meaning specified in Section 102 of the First
Supplemental Indenture.

         "Applicable Procedures" means, with respect to any transfer or exchange
of beneficial ownership interests in a Global Security, the rules and procedures
of the Depositary that are applicable to such transfer or exchange.

         "Certificated Security" means a Security that is in substantially the
form attached to the First Supplemental Indenture as Annex A and that does not
include the information or the schedule called for by footnotes 1, 2 and 3
thereof.

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         "Code" means the Internal Revenue Code of 1986, as amended from time to
time.

         "Conversion Shares" has the meaning specified in Section 1704(k) of
this Indenture.

         "Credit Agreement" means the Third Amended and Restated Secured Credit
Agreement, dated as of June 14, 1999, among the Company, the lenders named
therein and Bank of America, N.A., as administrative agent, including any notes,
guarantees, collateral documents, instruments and agreements executed in
connection therewith, and in each case as amended (including any amendment and
restatement thereof), modified, extended, renewed, refunded, substituted or
replaced or refinanced from time to time, including any agreement extending the
maturity of, refinancing, replacing or otherwise restructuring (including
increasing the amount of available borrowings thereunder or adding Subsidiaries
of the Company as additional borrowers or guarantors thereunder), all or any
portion of the Indebtedness under such agreement or any successor or replacement
agreement and whether by the same or any other agents, creditor, lender or group
of creditors or lenders.

         "Designated Senior Debt" means Senior Debt under the Credit Agreement
and the Senior Secured Note Agreement and the obligations of the Company under
any other particular Senior Debt that expressly provides that such Senior Debt
shall be "Designated Senior Debt" for purposes of this Indenture.

         "Expiration Time" has the meaning specified in Section 1704(e)(1) of
this Indenture.

         "First Supplemental Indenture" means the First Supplemental Indenture,
dated as of July 25, 2000, between the Company and the Trustee, amending and
supplementing the Original Indenture.

         "Fundamental Change" has the meaning specified in Section 1801(a) of
this Indenture.

         "Indebtedness" means:

         (1)      all indebtedness, obligations and other liabilities for
                  borrowed money, including commitment or standby fees,
                  enforcement expenses, collateral protection expenses and other
                  reimbursement indemnity obligations with respect to such
                  indebtedness, overdrafts, foreign exchange contracts, currency
                  exchange agreements, interest rate protection agreements, and
                  any loans or advances from banks, or evidenced by bonds,
                  debentures, notes or similar instruments, other than any
                  account payable or other accrued current liability or
                  obligation incurred in the ordinary course of business in
                  connection with the obtaining of materials or services;

         (2)      obligations with respect to letters of credit, bank guarantees
                  or bankers' acceptances;

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         (3)      obligations in respect of leases required in conformity with
                  generally accepted accounting principles to be accounted for
                  as capitalized lease obligations on the balance sheet of the
                  Company;

         (4)      all obligations and other liabilities under any lease or
                  related document in connection with the lease of real property
                  that provides that the Company is contractually obligated to
                  purchase or cause a third party to purchase the leased
                  property and thereby guarantee a minimum residual value of the
                  leased property to the lessor and the obligations of the
                  Company under the lease or related document to purchase or to
                  cause a third party to purchase the leased property;

         (5)      all obligations with respect to an interest rate or other
                  swap, cap or collar agreement or foreign currency hedge,
                  exchange or purchase agreement;

         (6)      all direct or indirect guaranties or similar agreements in
                  respect of the obligations or liabilities of the Company to
                  purchase, acquire or otherwise assure a creditor against loss
                  in respect of, indebtedness, obligations or liabilities of
                  others of the type described in (1) through (5) above;

         (7)      any obligations described in (1) through (5) above secured by
                  any mortgage, pledge, lien or other encumbrance existing on
                  property which is owned or held by the Company; and

         (8)      any renewals, extensions, refundings, refinancings,
                  restructurings, amendments or modifications to (1) through (7)
                  above.

         "Issue Date" means the first date on which the 2007 Notes were
originally issued, July 25, 2000.

         "Offer" has the meaning specified in section 1704(e)(1) of this
Indenture.

         "Offer Expiration Time" has the meaning specified in Section 1704(e)(2)
of this Indenture.

         "Original Indenture" has the meaning specified in the recitals of the
First Supplemental Indenture.

         "Purchased Shares" has the meaning specified in Section 1704(e)(2) of
this Indenture.

         "Repurchase Date" has the meaning specified in Section 1801.

         "Repurchase Notice" has the meaning specified in Section 1801.

         "Repurchase Price" has the meaning specified in Section 1801.

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         "Securities Custodian" means the Trustee, as custodian with respect to
the Securities in global form, or any successor thereto.

         "Senior Debt" means the principal, premium, if any, interest, including
any interest accruing after bankruptcy, and rent or termination payments on or
other amounts due on the current or future Indebtedness of the Company, whether
created, incurred, assumed, guaranteed or in effect guaranteed by it, but only
to the extent that the same are not treated as "unsecured indebtedness" for
purposes of section 279 of the Code. However, all amounts owing by the Company
under the Credit Agreement and the Senior Secured Note Agreement shall
constitute Senior Debt, and Senior Debt shall not include:

         (1)      Indebtedness that expressly provides that it shall not be
                  senior in right of payment to the 2007 Notes or expressly
                  provides that it is equal or junior in right of payment to the
                  2007 Notes;

         (2)      Indebtedness of the Company to any of the Subsidiaries of the
                  Company; and

         (3)      the 2007 Notes.

         "Senior Secured Note Agreement" means the Note Purchase Agreement,
dated as of March 1, 2000 among the Company and the lenders named therein,
including any notes, guarantees, collateral documents, instruments and
agreements executed in connection therewith, and in each case as amended
(including any amendment and restatement thereof), modified, extended, renewed,
refunded, substituted or replaced or refinanced from time to time, including any
agreement extending the maturity of, refinancing, replacing or otherwise
restructuring (including increasing the amount of available borrowings
thereunder or adding Subsidiaries of the Company as additional borrowers or
guarantors thereunder) all or any portion of the Indebtedness under such
agreement or any successor or replacement agreement and whether by the same or
any other agents, creditor, lender or group of creditors or lenders.

         "2007 Notes" has the meaning specified in the recitals of the First
Supplemental Indenture.

SECTION 203       Amendment of Article Eleven of the Original Indenture.

         Article Eleven of the Original Indenture is hereby supplemented and
amended, subject to Section 201 hereof and with respect to the 2007 Notes only,
by adding the following new Section 1108 thereto:

         "SECTION 1108. Condition to Right of Redemption of 2007 Notes; Press
Release.

                  The Company may not redeem any of the 2007 Notes pursuant to
         this Article Eleven if, at the time of giving the notice referred to in
         Section 1104 hereof, the

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         Company shall have failed to pay any interest or premium with respect
         to the 2007 Notes and such failure shall be continuing.

                  The Company shall issue a press release announcing a
         redemption of the 2007 Notes pursuant to this Article Eleven no later
         than the time notice thereof is given pursuant to Section 1104 hereof."

SECTION 204       Amendment of Article Seventeen of the Original Indenture.

         Article Seventeen of the Original Indenture is hereby supplemented and
amended, subject to Section 201 hereof and with respect to the 2007 Notes only,
by revising subsections (e) and (f) of Section 1704 of the Original Indenture to
read as follows:

         "(e) (1) In case there shall be completed a tender or exchange offer
made by the Company or any Subsidiary of the Company for all or any portion of
the Common Stock (any such tender or exchange offer being referred to as an
"Offer") that involves an aggregate consideration having a fair market value as
of the expiration of such Offer (the "Expiration Time") that, together with (i)
any cash and the fair market value of any other consideration payable in respect
of any other Offer, as of the expiration of such other Offer, expiring within
the 12 months preceding the expiration of such Offer and in respect of which no
Conversion Price adjustment pursuant to this subsection (e) has been made and
(ii) the aggregate amount of any all-cash distributions referred to in
subsection (d) of this Section 1704 to all holders of Common Stock within the 12
months preceding the expiration of such Offer for which no Conversion Price
adjustment pursuant to such subsection (d) has been made, exceeds 15% of the
product of the then current market price per share (determined as provided in
subsection (f) below) of the Common Stock at the Expiration Time times the
aggregate number of shares of Common Stock, Limited Vote Common Stock and
As-Converted Series A Preferred Shares outstanding (including any tendered
shares) at the Expiration Time, the Conversion Price shall be reduced by
multiplying such Conversion Price in effect immediately prior to the Expiration
Time by a fraction of which the numerator shall be the product of the then
current market price per share (determined as provided in subsection (f) below)
of the Common Stock at the Expiration Time times the aggregate number of shares
of Common Stock, Limited Vote Common Stock and As- Converted Series A Preferred
Shares outstanding (including any tendered shares) at the Expiration Time and
the denominator shall be the sum of (i) the fair market value of the aggregate
consideration payable to stockholders based on the acceptance (up to any maximum
specified in the terms of the Offer) of all shares validly tendered and not
withdrawn as of the Expiration Time (the shares deemed so accepted being
referred to as the "Purchased Shares") and (ii) the product of (x) such current
market price per share at the Expiration Time times (y) such number of
outstanding shares at the Expiration Time less the number of Purchased Shares,
such reduction to become effective immediately prior to the opening of business
on the day following the Expiration Time.

