<PAGE>
                                                                    Exhibit 4.11

                                 SIXTH AMENDMENT
             TO THIRD AMENDED AND RESTATED SECURED CREDIT AGREEMENT

         THIS SIXTH AMENDMENT TO THIRD AMENDED AND RESTATED SECURED CREDIT
AGREEMENT (this "AMENDMENT") is entered into as of October 17, 2001, 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, the Third Amendment and Consent dated as of June 15, 2000, the Fourth
Amendment dated as of October 27, 2000, the Fifth Amendment dated as of November
9, 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. In September, 1999 and in connection with the issuance by the
Borrower of the Preferred Stock to Utilicorp, the Borrower entered into a
management services agreement under which Utilicorp agreed to provide certain
advice regarding (i) corporate and strategic planning, (ii) the development,
evaluation, and marketing of the Borrower's products and services, (iii)
identifying potential acquisition candidates and additional business
opportunities, and (iv) other similar or related services in exchange for the
payment by the Borrower of the Management Fee (the "MANAGEMENT SERVICES
AGREEMENT").

         C. In December, 2000, the Borrower terminated the Management Services
Agreement in exchange for a one-time payment by the Borrower to Utilicorp of
approximately $28,600,000 (the "MANAGEMENT FEE TERMINATION PAYMENT").

         D. The Borrower has requested that the Minimum Interest Coverage Ratio
be amended to exclude the Management Fee Termination Payment from the
calculation of such ratio.

         E. The Borrower has also requested that the LC Commitment Amount be
increased from $50,000,000 to $100,000,000.

         F. The Borrower has also requested that the limitation on Borrower's
Investments in other Persons be increased from $10,000,000 to $60,000,000.

         G. The Borrower, Agent and the Lenders have agreed to amend the Credit
Agreement to accommodate the foregoing requests and to make other related
modifications, in each case subject to the terms and conditions set out in this
Amendment.

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



<PAGE>



         1. Amendments to Section 1.1 (Definitions). SECTION 1.1 of the Credit
Agreement is hereby amended by (a) amending and restating in their entirety the
defined terms "FUNDED DEBT", "LC COMMITMENT AMOUNT", and "MINIMUM INTEREST
COVERAGE RATIO", and (b) adding the defined term "MANAGEMENT FEE TERMINATION
PAYMENT" in its appropriate alphabetical order, in each case as follows (with
the new or modified language underlined):

                  ""FUNDED DEBT" means, as of any date of determination, the
         sum, without duplication, of the following for the Borrower and its
         Subsidiaries: (i) Indebtedness for borrowed money, all obligations
         evidenced by bonds, debentures, notes or similar instruments, and
         purchase money obligations which in accordance with GAAP would be shown
         on the consolidated balance sheet of the Borrower as a liability, (ii)
         all LC Obligations, and all reimbursement obligations relative to the
         face amount of all other letters of credit issued for the account of
         the Borrower or any of its Subsidiaries, and (iii) all Capitalized
         Lease Obligations.

                  "LC COMMITMENT AMOUNT" means $100,000,000 as such amount may
         be reduced from time to time pursuant to the terms of this Agreement.

                  "MANAGEMENT FEE TERMINATION PAYMENT" means the one-time
         payment by Borrower in December, 2000 of approximately $28,600,000 to
         Utilicorp in connection with the termination of the management services
         agreement between Borrower and Utilicorp under which the Borrower paid
         the Management Fee.

                  "MINIMUM INTEREST COVERAGE RATIO" means, for any period, the
         ratio of (a) EBIT plus the amount of the Management Fee expensed during
         such period to (b) the sum of Consolidated Interest Expense, plus the
         amount of any dividend or distribution in respect of the Preferred
         Stock paid or scheduled to be paid during such period, plus the amount
         of the Management Fee paid during such period (other than the
         Management Fee Termination Payment)."

         2. Amendment to Section 6.15 (Loans, Advances and Investments). SECTION
6.15(G) of the Credit Agreement is hereby amended and restated in its entirety
as follows (with the new or modified language underlined):

                  "(g) Investments in Persons other than Borrower or its
                  Subsidiaries, provided that all such Investments shall not
                  exceed $60,000,000 at any one time;"

         3. Conditions. This Amendment shall not be effective until (a) it has
been duly executed and delivered by Borrower, each Guarantor, and at least the
Majority Lenders, and (b) Borrower has delivered to Agent for the benefit of
Lenders (i) a certificate from the Borrower that no Default or Event of Default
exists under the Credit Agreement, and (ii) such other documents, if any, as the
Agent may reasonably request.

         4. Fees and Expenses. The Borrower agrees to pay (a) to Agent for the
benefit of each Lender that executes and delivers this Amendment on or before
October 12, 2001, an amendment fee equal to .10% of such Lender's Commitment,
and (b) the reasonable fees and expenses of counsel to Agent for services
rendered in connection with the preparation, negotiation and execution of this
Amendment.



                                        2
<PAGE>
         5. Representations and Warranties. The Borrower and the 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 the Guarantors, for which no consent of any Person is
required, and which will not violate their respective organizational documents,
and agree to furnish the Agent with evidence of such authorization and approval
upon request. The Borrower and the Guarantors further represent and warrant to
the Agent and 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.

         6. Scope of Amendment and Consent; 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 the 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 the
Guarantors hereby release the Agent and 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.

         7. 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, the Guarantors, the Agent, and the Lenders
         and their respective successors and assigns.

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




                                        3
<PAGE>
            (e) Entirety. THE CREDIT DOCUMENTS, AS AMENDED HEREBY, REPRESENT THE
         FINAL AGREEMENT BETWEEN THE BORROWER, GUARANTORS, THE AGENT 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.]




                                        4
<PAGE>



         EXECUTED as of the date first written above.


                                      QUANTA SERVICES, INC.


                                      By: /s/ Nick Grindstaff
                                         ---------------------------------------
                                               Nick Grindstaff
                                               Treasurer


                                      BANK OF AMERICA, N.A., as Administrative
                                      Agent


                                      By: /s/ Suzanne M. Paul
                                         ---------------------------------------
                                               Suzanne M. Paul, Vice President


                                      BANK OF AMERICA, N.A., as a Lender

                                      By: /s/ Gary L. Mingle
                                         ---------------------------------------
                                               Gary L. Mingle
                                               Senior Vice President

                                      BANK ONE, NA, as a Documentation Agent and
                                      as a Lender


                                      By: /s/ Kathy Turner
                                         ---------------------------------------
                                               Kathy Turner
                                               Director

                                      FLEET NATIONAL BANK (F/K/A BANK BOSTON,
                                      N.A.),  as a Documentation Agent and as a
                                      Lender


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


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


                                      By: /s/ Attila Koc
                                         ---------------------------------------
                                               Attila Koc
                                               Senior Vice President




                                        5
<PAGE>
                                      THE BANK OF NOVA SCOTIA, as a Managing
                                      Agent and as a Lender


                                      By: /s/ M. D. Smith
                                         ---------------------------------------
                                               M. D. Smith
                                               Agent

                                      NATIONAL CITY BANK, as a Lender


                                      By: /s/ Michael J. Durbin
                                         ---------------------------------------
                                               Michael J. Durbin
                                               Vice President


                                      LASALLE BANK NATIONAL ASSOCIATION, as a
                                      Lender


                                      By: /s/ Richard J. Kress
                                         ---------------------------------------
                                               Richard J. Kress
                                               First Vice President


                                      FIRST UNION NATIONAL BANK, as a Lender


                                      By: /s/ Mark B. Felkler
                                         ---------------------------------------
                                               Mark B. Felkler
                                               Senior Vice President


                                      COMERICA BANK, as a Lender

                                      By: /s/ Gerald R. Finney, Jr.
                                         ---------------------------------------
                                               Gerald R. Finney, Jr.
                                               Vice President


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

By: /s/ J. Fort                                By: /s/ J. Mearns
   ----------------------------------             ------------------------------
         J. Fort                               J. Mearns
         Vice President                        Vice President and Manager






                                        6
<PAGE>
                                      THE CHASE MANHATTAN BANK, 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/ Scott L. Brewer
                                         ---------------------------------------
                                               Scott L. Brewer
                                               Vice President