         "(e) (2) In case there shall be completed an Offer made by a Person
other than the Company, any of its Subsidiaries or UtiliCorp United, Inc. for an
amount that increases the offeror's ownership

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of Common Stock to more than twenty-five percent (25%) of the Common Stock
outstanding and shall involve the payment by such Person of consideration per
share of Common Stock having a fair market value that as of the last time (the
"Offer Expiration Time") tenders or exchanges may be made pursuant to such Offer
(as it shall have been amended) that exceeds the current market price per share
of the Common Stock on the Trading Day next succeeding the Offer Expiration
Time, and in which, as of the Offer Expiration Time, the Board of Directors is
not recommending rejection of the Offer, the Conversion Price shall be reduced
so that the same shall equal the price determined by multiplying the Conversion
Price in effect immediately prior to the Offer Expiration Time by a fraction the
numerator of which shall be the aggregate number of shares of Common Stock,
Limited Vote Common Stock and As-Converted Series A Preferred Shares outstanding
(including any tendered or exchanged shares) at the Offer Expiration Time
multiplied by the current market price per share of the Common Stock on the
Trading Day next succeeding the Offer Expiration Time and the denominator of
which shall be the sum of (x) the fair market value of the aggregate
consideration payable to stockholders based on the acceptance (up to any maximum
specified in the terms of the Offer) of all shares validly tendered or exchanged
and not withdrawn as of the Offer Expiration Time (the shares deemed so
accepted, up to any such maximum, being referred to as the "Accepted Purchased
Shares") and (y) the product of such number of shares outstanding (less any
Accepted Purchased Shares) at the Offer Expiration Time and the current market
price per share of the Common Stock on the Trading Day next succeeding the Offer
Expiration Time, such reduction to become effective immediately prior to the
opening of business on the Trading Day following the Offer Expiration Time.
Notwithstanding the foregoing, the adjustment described in this clause (2) of
subsection (e) shall not be made if, as of the Offer Expiration Time, the
offering documents with respect to such Offer disclose a plan or intention to
cause the Company to engage in any transaction described in Article Eight.

         "For purposes of this subsection (e), the fair market value of any
consideration with respect to an Offer shall be determined by the Board of
Directors of the Company (whose determination shall, if made in good faith, be
conclusive evidence of such fair market value) and described in a Board
Resolution.

         "In no case shall the Conversion Price be increased pursuant to this
subsection (e).

         "(f) For the purpose of any computation under subsections (b), (c), (d)
and (e) (1) above, the current market price per share of Common Stock on any
date shall be deemed to be the average of the Last Sale Prices of a share of
Common Stock for the five consecutive Trading Days selected by the Company
commencing not more than 20 Trading Days before, and ending not later than, the
earlier of the date in question and the date before the "ex date," with respect
to the issuance, distribution or Offer requiring such computation. For purposes
of any computation under subsection (e)(2) above, the current market price per
share of Common Stock on any date shall be deemed to be the average of the Last
Sale Prices per share of Common Stock for such day and each of the next two
succeeding Trading Days. For purposes of this paragraph, the term "ex date,"
when used with respect to any issuance, distribution or payments with respect to
an Offer, means the first date on which the Common Stock trades regular way on
the New York Stock Exchange (or if not listed or

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admitted to trading thereon, then on the principal national securities exchange
on which the Common Stock is listed or admitted to trading) without the right to
receive such issuance, distribution or Offer."

         Article Seventeen of the Original Indenture is hereby further
supplemented and amended, subject to Section 201 hereof and with respect to the
2007 Notes only, by revising subsection (e) of Section 1706 to read as follows:

         "(e) the Company, any of its Subsidiaries or any other Person described
         in clause (2) of subsection (e) of Section 1704 shall complete an
         Offer;".

SECTION 205      Addition of Article Eighteen to the Original Indenture
                 Applicable Only to 2007 Notes.

         The Original Indenture is hereby supplemented and amended, subject to
Section 201 hereof and with respect to the 2007 Notes only, by adding the
following Article Eighteen:

                                "ARTICLE EIGHTEEN

                      REPURCHASE OF SECURITIES AT OPTION OF
                       THE HOLDER UPON FUNDAMENTAL CHANGE

         SECTION 1801.     Right to Require Repurchase.

                  (a) If at any time prior to July 1, 2007 that 2007 Notes
         remain Outstanding there shall occur a Fundamental Change, then all, or
         such portion as is specified by each Holder, of such 2007 Notes shall
         be repurchased by the Company at the option of the Holders thereof as
         of the date that is 30 days after the date of mailing of the notice
         referred to in Section 1801(b) hereof or if such 30th day is not a
         Business Day, then on the next succeeding Business Day (the "Repurchase
         Date"), at a price equal to 100% of the principal amount of the 2007
         Notes to be repurchased, plus accrued and unpaid interest to, but
         excluding, the Repurchase Date (the "Repurchase Price"), subject to
         satisfaction by or on behalf of any Holder of the requirements set
         forth in subsection (c) of this Section 1801. If the Repurchase Date is
         an Interest Payment Date, the Company shall pay such accrued and unpaid
         interest to the Holder on the preceding Regular Record Date.

                  A "Fundamental Change" means any transaction or event in
         connection with which all or substantially all of the Common Stock of
         the Company is exchanged for, converted into, acquired for or
         constitutes solely the right to receive, consideration, whether by
         means of an exchange offer, liquidation, tender offer, consolidation,
         merger, combination, reclassification, recapitalization or

                                       12

<PAGE>   16

         otherwise, which is not all or substantially all common stock listed
         on, or that will be listed on or immediately after the transaction or
         event on:

                  (1)      A United States national securities exchange, or

                  (2)      approved for quotation on the Nasdaq National Market
                           or any similar United States system of automated
                           dissemination of quotations of securities prices.

         (b) Within 10 days after the occurrence of a Fundamental Change, the
Company shall mail a written notice of the Fundamental Change to the Trustee and
to each Holder. The notice shall include the form of a Repurchase Notice to be
completed by the Holder and shall state:

                           (1) the date of such Fundamental Change and, briefly,
                  the events causing such Fundamental Change;

                           (2) the date by which the Repurchase Notice pursuant
                  to this Section 1801 must be given;

                           (3) the Repurchase Date;

                           (4) the Repurchase Price;

                           (5) briefly, the conversion rights of the 2007 Notes;

                           (6) the name and address of each Paying Agent and
                  Conversion Agent;

                           (7) the Conversion Price and any adjustments thereto;

                           (8) that 2007 Notes as to which a Repurchase Notice
                  has been given may be converted into Common Stock pursuant to
                  Article Seventeen only to the extent that the Repurchase
                  Notice has been withdrawn in accordance with the terms of this
                  Indenture;

                           (9) the procedures that the Holder must follow to
                  exercise rights under this Section 1801;

                           (10) the procedures for withdrawing a Repurchase
                  Notice, including a form of notice of withdrawal; and

                           (11) that the Holder must satisfy the requirements
                  set forth in the 2007 Notes in order to convert the
                  Securities.

                                       13

<PAGE>   17

                  If any of the 2007 Notes is in the form of a Global Security,
         then the Company shall modify such notice to the extent necessary to
         accord with the procedures of the Depositary applicable to the
         repurchase of Global Securities.

                  (c) A Holder may exercise its rights specified in subsection
         (a) of this Section 1801 upon delivery of a written notice (which shall
         be in substantially the form included as an attachment to the 2007
         Notes and which may be delivered by letter, overnight courier, hand
         delivery, facsimile transmission or in any other written form and, in
         the case of Global Securities, may be delivered electronically or by
         other means in accordance with the Depositary's customary procedures)
         of the exercise of such rights (a "Repurchase Notice") to any Paying
         Agent at any time prior to the close of business on the Business Day
         next preceding the Repurchase Date.

                  The delivery of a 2007 Note to any Paying Agent (together with
         all necessary endorsements) at the office of such Paying Agent shall be
         a condition to the receipt by the Holder of the Repurchase Price
         therefor.

                  The Company shall repurchase from the Holder thereof, pursuant
         to this Section 1801, a portion of a 2007 Note if the principal amount
         of such portion is $1,000 or an integral multiple of $1,000. Provisions
         of this Indenture that apply to the repurchase of all of a 2007 Note
         pursuant to Sections 1801 through 1806 also apply to the repurchase of
         such portion of such 2007 Note.

                  Notwithstanding anything herein to the contrary, any Holder
         delivering to a Paying Agent the Repurchase Notice contemplated by this
         subsection (c) shall have the right to withdraw such Repurchase Notice
         in whole or as to a portion thereof that is a principal amount of
         $1,000 or an integral multiple thereof at any time prior to the close
         of business on the Business Day next preceding the Repurchase Date by
         delivery of a written notice of withdrawal to the Paying Agent in
         accordance with Section 1802.

                  A Paying Agent shall promptly notify the Company of the
         receipt by it of any Repurchase Notice or written withdrawal thereof.

                  Anything herein to the contrary notwithstanding, in the case
         of Global Securities, any Repurchase Notice may be delivered or
         withdrawn and such 2007 Notes may be surrendered or delivered for
         repurchase in accordance with the Applicable Procedures of the
         Depositary as in effect from time to time.