                                      SUNTRUST BANK, ATLANTA, as a Lender


                                      By: /s/ Frank A. Coe
                                         ---------------------------------------
                                               Frank A. Coe
                                               Vice President



                                      BANKERS TRUST COMPANY, as a Lender


                                      By: /s/ Patrick Dowling
                                         ---------------------------------------
                                               Patrick Dowling
                                               Vice President




                                        7

<PAGE>
                        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 TECHNOLOGIES AND INSTALLATION, INC.
                                ARBY CONSTRUCTION, INC.
                                AUSTIN TRENCHER, INC.
                                CCLC, INC.
                                COMMUNICATION MANPOWER, INC.
                                COMPUTAPOLE, INC.
                                CONTI COMMUNICATIONS, INC.
                                CROCE ELECTRIC COMPANY, INC.
                                CROWN FIBER COMMUNICATIONS, INC.
                                DIGCO UTILITY CONSTRUCTION, 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.
                                H.L. CHAPMAN PIPELINE CONSTRUCTION, INC.
                                HAINES CONSTRUCTION COMPANY
                                HARKER & HARKER, INC.
                                INTERMOUNTAIN ELECTRIC, INC.
                                IRBY CONSTRUCTION COMPANY
                                LINE EQUIPMENT SALES CO., INC.
                                LOGICAL LINK, INC.
                                MANUEL BROS., INC.
                                MEARS GROUP, INC.
                                MEARS PIPELINE SERVICES, INC.
                                MEJIA PERSONNEL SERVICES, INC.
                                METRO UNDERGROUND SERVICES, INC.
                                NETCOM MANAGEMENT GROUP, INC.
                                NETWORK COMMUNICATIONS SERVICES, INC.
                                NETWORK ELECTRIC COMPANY
                                NORTH HOUSTON POLE LINE CORP.
                                NORTH PACIFIC CONSTRUCTION CO., INC.
                                NORTH SKY COMMUNICATIONS, INC.
                                NORTHERN LINE LAYERS, INC.
                                PAC WEST CONSTRUCTION, INC.
                                PAR ELECTRICAL CONTRACTORS, INC.
                                PARKSIDE SITE & UTILITY COMPANY CORPORATION
                                PARKSIDE UTILITY CONSTRUCTION CORP.
                                P.D.G. ELECTRIC COMPANY




                                        8
<PAGE>
                                 POTELCO, INC.
                                 PROFESSIONAL TELECONCEPTS, INC. (IL)
                                 PROFESSIONAL TELECONCEPTS, INC. (NY)
                                 QSI, INC.
                                 QUANTA XLVI ACQUISITION, INC.
                                 QUANTA XLVIII ACQUISITION, INC.
                                 QUANTA LI ACQUISITION, INC.
                                 QUANTA LIV ACQUISITION, INC.
                                 QUANTA LV ACQUISITION, INC.
                                 QUANTA LVI ACQUISITION, INC.
                                 QUANTA LVII ACQUISITION, INC.
                                 QUANTA LVIII ACQUISITION, INC.
                                 QUANTA LIX ACQUISITION, INC.
                                 QUANTA LX ACQUISITION, INC.
                                 QUANTA LXI ACQUISITION, INC.
                                 QUANTA LXII ACQUISITION, INC.
                                 QUANTA LXIII ACQUISITION, INC.
                                 QUANTA LXIV ACQUISITION, INC.
                                 QUANTA LXV ACQUISITION, INC.
                                 QUANTA LXVI ACQUISITION, INC.
                                 QUANTA LXVII ACQUISITION, INC.
                                 QUANTA LXVIII ACQUISITION, INC.
                                 QUANTA LXIX ACQUISITION, INC.
                                 QUANTA LXX ACQUISITION, INC.
                                 QUANTA LXXI ACQUISITION, INC.
                                 QUANTA LXXII ACQUISITION, INC.
                                 QUANTA LXXIII ACQUISITION, INC.
                                 QUANTA UTILITY INSTALLATION CO., INC,
                                 R. A. WAFFENSMITH & CO., INC.
                                 RANGER DIRECTIONAL, INC.
                                 RIGGIN & DIGGIN LINE CONSTRUCTION, INC.
                                 SEAWARD CORPORATION
                                 SERVICE CORPORATION OF THE SOUTHWEST
                                 SOUTHEAST PIPELINE CONSTRUCTION, INC.
                                 SOUTHWEST TRENCHING COMPANY, INC.
                                 SPALJ CONSTRUCTION COMPANY
                                 SPECIALTY DRILLING TECHNOLOGY, INC.
                                 SULLIVAN WELDING, INC.
                                 SUMTER UTILITIES, INC.
                                 TELECOM NETWORK SPECIALISTS, INC.
                                 THE RYAN COMPANY, INC.
                                 TOM ALLEN CONSTRUCTION COMPANY
                                 TRAWICK CONSTRUCTION COMPANY, INC.
                                 TTGP, INC.
                                 TTLP, INC.
                                 TTM, INC.
                                 TXLP, INC.
                                 TVS SYSTEMS, INC.
                                 UNDERGROUND CONSTRUCTION CO., INC.
                                 UTILCO, INC.
                                 VCI TELECOM, INC.
                                 W.C. COMMUNICATIONS, INC.
                                 W.H.O.M. CORPORATION