                  (d) The Company shall not be required to offer to repurchase
         any 2007 Notes pursuant to this Section 1801 after the occurrence of a
         Fundamental Change if a third party shall make such offer in the
         manner, at the times and otherwise in

                                       14

<PAGE>   18

         compliance with the requirements applicable to a repurchase offer made
         following a Fundamental Change pursuant to this Article Eighteen and
         purchases all 2007 Notes validly tendered and not withdrawn under such
         offer.

         SECTION 1802.     Effect of Repurchase Notice.

                  Upon receipt by any Paying Agent of the Repurchase Notice
         specified in Section 1801(c), the Holder of the 2007 Notes in respect
         of which such Repurchase Notice was given shall (unless such Repurchase
         Notice is withdrawn as specified below) thereafter be entitled to
         receive the Repurchase Price with respect to such 2007 Notes. Such
         Repurchase Price shall be paid to such Holder promptly following the
         Repurchase Date (provided the conditions in Section 1801(c) have been
         satisfied). Any 2007 Notes in respect of which a Repurchase Notice has
         been given by the Holder thereof may not be converted into Common Stock
         on or after the date of the delivery of such Repurchase Notice unless
         such Repurchase Notice has first been validly withdrawn or unless the
         Company shall default on the payment of the Repurchase Price, in which
         case such 2007 Notes will continue to be convertible.

                  A Repurchase Notice may be withdrawn by means of a written
         notice (which may be delivered by letter, overnight courier, hand
         delivery, facsimile transmission or in any other written form and, in
         the case of Global Securities, may be delivered electronically or by
         other means in accordance with the Depositary's customary procedures)
         of withdrawal delivered by the Holder to a Paying Agent at any time
         prior to the close of business on the Business Day immediately
         preceding the Repurchase Date, specifying the principal amount of the
         2007 Note or portion thereof (which must be a principal amount of
         $1,000 or an integral multiple of $1,000 in excess thereof) with
         respect to which such notice of withdrawal is being submitted.

         SECTION 1803.     Deposit of Repurchase Price.

                  On or before 11:00 a.m. New York City time on the Repurchase
         Date, the Company shall deposit with the Trustee or with a Paying Agent
         (other than the Company or an Affiliate of the Company) an amount of
         money (in immediately available funds if deposited on such Business
         Day) sufficient to pay the aggregate Repurchase Price of all the 2007
         Notes or portions thereof that are to be purchased as of such
         Repurchase Date. The manner in which the deposit required by this
         Section 1803 is made by the Company shall be at the option of the
         Company, provided, however, that such deposit shall be made in a manner
         such that the Trustee or a Paying Agent shall have immediately
         available funds on the Repurchase Date.

                                       15

<PAGE>   19

                  If a Paying Agent holds, in accordance with the terms hereof,
         money sufficient to pay the Repurchase Price of any 2007 Notes for
         which a Repurchase Notice has been validly tendered and not withdrawn
         in accordance with this Indenture then, on the Repurchase Date, such
         2007 Notes will cease to be Outstanding and the rights of the Holder in
         respect thereof shall terminate (other than the right to receive the
         Repurchase Price as aforesaid), unless such payment is prohibited by
         Article Twelve hereof, in which case the 2007 Notes shall continue to
         accrue interest and be convertible until such prohibition no longer
         exists and the 2007 Notes are repurchased. The Company shall publicly
         announce the principal amount of 2007 Notes repurchased as a result of
         such Fundamental Change on or as soon as practicable after the
         Repurchase Date.

         SECTION 1804.     Securities Purchased In Part.

                  Any 2007 Note that is to be purchased only in part shall be
         surrendered at the office of a Paying Agent and promptly after the
         Repurchase Date the Company shall execute and the Trustee shall
         authenticate and deliver to the Holder of such 2007 Note, without
         service charge, a new 2007 Note or 2007 Notes, of such authorized
         denomination or denominations as may be requested by such Holder, in
         aggregate principal amount equal to, and in exchange for, the portion
         of the principal amount of the 2007 Note so surrendered that is not
         repurchased.

         SECTION 1805. Compliance With Securities Laws Upon Purchase of
         Securities.

                  In connection with any offer to repurchase or repurchase of
         2007 Notes under Section 1801, the Company shall (a) comply with Rule
         13e-4 (or any successor to such Rule), if applicable, under the
         Exchange Act, (b) file the related Schedule TO (or any successor or
         similar schedule, form or report) if required under the Exchange Act,
         and (c) otherwise comply with all federal and state securities laws in
         connection with such offer, all so as to permit the rights of the
         Holders and obligations of the Company under Sections 1801 through 1806
         to be exercised in the time and in the manner specified therein.

         SECTION 1806.     Repayment to the Company.

                  To the extent that the aggregate amount of cash deposited by
         the Company pursuant to Section 1803 exceeds the aggregate Repurchase
         Price of the 2007 Notes or portions thereof that the Company is
         obligated to repurchase, then promptly after the Repurchase Date the
         Trustee or a Paying Agent, as the case may be, shall return any such
         excess to the Company."

                                       16

<PAGE>   20

                                  ARTICLE THREE

                            MISCELLANEOUS PROVISIONS

SECTION 301        Integral Part.

         This First Supplemental Indenture constitutes an integral part of the
Indenture with respect to the 2007 Notes only.

SECTION 302        General Definitions.

         For all purposes of this First Supplemental Indenture:

                   (a) capitalized terms used herein without definition shall
have the meanings specified in the Original Indenture; and

                   (b) the terms "herein", "hereof", "hereunder" and other words
of similar import refer to this First Supplemental Indenture.

SECTION 303        Adoption, Ratification and Confirmation.

         The Original Indenture, as supplemented and amended by this First
Supplemental Indenture, is in all respects hereby adopted, ratified and
confirmed, and this First Supplemental Indenture shall be deemed part of the
Indenture in the manner and to the extent herein and therein provided. The
provisions of this Supplemental Indenture shall, subject to the terms hereof,
supersede the provisions of the Original Indenture to the extent the Original
Indenture is inconsistent herewith.

SECTION 304        Counterparts.

         This First Supplemental Indenture may be executed in any number of
counterparts, each of which when so executed shall be deemed an original; and
all such counterparts shall together constitute but one and the same instrument.

SECTION 305        Governing Law.

         THIS FIRST SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

SECTION 306        Conflict With Any Provision of Trust Indenture Act.

         If and to the extent that any provision of this First Supplemental
Indenture limits, qualifies or conflicts with a provision required under the
terms of the Trust Indenture Act such Trust Indenture Act provision shall
control.

                                       17

<PAGE>   21

SECTION 307        Effect of Headings and Table of Contents.

         The Article and Section headings herein are for convenience only and
shall not affect the construction hereof.

SECTION 308        Seperability Clause.

         In case any provision in this First Supplemental Indenture or in the
2007 Notes shall be invalid, illegal or unenforceable, the validity, legality
and enforceability of the remaining provisions shall not in any way be affected
or impaired thereby.

SECTION 309        Successors and Assigns.

         All covenants and agreements in this First Supplemental Indenture by
the parties hereto shall bind their respective successors and assigns and inure
to the benefit of their respective successors and assigns, whether so expressed
or not.

SECTION 310        Benefit of First Supplemental Indenture.

         Nothing in this First Supplemental Indenture, express or implied, shall
give to any Person, other than the parties hereto, any Security Registrar, any
Paying Agent, any Conversion Agent and their successors hereunder, and the
Holders of the 2007 Notes, any benefit or any legal or equitable right, remedy
or claim under this First Supplemental Indenture.

SECTION 311        Acceptance by Trustee.

         The Trustee accepts the amendments and supplements to the Original
Indenture effected by this First Supplemental Indenture and agrees to execute
the trusts created by the Original Indenture as hereby amended, but only upon
the terms and conditions set forth in the Indenture. Without limiting the
generality of the foregoing, the Trustee assumes no responsibility for the
correctness of the recitals contained herein, which shall be taken as the
statements of the Company and, except as provided in the Indenture, the Trustee
shall not be responsible or accountable in any way whatsoever for or with
respect to the validity or execution or sufficiency of this First Supplemental
Indenture and the Trustee makes no representation with respect thereto.

                                       18

<PAGE>   22

         IN WITNESS WHEREOF, the parties hereto have caused this First
Supplemental Indenture to be duly executed as of the day and year first written
above.

                                        QUANTA SERVICES, INC.

                                        By:  /s/ Brad Eastman
                                           ---------------------------------
                                                 Brad Eastman
                                                 Vice President

                                        CHASE BANK OF TEXAS, NATIONAL
                                          ASSOCIATION, as Trustee

                                        By:  /s/ John G. Jones
                                           ---------------------------------
                                                 John G. Jones
                                                 Vice President

                                       19

<PAGE>   23

                                                                         ANNEX A

                                                 [FORM OF FACE OF SECURITY]

         [UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE
OF THE DEPOSITORY TRUST COMPANY TO THE COMPANY OR ITS AGENT FOR REGISTRATION OF
TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE ISSUED IS REGISTERED IN THE
NAME OF CEDE & CO. OR IN SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY (AND ANY PAYMENT HEREON IS MADE
TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY), ANY TRANSFER, PLEDGE OR OTHER
USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL SINCE THE
REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

         THIS SECURITY IS A GLOBAL SECURITY WITHIN THE MEANING OF THE INDENTURE
HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF A DEPOSITARY OR A
NOMINEE THEREOF. THIS SECURITY IS EXCHANGEABLE FOR SECURITIES REGISTERED IN THE
NAME OF A PERSON OTHER THAN THE DEPOSITARY OR ITS NOMINEE ONLY IN THE LIMITED
CIRCUMSTANCES DESCRIBED IN THE INDENTURE AND, UNLESS AND UNTIL IT IS EXCHANGED
IN WHOLE OR IN PART FOR SECURITIES IN DEFINITIVE FORM, THIS SECURITY MAY NOT BE
TRANSFERRED EXCEPT AS A WHOLE BY THE DEPOSITARY TO A NOMINEE OF THE DEPOSITARY
OR BY A NOMINEE OF THE DEPOSITARY TO THE DEPOSITARY OR ANOTHER NOMINEE OF THE
DEPOSITARY OR BY THE DEPOSITARY OR ANY SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR
A NOMINEE OF SUCH SUCCESSOR DEPOSITARY.](1)

----------

(1) Include only if the Security is a Global Security.

                                       A-1

<PAGE>   24

                              QUANTA SERVICES, INC.

                    4% CONVERTIBLE SUBORDINATED NOTE DUE 2007

                                                              CUSIP: 747 62E AA0
                                                               $
                                                                ----------------
Registered: No. R-

         Quanta Services Inc., a Delaware corporation (herein called the
"Company", which term includes any successor corporation under the Indenture
hereinafter referred to), for value received, hereby promises to pay to
_____________, or registered assigns, the principal sum of __________________
DOLLARS ($_________)[(or such greater or lesser amount as is indicated on the
Schedule of Exchanges of Securities on the other side of this Security)](2), on
July 1, 2007, and to pay interest thereon from July 25, 2000 or from the most
recent Interest Payment Date to which interest has been paid or duly provided
for, semi-annually on July 1 and December 31 in each year, commencing December
31, 2000, at the rate of 4 % per annum, until the principal hereof is paid or
made available for payment, provided that any principal and premium, and any
such installment of interest, that is overdue shall bear interest at the rate of
4% per annum (to the extent that the payment of such interest shall be legally
enforceable), from the dates such amounts are due until they are paid or made
available for payment, and such overdue interest shall be payable on demand. The
interest so payable, and punctually paid or duly provided for, on any Interest
Payment Date shall, as provided in such Indenture, be paid to the Person in
whose name this Security (or one or more Predecessor Securities) is registered
at the close of business on the Regular Record Date for such interest, which
shall be the June 15 or December 15 (whether or not a Business Day), as the case
may be, next preceding such Interest Payment Date. Any such interest not so
punctually paid or duly provided for will forthwith cease to be payable to the
Holder on such Regular Record Date and may either be paid to the Person in whose
name this Security (or one or more Predecessor Securities) is registered at the
close of business on a Special Record Date for the payment of such Defaulted
Interest, notice whereof shall be given to Holders of Securities of this series
not less than 10 days prior to such Special Record Date, or be paid at any time
in any other lawful manner not inconsistent with the requirements of any
securities exchange on which the Securities of this series may be listed, and
upon such notice as may be required by such exchange, all as more fully provided
in said Indenture.

         This Security is convertible as specified on the reverse of this
Security.

         Payment of the principal of, premium, if any, and interest on this
Security will be made at the office or agency of the Company maintained for that
purpose in The City of New York, in such coin or currency of the United States
of America as at the time of payment is legal tender for

----------

(2) Include only if the Security is a Global Security.

                                       A-2

<PAGE>   25

payment of public and private debts; provided, however, that at the option of
the Company, payment of interest may be made by check mailed to the address of
the Person entitled thereto as such address shall appear in the Security
Register (provided that any Holder of Notes with an aggregate principal amount
in excess of $2,000,000 shall, at the written election of such Holder, be paid
by wire transfer of immediately available funds).

         Reference is hereby made to the further provisions of this Security set
forth on the reverse hereof, which further provisions shall for all purposes
have the same effect as if set forth at this place.

         Unless the certificate of authentication hereon has been executed by
the Trustee referred to on the reverse hereof by manual signature, this Security
shall not be entitled to any benefit under the Indenture or be valid or
obligatory for any purpose.

         IN WITNESS WHEREOF, the Company has caused this instrument to be duly
executed under its corporate seal.

                                             QUANTA SERVICES, INC.

                                             By:
                                                -------------------------------
                                                Name:
                                                Title:

                                       A-3

<PAGE>   26

                     TRUSTEE'S CERTIFICATE OF AUTHENTICATION

         This is one of the Securities of the series designated therein referred
to in the within- mentioned Indenture.

                                                 CHASE BANK OF TEXAS, NATIONAL
                                                 ASSOCIATION, as Trustee

                                                 By:
                                                    --------------------------
                                                      Authorized Signatory

Date of Authentication:
                       -------------------------

                                       A-4

<PAGE>   27

                          [FORM OF REVERSE OF SECURITY]

                              QUANTA SERVICES, INC.

                    4% CONVERTIBLE SUBORDINATED NOTE DUE 2007

         This Security is one of a duly authorized issue of debt securities of
the Company (herein called the "Securities"), issued and to be issued in one or
more series under a Subordinated Indenture, dated as of July 25, 2000, as
amended and supplemented by the First Supplemental Indenture thereto, dated as
of July 25, 2000 (as so amended and supplemented, herein called the
"Indenture"), between the Company and Chase Bank of Texas, National Association,
as Trustee (herein called the "Trustee", which term includes any successor
trustee under the Indenture), to which Indenture and all indentures supplemental
thereto reference is hereby made for a statement of the respective rights,
limitations of rights, duties and immunities thereunder of the Company, the
Trustee, the holders of Senior Debt and the Holders of the Securities and of the
terms upon which the Securities are, and are to be, authenticated and delivered.
This Security is one of the series designated on the face hereof, limited in
aggregate principal amount to $172,500,000.

OPTIONAL REDEMPTION

       No sinking fund is provided for the Securities. At any time on or after
July 3, 2003, the Securities are redeemable as a whole, or in part, at the
option of the Company, at the following Redemption Prices expressed as a
percentage of the principal amount:

<TABLE>
<CAPTION>

                                      Period                                               Redemption Price
                                      -------                                              ----------------
<S>                                                                                       <C>
Beginning on July 3, 2003 and ending on June 30, 2004.............................                   102.286%
Beginning on July 1, 2004 and ending on June 30, 2005.............................                   101.714%
Beginning on July 1, 2005 and ending on June 30, 2006.............................                   101.143%
Beginning on July 1, 2006 and ending on June 30, 2007.............................                   100.571%
</TABLE>

and 100% at July 1, 2007, together with accrued and unpaid interest thereon to,
but excluding, the Redemption Date. If the Redemption Date is also an Interest
Payment Date, interest shall be paid to the Holder on the preceding record date.

NOTICE OF REDEMPTION

         Notice of redemption will be mailed by first-class mail at least 30
days but not more than 60 days before the Redemption Date to each Holder of
Securities to be redeemed at its registered address. Securities in denominations
larger than $1,000 principal amount may be redeemed in part, but only in whole
multiples of $1,000. On and after the Redemption Date, interest shall cease to

                                       A-5

<PAGE>   28

accrue on Securities or portions thereof called for redemption, unless the
Company defaults in making such payment of the Redemption Price, in which case
interest shall continue to accrue until such default is cured and the Securities
are redeemed.

REPURCHASE OF SECURITIES AT OPTION OF HOLDER UPON A FUNDAMENTAL CHANGE IN
CONTROL

         At the option of the Holder and subject to the terms and conditions of
the Indenture, the Company shall become obligated to repurchase all or any part
specified by the Holder (so long as the principal amount of such part is $1,000
or an integral multiple of $1,000 in excess thereof) of the Securities held by
such Holder on the date that is 30 days after the date of the Company's notice
of the occurrence of a Fundamental Change at a Repurchase Price equal to 100% of
the principal amount of the Securities to be repurchased plus accrued and unpaid
interest to, but excluding, the Repurchase Date. If the Repurchase Date is also
an Interest Payment Date, interest shall be paid to the Holder on the preceding
Regular Record Date.

         The Holder shall have the right to withdraw any Repurchase Notice (in
whole or in a portion thereof that is $1,000 principal amount or an integral
multiple of $1,000 in excess thereof) at any time prior to the close of business
on the Business Day prior to the Repurchase Date by delivering a written notice
of withdrawal to the Paying Agent in accordance with the terms of the Indenture.

CONVERSION

         Subject to the provisions of the Indenture, a Holder of a Security may
convert the Security into shares of Common Stock at any time until the close of
business on the second Business Day prior to the Stated Maturity; provided,
however, that if the Security is called for redemption, the conversion right
will terminate at the close of business on the Business Day immediately
preceding the Redemption Date for such Security or such earlier date as the
Holder presents such Security for redemption (unless the Company shall default
in making the redemption payment when due, in which case the conversion right
shall terminate at the close of business on the date such default is cured and
such Security is redeemed). A Security in respect of which a Holder has
delivered a Repurchase Notice exercising the option of such Holder to require
the Company to repurchase such Security may be converted only if such notice of
exercise is withdrawn in accordance with the terms of the Indenture. The initial
Conversion Price is $ 54.53, subject to adjustment as a result of certain events
described in the Indenture. The Company will deliver cash or a check in lieu of
any fractional share of Common Stock.