                                        9
<PAGE>



                                 By:       /s/ Dana Gordon
                                          --------------------------------------
                                          Dana Gordon, President or Vice
                                          President of each Guarantor

                                 QUANTA DELAWARE, INC.

                                 By:       /S/ Linda Bubacz
                                          --------------------------------------
                                          Linda Bubacz, President

                                 BROWN ENGINEERING, LLC

                                 By:      Haines Construction Company, Its
                                          Member

                                          By:       /s/ Dana Gordon
                                                   -----------------------------
                                                   Dana Gordon, Vice President

                                 COAST TO COAST, LLC

                                 By:      Environmental Professional Associates,
                                          Limited, a California corporation,
                                          Its Member

                                          By:       /s/ Dana Gordon
                                                   -----------------------------
                                                   Dana Gordon, Vice President

                                 DOT 05, LLC

                                 By:      Seaward Corporation, Its Member

                                          By:       /s/ Dana Gordon
                                                   -----------------------------
                                                   Dana Gordon, Vice President

                                 LAKE NORMAN PIPELINE, LLC

                                 By:      Edwards Pipeline Company, Inc., Its
                                          Member

                                          By:       /s/ Dana Gordon
                                                   -----------------------------
                                                   Dana Gordon, Vice President

                                 MEARS/CPG, LLC
                                 MEARS ENGINEERING, LLC
                                 MEARS/HDD, LLC
                                 MEARS SERVICES, LLC

                                 By:      Mears Group, Inc., The Sole Member of
                                          each of the foregoing limited
                                          liability companies

                                          By:       /s/ Dana Gordon
                                                   -----------------------------
                                                   Dana Gordon, Vice President





                                       10
<PAGE>


                            S.K.S. PIPELINERS, LLC

                            By:      Arby Construction, Inc., Its Member

                                     By:       /s/ Dana Gordon
                                              ----------------------------------
                                              Dana Gordon, Vice President
                            TJADER, L.L.C.

                            By:      Spalj Construction Company, Its Member

                                     By:       /s/ Dana Gordon
                                              ----------------------------------
                                              Dana Gordon, Vice President

                            TNS-VA, LLC

                            By:      Professional Teleconcepts, Inc., Its Member

                                     By:       /s/ Dana Gordon
                                              ----------------------------------
                                              Dana Gordon, Vice President

                            AIRLAN TELECOM SERVICES, L.P.

                            By:      Mejia Personnel Services, Inc., Its General
                                     Partner


                                     By:       /s/ Dana Gordon
                                              ----------------------------------
                                              Dana Gordon, Vice President

                            QUANTA SERVICES MANAGEMENT PARTNERSHIP, L.P.

                            By:      QSI, Inc., Its General Partner


                                     By:       /s/ Dana Gordon
                                              ----------------------------------
                                              Dana Gordon, Vice President

                            TRANS TECH ELECTRIC, L.P.

                            By:      TTGP, Inc., Its General Partner


                                     By:       /s/ Dana Gordon
                                              ----------------------------------
                                              Dana Gordon, Vice President






                                       11