         To convert a Security, a Holder must (a) complete and manually sign the
conversion notice in substantially the form attached hereto and deliver such
notice to a Conversion Agent, (b) surrender the Security to the Conversion
Agent, (c) furnish appropriate endorsements and transfer documents (including
any certification that may be required under applicable law) if required by the
Conversion Agent,(d) pay any transfer or similar tax, if required, and (e) pay
an amount equal to accrued interest payable on the Security, if required by the
Indenture.

                                       A-6

<PAGE>   29

SUBORDINATION

         The indebtedness evidenced by this Security is, to the extent provided
in the Indenture, subordinate and subject in right of payment to the prior
payment in full of all Senior Debt of the Company, and this Security is issued
subject to the provisions of the Indenture with respect thereto. Each Holder of
this Security, by accepting the same, (a) agrees to and shall be bound by such
provisions, (b) authorizes and directs the Trustee on its behalf to take such
action as may be necessary or appropriate to effectuate the subordination so
provided and (c) appoints the Trustee its attorney-in-fact for any and all such
purposes.

DEFEASANCE AND COVENANT DEFEASANCE

         The Indenture contains provisions for Defeasance and Covenant
Defeasance at any time of the entire indebtedness of this Security and certain
restrictive covenants and Events of Default with respect to this Security, in
each case upon compliance with certain conditions set forth in the Indenture.

TRANSFER

         As provided in the Indenture and subject to certain limitations therein
set forth, the transfer of this Security is registrable in the Security
Register, upon surrender of this Security for registration or transfer at the
office or agency in a Place of Payment for Securities of this series, duly
endorsed by, or accompanied by a written instrument of transfer in form
satisfactory to the Company and the Security Registrar duly executed by, the
Holder hereof or its attorney duly authorized in writing, and thereupon one or
more new Securities of this series and of like tenor, of any authorized
denominations and for the same aggregate principal amount, executed by the
Company and authenticated and delivered by the Trustee, will be issued to the
designated transferee or transferees.

         The Securities of this series are issuable only in registered form
without coupons in denominations of $1,000 and any integral multiple thereof. As
provided in the Indenture and subject to certain limitations set forth therein
and on the face of this Security, Securities of this series are exchangeable for
a like aggregate principal amount of Securities of this series of like tenor and
of a different authorized denomination, as requested by the Holder surrendering
the same.

         No service charge shall be made for any such registration of transfer
or exchange, but the Company may require payment of a sum sufficient to cover
any tax or other governmental charge payable in connection therewith.

         Prior to due presentment of this Security for registration of transfer,
the Company, the Trustee or any agent of the Company or the Trustee may treat
the Person in whose name this Security

                                       A-7

<PAGE>   30

is registered as the owner hereof for all purposes, whether or not this Security
be overdue, and neither the Company, the Trustee nor any such agent shall be
affected by notice to the contrary.

AMENDMENT, SUPPLEMENT AND WAIVER

         The Indenture permits, with certain exceptions as therein provided, the
amendment thereof and the modification of the rights and obligations of the
Company and the rights of the Holders of the Securities of each series to be
affected under the Indenture at any time by the Company and the Trustee with the
consent of the Holders of a majority in principal amount of the Securities at
the time Outstanding of each series to be affected. The Indenture also contains
provisions permitting the Holders of specified percentages in principal amount
of the Securities of each series at the time Outstanding, on behalf of the
Holders of all Securities of such series at the time Outstanding, to waive
compliance by the Company with certain provisions of the Indenture and certain
past defaults under the Indenture and their consequences. Any such consent or
waiver by the Holder of this Security shall be conclusive and binding upon such
Holder and upon all future Holders of this Security and of any Security issued
upon the registration of transfer hereof or in exchange hereof or in lieu
hereof, whether or not notation of such consent or waiver is made upon this
Security.

EVENTS OF DEFAULT

         If an Event of Default with respect to Securities of this series shall
occur and be continuing, the principal of Securities of this series may be
declared due and payable in the manner and with the effect provided in the
Indenture. If an Event of Default occurs as a result of certain events of
bankruptcy, insolvency or reorganization of the Company, any Significant
Subsidiary or any group of Subsidiaries that together would constitute a
Significant Subsidiary, the principal amount of the Securities shall become due
and payable immediately without any declaration or other act on the part of the
Trustee or any Holder, all as and to the extent provided in the Indenture.

INDENTURE TO CONTROL; GOVERNING LAW

         In the case of any conflict between the provisions of this Security and
the Indenture, the provisions of the Indenture shall control.

         THE INDENTURE AND THE SECURITIES SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

                                       A-8

<PAGE>   31

ABBREVIATIONS AND DEFINITIONS

         Customary abbreviations may be used in the name of the Holder or an
assignee, such as: TEN COM (= tenants in common), TEN ENT (= tenants by the
entireties), JT TEN (= joint tenants with right of survivorship and not as
tenants in common),

CUST (= Custodian) and U/G/M/A (= Uniform Gifts to Minors Act).

         All terms defined in the Indenture and used in this Security but not
specifically defined herein are defined in the Indenture and are used herein as
so defined.

                                       A-9

<PAGE>   32

                                CONVERSION NOTICE

         To convert this Security into Common Stock of the Company, check the
box: [ ]

         To convert only part of this Security, state the principal amount to be
converted (must be $1,000 or a multiple of $1,000): $______________.

         If you want the stock certificate made out in another person's name,
fill in the form below:

--------------------------------------------------------------------------------
                (Insert other person's soc. sec. or tax I.D. no.)

--------------------------------------------------------------------------------
            (Print or type other person's name, address and zip code)

Your Signature:                                     Date:
                -----------------------------             ---------------------
(Sign exactly as your name appears on the other side of this Security.)

*Signature guaranteed by:
                         ------------------------------------------------------

By:
   ----------------------------------------------

----------

*    The signature must be guaranteed by an institution which is a member of one
     of the following recognized signature guaranty programs: (i) the Securities
     Transfer Agent Medallion Program (STAMP); (ii) the New York Stock Exchange
     Medallion Program (MSP); (iii) the Stock Exchange Medallion Program (SEMP);
     or (iv) such other guaranty program acceptable to the Trustee.

                                      A-10

<PAGE>   33

               OPTION OF HOLDER TO ELECT REPURCHASE ON FUNDAMENTAL
                                     CHANGE

         If you want to elect to have this Security repurchased, in whole or in
part, by the Company pursuant to Section 1801 of the Indenture, check the
following box: [ ]

         If you want to have only part of this Security purchased by the Company
pursuant to Section 1801 of the Indenture, state the principal amount you want
to be purchased (must be $1,000 or a multiple of $1,000): $________________

Your Signature:                                         Date:
                ----------------------------                 ------------------
(Sign exactly as your name appears on the face of this Security.)

*Signature guaranteed by:
                         ------------------------------------------------------

By:
   ------------------------------------------

----------

*    The signature must be guaranteed by an institution which is a member of one
     of the following recognized signature guaranty programs: (i) the Securities
     Transfer Agent Medallion Program (STAMP); (ii) the New York Stock Exchange
     Medallion Program (MSP); (iii) the Stock Exchange Medallion Program (SEMP);
     or (iv) such other guaranty program acceptable to the Trustee.

                                      A-11

<PAGE>   34

                                 ASSIGNMENT FORM

     To assign this Security, fill in the form below: (I) or (we) assign and
     transfer this Security to

--------------------------------------------------------------------------------
                  (Insert assignee's soc. sec. or tax I.D. no.)

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
              (Print or type assignee's name, address and zip code)

and irrevocably appoint
                       ---------------------------------------------------------
to transfer this Security on the books of the Company. The agent may substitute
another to act for him.

Date:

                     Your Signature:
                                    -------------------------------------------
               (Sign exactly as your name appears on the face of this Security)

                     Signature Guarantee:

                                      A-12

<PAGE>   35

                     SCHEDULE OF EXCHANGES OF SECURITIES(3)

         The following exchanges, redemptions, repurchases or conversions of a
part of this Global Security have been made:

<TABLE>
                                                               Principal Amount
                Amount of decrease      Amount of increase      of this Global             Signature of
                   in Principal            in Principal       Security following        authorized officer
    Date of       Amount of this          Amount of this      such decrease (or            of Trustee or
  Transaction    Global Security         Global Security          increase)                 Custodian
  -----------   ------------------      ------------------    -------------------      -------------------
<S>             <C>                     <C>                   <C>                      <C>
</TABLE>

----------

(3)      This schedule should be included only if the Security is a Global
         Security.

                                      A-13<PAGE>   1
                                                                     EXHIBIT 4.3

                           THIRD AMENDMENT AND CONSENT
             TO THIRD AMENDED AND RESTATED SECURED CREDIT AGREEMENT

         THIS THIRD AMENDMENT AND CONSENT TO THIRD AMENDED AND RESTATED SECURED
CREDIT AGREEMENT (this "Amendment") is entered into as of June 15, 2000, among
QUANTA SERVICES, INC., a Delaware corporation ("Borrower"), the Lenders, as
defined below, and BANK OF AMERICA, N.A., f/k/a NationsBank, N.A., as
administrative agent for the Lenders (in such capacity, the "Agent").
Capitalized terms used but not defined in this Amendment have the meaning given
such terms in the Credit Agreement (defined below).

                                    RECITALS

         A. The Borrower is party to that certain Third Amended and Restated
Secured Credit Agreement dated as of June 14, 1999 (as amended by the First
Amendment dated as of September 21, 1999, the Second Amendment dated as of March
21, 2000, and as may be amended, restated or supplemented from time to time, the
"Credit Agreement"), among the Borrower, Agent, and the lenders from time to
time parties thereto (each a "Lender" collectively, "Lenders").

         B. The Borrower proposes to engage in a private offering of convertible
subordinated debt securities (the "Proposed Offering") pursuant to the
Subordinated Indenture (defined below).

         C. Such convertible debt securities will constitute Subordinated Debt
Investments, and will be permitted Indebtedness under Section 6.14(e) of the
Credit Agreement, provided that such Indebtedness is subordinated in right of
payment and collection to the Obligations on terms acceptable to Majority
Lenders.

         D. Attached as Exhibit "A" to this Amendment are extracts of terms of
the Proposed Offering as delivered by the Borrower to the Agent, which among
other things, sets forth the subordination terms (the "Subordination Terms") of
the Convertible Senior Notes (defined below).

         E. Section 6.11(a)(v)(y) of the Credit Agreement requires, among other
things, Majority Lender approval of any Acquisition where the cash related
purchase price exceeds 7.5% of Consolidated Net Worth as of the end of the
immediately preceding fiscal quarter (the "Consolidated Net Worth Test").

         F. The Borrower is contemplating the Acquisition of Irby Construction
Company, a Mississippi corporation for a proposed cash purchase price of
approximately $68,000,000, with an anticipated closing date in July, 2000 (the
"Proposed Acquisition").

         G. UtiliCorp United, the current holder of the $49,350,000 subordinated
Indebtedness (the "Enron Subordinated Debt") pursuant to the Enron Subordinated
Debt Documents, has given notice that it intends to convert such Indebtedness to
equity (the "Enron Subordinated Debt Conversion"), which conversion will result
in the Borrower's compliance with the Consolidated Net Worth Test for purposes
of the Proposed Acquisition.

         H. For purposes of the Proposed Acquisition only, the Borrower has
requested that the Enron Subordinated Debt be included into the calculation of
the Borrower's Consolidated Net Worth for the fiscal quarter ending June 30,
2000, to the extent that such Indebtedness is not otherwise converted on or
prior to such date.

         I. The Borrower and the Lenders have agreed (1) to amend the Credit
Agreement to accommodate the Proposed Offering, and (2) for purposes of the
Proposed Acquisition only, to include

<PAGE>   2

the Enron Subordinated Debt into the Consolidated Net Worth Test for purposes of
Section 6.11(a)(v)(y) of the Credit Agreement, in each case subject to the terms
and conditions set out in this Amendment.

         NOW THEREFORE, in consideration of the premises and other good and
valuable consideration, the receipt and sufficiency of which are acknowledged,
the undersigned agree as follows:

         1.       New Definitions. Section 1.1 of the Credit Agreement shall be
amended to add the following new definitions:

                  "Convertible Subordinated Notes" means the notes, guarantees,
         and all other obligations now or hereafter arising under, or pursuant
         to, the First Supplemental Indenture, including, without limitation,
         the "2007 Notes" (as defined in the Supplemental Indenture)."

                  "First Supplemental Indenture" means that certain First
         Supplemental Indenture dated as of July 25, 2000, by the Borrower, as
         Issuer, to Chase Bank of Texas, National Association, as Trustee,
         executed and delivered pursuant to the Subordinated Indenture, as the
         same may be amended, restated or supplemented from time to time.

                  "Subordinated Indenture" means that certain Subordinated
         Indenture dated as of July 25, 2000, by the Borrower, as Issuer, to
         Chase Bank of Texas, National Association, as Trustee, as the same may
         be amended, restated or supplemented from time to time.

         2.       Notices. Section 6.6(e) of the Credit Agreement shall be
amended to delete the "and" at the end of clause (iii); to delete the period at
the end of clause (iv); to add a semicolon and "and" at the end of clause (iv);
to add a clause (v) so that clauses (iii), (iv) and (v) read as follows (with
the underlined portions showing the new or revised language):

                  "....(iii) any circumstance that has had a Material Adverse
         Effect; (iv) any event which would result in a breach of Sections 6.20,
         6.21, 6.22, 6.23, or 6.24; and (v) to the extent not covered by clauses
         (i) through (iv) above, any notice received by, or required to be
         delivered by, the Borrower under the Subordinated Indenture and the
         First Supplemental Indenture."

         3.       Indebtedness. Section 6.14 of the Credit Agreement shall be
amended to delete the "and" at the end of clause (f); to delete the period at
the end of clause (g); to add a semicolon and "and" at the end of clause (g);
and add a clause (h) so that clauses (f), (g) and (h) read as follows (with the
underlined portions showing new or revised language):

                 "(f) other Indebtedness not included within subsections (a)
         through (e) above, provided that such Indebtedness shall not exceed, at
         any one time outstanding, an amount equal to 8.5% of Consolidated Net
         Worth as of the end of the immediately preceding fiscal quarter;

                  (g) Indebtedness not to exceed $400,000,000 at any time under
         the Senior Notes and the Note Purchase Agreement"; and ---

                  (h) Indebtedness not to exceed $300,000,000 at any time under
         the Subordinated Indenture, including without limitation, the
         Indebtedness under the Convertible Subordinated Notes and the First
         Supplemental Indenture."

                                       2
<PAGE>   3
         4.       Subordinated Debt Investment. Section 6.24 of the Credit
Agreement shall be amended to delete each reference to the term "Change of
Control" and in each case replace it with the defined term "Change in Control"
so that such section shall read in its entirety as follows (with the underlined
portions showing new or revised language):

                  "Section 6.24 Subordinated Debt Investment. The Borrower shall
         provide written notice to the Agent (by confirmed fax to each of the
         Agent and its legal counsel, Porter & Hedges, L.L.P., attention: Mr.
         Nick H. Sorensen (fax no.: 713-226-0277)) of (i) any Change in Control
         within two (2) Business Days following any such Change in Control, and
         (ii) any notice received by the Borrower from any holder of a
         Subordinated Debt Investment exercising any right to (a) require the
         Borrower to redeem or (b) in respect of the Convertible Subordinated
         Notes, convert, in either case as applicable, all or any part of a
         Subordinated Debt Investment within two (2) Business Days of the
         Borrower's receipt thereof. The Borrower shall not redeem all or any
         part of the Indebtedness evidenced by the Enron Subordinated Debt
         Documents as a result of a Change in Control before ten (10) days
         following the date of a Redemption Notice (as defined in the Enron
         Subordinated Debt Documents) or if prohibited by the subordination
         provisions contained therein. The Borrower shall not redeem, pursuant
         to any optional redemption right it may have, all or any part of a
         Subordinated Debt Investment before the Maturity Date. The Borrower
         shall not amend, modify or change in any way any provision of a
         Subordinated Debt Investment so as to change the stated maturity date
         of the principal of such Indebtedness, or any installment of interest
         thereon, to an earlier date, increase the rate of interest thereon or
         any premium payable on the redemption thereof, change any of the
         redemption or subordination provisions thereof (or the definitions of
         any defined terms contained therein) or otherwise change in any respect
         materially adverse to the interests of the Lenders any of the terms
         thereof, in each case, without the consent of the Majority Lenders."

         5.       Events of Default. Section 7.1 of the Credit Agreement shall
be amended to delete the "or" at the end of clause (l); to delete the period at
the end of clause (m); to add a semicolon and "or" at the end of clause (m); and
add a clause (n) so that clauses (l), (m) and (n) read as follows deleted and
replaced in its entirety as follows (with the underlined portions showing new or
revised language):

                  "(l) an Event of Default shall occur and be continuing under
         the Enron Subordinated Debt Documents or any other documents evidencing
         a Subordinated Debt Investment;

                  (m) an Event of Default shall occur and be continuing under
         the Note Purchase Agreement, the Senior Notes, or any other document
         evidencing Indebtedness under the Senior Notes or the Note Purchase
         Agreement"; or

                  (n) an Event of Default shall occur and be continuing under
         the Subordinated Indenture, the First Supplemental Indenture or the
         Convertible Subordinated Notes, or any other document evidencing
         Indebtedness under the Subordinated Indenture, the First Supplemental
         Indenture or the Convertible Subordinated Notes."

         6.       Consent. Each Lender which signs this Amendment consents to
(a) the Subordination Terms, provided that the same shall not waive or otherwise
modify the covenants contained in Section 6.10(a) of the Credit Agreement, and
the Agent entering into such agreements or other documents as the Agent or its
counsel deems reasonable and necessary in connection with therewith, and (b) for
purposes of the Proposed Acquisition only, the inclusion of the Enron
Subordinated Debt into the Consolidated Net

                                       3
<PAGE>   4
Worth Test for the fiscal quarter ended June 30, 2000 only pursuant to Section
6.11(a)(v)(y) of the Credit Agreement, to the extent that such Indebtedness is
not otherwise converted on or prior to June 30, 2000, provided that (i) the same
shall not waive or otherwise modify any other covenants contained in the Credit
Documents, and (ii) the Borrower otherwise complies with all other terms and
conditions of the Credit Documents in respect of such Proposed Acquisition.

         7.       Conditions. This Amendment shall not be effective until each
of the following have been delivered to the Agent or the following conditions
have been otherwise satisfied in the Agent's sole discretion:

                  (a) this Amendment signed by the Borrower, the Guarantors, and
         the Majority Lenders;

                  (b) the Borrower has delivered to the Agent for the benefit of
         the Lenders a certificate from the Borrower that (i) no Default or
         Potential Default exists under the Credit Agreement, (ii) in respect of
         the Proposed Offering, the Convertible Subordinated Notes have been
         issued, (iii) no default or event of default exists (y) in respect of
         the Proposed Offering, under the Subordinated Indenture or the First
         Supplemental Indenture, and (z) in respect of the consent granted
         herein regarding clause (b) of Section 6 above, under the Enron
         Subordinated Debt Documents, and (iv) contains a true and complete copy
         of the Subordinated Indenture, the First Supplemental Indenture duly
         executed and delivered, together with copies of all other material
         documents executed and delivered in connection therewith; and

                  (c) such other documents as the Agent may reasonably request.

         8.       Fees and Expenses.  The Borrower agrees to pay the reasonable
fees and expenses of counsel to Agent for services rendered in connection with
the preparation, negotiation and execution of this Amendment.

         9.       Representations and Warranties. The Borrower and Guarantors
represent and warrant to the Lenders that they possess all requisite power and
authority to execute, deliver and comply with the terms of this Amendment, which
has been duly authorized and approved by all requisite corporate action on the
part of the Borrower and Guarantors, for which no consent of any Person is
required, and which will not violate their respective organizational documents,
and agree to furnish the Lenders with evidence of such authorization and
approval upon request. The Borrower and Guarantors further represent and warrant
to the Lenders that (a) the representations and warranties in each Credit
Document to which they are a party are true and correct in all material respects
on and as of the date of this Amendment as though made on the date of this
Amendment (except to the extent that (i) such representations and warranties
speak to a specific date or (ii) the facts on which such representations and
warranties are based have been changed by transactions contemplated by the
Credit Agreement), (b) it is in full compliance with all covenants and
agreements contained in each Credit Document to which it is a party, and (c) no
Default or Event of Default has occurred and is continuing.

         10.       Scope of Amendment; Reaffirmation; Release. Except as
affected by this Amendment, the Credit Documents are unchanged and continue in
full force and effect. However, in the event of any inconsistency between the
terms of the Credit Agreement as hereby amended and any other Credit Document,
the terms of the Credit Agreement shall control and such other document shall be
deemed to be amended hereby to conform to the terms of the Credit Agreement. All
references to the Credit Agreement shall refer to the Credit Agreement as
amended by this Amendment. The Borrower and

                                       4
<PAGE>   5
Guarantors hereby reaffirm their respective obligations under, and agree that,
all Credit Documents to which they are a party remain in full force and effect
and continue to evidence their respective legal, valid and binding obligations
enforceable in accordance with their terms (as the same are affected by this
Amendment). The Borrower and Guarantors hereby release the Lenders from any
liability for actions or failures to act in connection with the Credit Documents
prior to the date hereof. This Amendment shall be binding upon and inure to the
benefit of each of the undersigned and their respective successors and permitted
assigns.

         11. Miscellaneous.

                  (a) No Waiver of Defaults. This Amendment does not constitute
         a waiver of, or a consent to, any present or future violation of or
         default under, any provision of the Credit Documents, or a waiver of
         the Lenders' right to insist upon future compliance with each term,
         covenant, condition and provision of the Credit Documents, and the
         Credit Documents shall continue to be binding upon, and inure to the
         benefit of, the Borrower, Guarantors, and the Lenders and their
         respective successors and assigns.

                  (b) Form. Each agreement, document, instrument or other
         writing to be furnished Agent under any provision of this instrument
         must be in form and substance satisfactory to Agent and its counsel.

                  (c) Multiple Counterparts. This Amendment may be executed in
         any number of counterparts with the same effect as if all signatories
         have signed the same document. All counterparts must be construed
         together to constitute one and the same instrument.

                  (d) Governing Law. This Amendment and the other Credit
         Documents must be construed-and their performance enforced-under Texas
         law.

         12.       Entirety .  THE CREDIT DOCUMENTS REPRESENT THE FINAL
AGREEMENT BETWEEN THE BORROWER, GUARANTORS AND THE LENDERS AND MAY NOT BE
CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL
AGREEMENTS BY THE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS AMONG THE
PARTIES.

 [Signatures and Guarantors' Consent and Agreement appear on following pages.]

                                       5
<PAGE>   6
        The Amendment is executed as of the date set out in the preamble to
this Amendment.

                                       QUANTA SERVICES, INC.

                                       By:/S/ James H. Haddox
                                          --------------------------------------
                                              James H. Haddox
                                              Chief Financial Officer

                                       BANK OF AMERICA, N.A., as Administrative
                                       Agent and as a Lender

                                       By: /S/ CRAIG S. WALL
                                           ------------------------------------
                                               Craig S. Wall
                                               Senior Vice President

                                       BANK ONE, TEXAS, NATIONAL ASSOCIATION,
                                       as a Documentation Agent and as a Lender

                                       By: /S/ GREG SMOTHERS
                                           ------------------------------------
                                               Greg Smothers
                                               Vice President

                                       FLEET NATIONAL BANK (f/k/a Bank Boston,
                                       N.A.), as a Documentation Agent and as a
                                       Lender

                                       By: /S/ RITA M. CAHILL
                                           ------------------------------------
                                               Rita M. Cahill
                                               Managing Director

                                       CREDIT LYONNAIS NEW YORK BRANCH, as a
                                       Managing Agent and as a Lender

                                       By: /S/ ROBERT IVOSEVICH
                                           -------------------------------------
                                               Robert Ivosevich
                                               Senior Vice President

<PAGE>   7

                                       THE BANK OF NOVA SCOTIA, as a Managing
                                       Agent and as a Lender

                                       By: /S/ F.C.H. ASHBY
                                          --------------------------------------
                                              F.C.H. Ashby
                                              Senior Manager Loan Operations

                                       NATIONAL CITY BANK, as a Lender

                                       By: /S/ MICHAEL J. DURBIN
                                          --------------------------------------
                                              Michael J. Durbin
                                              Vice President

                                        LASALLE BANK NATIONAL ASSOCIATION, as a
                                        Lender

                                       By:
                                          --------------------------------------
                                       Name:
                                          --------------------------------------
                                       Title:
                                           -------------------------------------

                                        FIRST UNION NATIONAL BANK, as a Lender

                                        By: /S/ MARK B. FELKER
                                           -------------------------------------
                                               Mark B. Felker
                                               Senior Vice President

                                        COMERICA BANK, as a Lender

                                       By: /S/ MARK B. GROVER
                                            -----------------------------------
                                               Mark B. Grover
                                               First Vice President

                                       THE BANK OF TOKYO-MITSUBISHI, LTD., as a
                                       Lender

                                       By: /S/ JAY FORT
                                           ------------------------------------
                                               Jay Fort
                                               Vice President

                                       By: /S/ J. M. MEARNS
                                           -------------------------------------
                                               J. M. Mearns
                                               Vice President and Manager

<PAGE>   8
                                       CHASE BANK OF TEXAS, N.A., as a Lender

                                        By: /S/ JAMES R. DOLPHIN
                                            ------------------------------------
                                                James R. Dolphin
                                                Senior Vice President

                                       GUARANTY FEDERAL BANK, F.S.B., as a
                                       Lender

                                       By: /S/ RICHARD MENCHACA
                                           -------------------------------------
                                               Richard Menchaca
                                               Vice President

                                       SUNTRUST BANK, ATLANTA, as a Lender

                                       By: /S/ DAVID EDGE
                                           -------------------------------------
                                               David Edge
                                               Vice President

                                       By: /S/ M. ANN FORD
                                           -------------------------------------
                                               M. Ann Ford
                                               Officer

                                       BANKERS TRUST COMPANY, as a Lender

                                       By: /S/ PADDY DOWLING
                                           -------------------------------------
                                               Paddy Dowling
                                               Vice President

Exhibit "A" - Subordination Terms

<PAGE>   9
                        GUARANTORS' CONSENT AND AGREEMENT

         As an inducement to the Lenders to execute, and in consideration of the
Lenders' execution of this Amendment, each of the undersigned hereby consents to
this Amendment and agrees that the same shall in no way release, diminish,
impair, reduce or otherwise adversely affect the obligations and liabilities of
the undersigned under their respective Guaranties described in the Credit
Agreement executed by the undersigned, or any agreements, documents or
instruments executed by any of the undersigned, all of which obligations and
liabilities are, and shall continue to be, in full force and effect. This
consent and agreement shall be binding upon the undersigned, and their
respective successors and assigns, and shall inure to the benefit of the
Lenders, and their respective successors and assigns.

                             Advanced Communication Technologies, Inc.
                             Arby Construction, Inc.
                             Austin Trencher, Inc.
                             CCLC, Inc.
                             Computapole, Inc.
                             Conti Communications, Inc.
                             Croce Electric Company, Inc.
                             Crown Fiber Communications, Inc.
                             Dillard Smith Construction Company
                             Driftwood Electrical Contractors, Inc.
                             Edwards Pipeline Company,  Inc.
                             Environmental Professional Associates, Limited
                             Fiber Technology, Inc.
                             Five Points Construction Company
                             GEM Engineering Co., Inc.
                             Golden State Utility Co.
                             Grand Electric Company
                             H.L. Chapman Pipeline Construction, Inc.
                             Haines Construction Company
                             Harker & Harker, Inc.
                             Intermountain Electric, Inc.
                             Manuel Bros., Inc.
                             Network Communications Services, Inc.
                             NorAm Telecommunications, Inc.
                             North Pacific Construction Company
                             North Sky Communications
                             Northern Line Layers, Inc.
                             Pac West Construction, Inc.
                             PAR Electrical Contractors, Inc.
                             PDG Electric Company
                             Potelco, Inc.
                             QSI, Inc.
                             Quanta Acquisition XLV, Inc.
                             Quanta Delaware, Inc.
                             Quanta L Acquisition, Inc.
                             Quanta LI Acquisition, Inc.
                             Quanta LII Acquisition, Inc.
                             Quanta LIII Acquisition, Inc.
                             Quanta Services Management Partnership, L.P.

<PAGE>   10

                             Quanta Utility Installation Co., Inc.
                             Quanta XLI Acquisition, Inc.
                             Quanta XLII Acquisition, Inc.
                             Quanta XLIII Acquisition, Inc.
                             Quanta XLIV Acquisition, Inc.
                             Quanta XLIX Acquisition, Inc.
                             Quanta XLVI Acquisition, Inc.
                             Quanta XLVII Acquisition, Inc.
                             Quanta XLVIII Acquisition, Inc.
                             Quanta XVII Acquisition, Inc.
                             Quanta XXXIX Acquisition, Inc.
                             R. A. Waffensmith & Co., Inc.
                             Ranger Directional, Inc.
                             S.K.S. Pipeliners, Inc.
                             Seaward Corporation
                             Spalj Construction Company
                             Specialty Drilling, Inc.
                             Sullivan Welding, Inc.
                             Sumter Builders, Inc.
                             Telecom Network Specialists, Inc.
                             The Ryan Company, Inc.
                             Tom Allen Construction Company
                             TRANS TECH Electric, Inc.
                             Trawick Construction Co.
                             TTM, Inc.
                             TVS Systems, Inc.
                             Underground Construction Co., Inc.
                             Utilco, Inc.
                             VCI Telecom, Inc.
                             W.H.O.M. Corporation
                             Wade D. Taylor, Inc.
                             West Coast Communications, Inc.
                             World Fiber, Inc.

                             By: /s/ BRAD EASTMAN
                                 -----------------------------------------------
                                     Brad Eastman, President or Vice President
                                     of each Guarantor

                             Coast To Coast, LLC

                             By:       Environmental Professional Associates,
                                       Limited, Its Member

                                       By: /s/ BRAD EASTMAN
                                           -------------------------------------
                                               Brad Eastman, Vice President

                             By:      Quanta Services, Inc., Its Member

                                      By: /s/ BRAD EASTMAN
                                          --------------------------------------
                                              Brad Eastman, Vice President

                             Quanta Services Management Partnership, L.P.

                             By:       QSI, Inc., Its General Partner

                                       By: /s/ BRAD EASTMAN
                                           -------------------------------------
                                                Brad Eastman, Vice President

<PAGE>   11

         EXHIBIT "A"

                               SUBORDINATION TERMS

SUBORDINATION

         The payment of the principal of, premium, if any, and interest on the
Notes will, to the extent described in the Indenture, be subordinated in right
of payment to the prior payment in full of all our Senior Indebtedness. The
holders of all Senior Indebtedness will first be entitled to receive payment in
full of all amounts due or to become due on the Senior Indebtedness, or
provision for payment in money or money's worth, before the holders of the Notes
will be entitled to receive any payment in respect of the Notes, when there is a
payment or distribution of assets to creditors upon our:

         o        liquidation;
         o        dissolution;
         o        winding up;
         o        reorganization;
         o        assignment for the benefit of creditors;
         o        marshaling of assets;
         o        bankruptcy;
         o        insolvency; or
         o        similar proceedings.

         No payments on account of the Notes or on account of the purchase or
acquisition of Notes may be made if a default in any payment with respect to
Senior Indebtedness has occurred and is continuing. If (1) there is a default on
any Senior Indebtedness other than a payment default that occurs that permits
the Holders of that Senior Indebtedness to accelerate its maturity and (2) the
Trustee and the Borrower receive the notice required by the Indenture, no
payments may be made on the Notes for up to 179 days in any 365-day period
unless the default is cured or waived. By reason of this subordination, in the
event of our insolvency, holders of the Notes may recover less, ratably, than
holders of our Senior Indebtedness.

         "Senior Indebtedness" means:

         o        the principal of and premium, if any, and interest on, and
                  fees, costs, enforcement expenses, collateral protection
                  expenses and other reimbursement or indemnity obligations in
                  respect of all of our indebtedness or obligations of us to any
                  person for money borrowed that is evidenced by a note, bond,
                  debenture, loan agreement, or similar instrument or agreement;

         o        commitment or standby fees due and payable to lending
                  institutions with respect to credit facilities available to
                  us;

         o        all of our noncontingent obligations (1) for the reimbursement
                  of any obligor on any letter of credit, banker's acceptance,
                  or similar credit transaction, (2) under interest rate swaps,
                  caps, collars, options, and similar arrangements, and (3)
                  under any foreign exchange contract, currency swap agreement,
                  futures contract, currency option contract, or other foreign
                  currency hedge;

<PAGE>   12

         o        all of our obligations for the payment of money relating to
                  capitalized lease obligations;

         o        any liabilities of others described in the preceding clauses
                  that we have guaranteed or which are otherwise our legal
                  liability; and

         o        renewals, extensions, refundings, refinancings,
                  restructurings, amendments, and modifications of any such
                  indebtedness or guarantee; other than any indebtedness or
                  other obligation of us that by its terms is not superior in
                  right of payment to the Notes.

EVENTS OF DEFAULT

         Each of the following is an "event of default":

         (1)      a default in the payment of any interest upon any of the Notes
                  when due and payable, continued for 30 days;

         (2)      a default in the payment of the principal of and premium, if
                  any, on any of the Notes when due, including on a redemption
                  date;

         (3)      failure to pay when due the principal of or interest on
                  indebtedness for money borrowed by us or our subsidiaries in
                  excess of $20.0 million, or the acceleration of that
                  indebtedness that is not withdrawn within 15 days after the
                  date of written notice to us by the Trustee or to us and the
                  Trustee by the holders of at least 25% in principal amount of
                  the outstanding Notes;

         (4)      a default by us in the performance, or breach, of any of our
                  other covenants in the indenture which are not remedied by the
                  end of a period of 60 days after written notice to us by the
                  Trustee or to us and the Trustee by the holders of at least
                  25% in principal amount of the outstanding Notes; or

         (5)      events of bankruptcy, insolvency or reorganization of the
                  Borrower.

         If an event of default described in clauses (1), (2), (3) or (4) occurs
and is continuing, either the Trustee or the holders of at least 25% in
principal amount of the outstanding Notes may declare the principal amount of an
accrued interest on all Notes to be immediately due and payable. This
declaration may be rescinded if the conditions described in the Indenture are
satisfied. If an event of default of the type referred to in clause (5) occurs,
the principal amount of and accrued interest on the outstanding Notes will
automatically become immediately due and payable.

         Within 90 days after a default, the Trustee must give to the registered
holders of Notes notice of all uncured defaults known to it. The Trustee will be
protected in withholding the notice if it in good faith determines that the
withholding of the notice is in the best interests of the registered holders,
except in the case of a default in the payment of the principal of, or premium,
if any, or interest on, any of the Notes when due or in the payment of any
redemption obligation.

         The holders of not less than a majority in principal amount of the
outstanding Notes may direct the time, method and place of conducting any
proceedings for any remedy available to the Trustee, or exercising any trust or
power conferred on the Trustee. Subject to the provisions of the Indenture
relating

<PAGE>   13

to the duties of the Trustee, if an event of default occurs and is continuing,
the Trustee will be under no obligation to exercise any of the rights or powers
under the Indenture at the request or direction of any of the holders of the
Notes unless the holder have offered to the Trustee reasonable indemnity or
security against any loss, liability or expense. Except to enforce the right to
receive payment of principal, premium, if any, or interest when due or the right
to convert a note in accordance with the Indenture, no holder may institute any
proceeding or pursue any remedy with respect to the Indenture or the Notes
unless it complies with the conditions provided in the Indenture, including:

         o        holders of at least 25% in principal amount of the outstanding
                  Notes have requested the Trustee to pursue the remedy; and

         o        holders have offered the Trustee security or indemnity
                  satisfactory to the Trustee against any loss, liability or
                  expense.

         We are required to deliver to the Trustee annually a certificate
indicating whether the officers signing the certificate know of any default by
us in the performance or observance of any of the terms of the Indenture. If the
officers know of a default, the certificate must specify the status and nature
of all defaults.

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