<PAGE>   1
                                                                     EXHIBIT 4.2





                             TAMPA ELECTRIC COMPANY


                                       and


                              THE BANK OF NEW YORK
                                   As Trustee



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



                          THIRD SUPPLEMENTAL INDENTURE

                            dated as of June 15, 2001

                           Supplementing the Indenture

                            dated as of July 1, 1998



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



                                  $250,000,000

                              6.875% Notes Due 2012



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                                TABLE OF CONTENTS

<TABLE>
<CAPTION>

                                                                                                     PAGE
                                                                                                     ----
<S>                        <C>                                                                        <C>
ARTICLE ONE                DEFINITIONS AND OTHER PROVISIONS OF GENERAL APPLICATION...................  2

         Section 101.      Definitions...............................................................  2

         Section 102.      Section References........................................................  2

ARTICLE TWO                DESIGNATION AND TERMS OF THE NOTES........................................  2

         Section 201.      Establishment of Series...................................................  2

         Section 202.      Variations in Terms of Notes..............................................  3

         Section 203.      Amount and Denominations; the Depositary..................................  3

         Section 204.      Interest Rates and Interest Payment Dates.................................  3

         Section 205.      Form and Other Terms of the Notes.........................................  3

         Section 206.      Authentication and Delivery...............................................  4

         Section 207.      Redemption, No Sinking Fund...............................................  4

ARTICLE THREE              AMENDMENTS TO ORIGINAL INDENTURE..........................................  6

         Section 301.      Amendment to Correct Section 610 of Original Indenture....................  6

         Section 302.      Additional Amendment to Modify Definition in the Original Indenture.......  6

         Section 303.      Amendment to Section 801 of Original Indenture............................  6

         Section 304.      Effectiveness of Amendment under Section 303..............................  7

ARTICLE FOUR               MISCELLANEOUS.............................................................  7

         Section 401.      Effect On Original Indenture..............................................  7

         Section 402.      Counterparts..............................................................  7

         Section 403.      Recitals..................................................................  7

         Section 404.      Governing Law.............................................................  7
</TABLE>

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         This Third Supplemental Indenture, dated as of June 15, 2001, between
Tampa Electric Company, a corporation duly organized and existing under the laws
of the State of Florida (hereinafter called the "COMPANY") and having its
principal office at TECO Plaza, 702 North Franklin Street, Tampa, Florida 33602,
and The Bank of New York, as trustee (hereinafter called the "TRUSTEE") and
having its principal corporate trust office at 101 Barclay Street, 21st Floor
West, New York, New York 10286.

                                   WITNESSETH:

         WHEREAS, the Company and the Trustee entered into an Indenture, dated
as of July 1, 1998 (the "ORIGINAL INDENTURE"), pursuant to which one or more
series of debt of the Company (the "SECURITIES") may be issued from time to
time; and

         WHEREAS, Section 201 of the Original Indenture permits the terms of any
series of Securities to be established in an indenture supplemental to the
Original Indenture; and

         WHEREAS, Section 901(7) of the Original Indenture provides that a
supplemental indenture may be entered into by the Company and the Trustee
without the consent of any Holders of the Securities to establish the form and
terms of the Securities of any series; and

         WHEREAS, the Company has requested the Trustee to join with it in the
execution and delivery of this Third Supplemental Indenture in order to
supplement and amend the Original Indenture by, among other things, establishing
the form and terms of one series of Securities to be known as the Company's
"6.875% Notes Due 2012" (the "NOTES") and amending and adding certain provisions
thereof for the benefit of the Holders of the Notes; and

         WHEREAS, the Company and the Trustee desire to enter into this Third
Supplemental Indenture for the purposes set forth in Sections 201 and 901 of the
Original Indenture as referred to above; and

         WHEREAS, the Company has furnished the Trustee with a Board Resolution
authorizing the execution of this Third Supplemental Indenture; and

         WHEREAS, all things necessary to make this Third Supplemental Indenture
a valid agreement of the Company and the Trustee and a valid supplement to the
Original Indenture have been done,

         NOW, THEREFORE, THIS THIRD SUPPLEMENTAL INDENTURE WITNESSETH:

         For and in consideration of the premises and the purchase of the Notes
to be issued hereunder by holders thereof, the Company and the Trustee mutually
covenant and agree, for the equal and proportionate benefit of the respective
holders from time to time of the Notes, as follows:

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                                  ARTICLE ONE
             DEFINITIONS AND OTHER PROVISIONS OF GENERAL APPLICATION


SECTION 101.  DEFINITIONS

         All capitalized terms that are used herein and not otherwise defined
herein shall have the meanings assigned to them in the Original Indenture. The
Original Indenture together with this Third Supplemental Indenture are
hereinafter sometimes collectively referred to as the "INDENTURE."

         "BUSINESS DAY" shall mean any day other than a Saturday or Sunday that
is neither a legal holiday nor a day on which banking institutions are
authorized or required by law or regulations to close in the City of New York.

         "DEPOSITARY" shall have the meaning specified in Section 203 hereof.

         "INTEREST RATE" shall mean the annual rate of interest applicable to
the Notes.

         "INTEREST PAYMENT DATE" shall have the meaning set forth in Section
204(a) hereof.

         "NOTES" shall have the meaning set forth in Section 201 hereof.

         "ORIGINAL ISSUE DATE" shall mean the date upon which the Notes are
initially issued by the Company, such date to be set forth on the face of the
Note.

         "RECORD DATE" shall mean the fifteenth calendar day (whether or not a
Business Day) immediately preceding the related Interest Payment Date.

         "STATED MATURITY DATE" shall mean June 15, 2012.

SECTION 102.  SECTION REFERENCES

         Each reference to a particular section set forth in this Third
Supplemental Indenture shall, unless the context otherwise requires, refer to
this Third Supplemental Indenture.

                                  ARTICLE TWO
                       DESIGNATION AND TERMS OF THE NOTES

SECTION 201.  ESTABLISHMENT OF SERIES

         There is hereby created a series of Securities to be known and
designated as the "6.875% Notes Due 2012" (the "NOTES"), which shall rank
equally with each other and all other unsecured and unsubordinated indebtedness
of the Company. For the purposes of the Original Indenture, the Notes shall
constitute a single series of Securities.

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SECTION 202.  VARIATIONS IN TERMS OF NOTES

         Subject to the terms and conditions set forth in the Original Indenture
and in this Third Supplemental Indenture, the terms of any particular Note may
vary from the terms of any other Note as contemplated by Section 301 of the
Original Indenture, and the terms for a particular Note will be set forth in
such Note as delivered to the Trustee or an Authenticating Agent for
authentication pursuant to Section 303 of the Original Indenture.

SECTION 203.  AMOUNT AND DENOMINATIONS; THE DEPOSITARY

         The aggregate principal amount of Notes that may be issued under this
Third Supplemental Indenture is limited to $250,000,000. The authorized
denominations of Notes shall be $1,000 or integral multiples of $1,000 in excess
thereof.

         The Notes shall be issuable only in fully registered form, without
coupons, and will initially be registered in the name of The Depository Trust
Company or its successor ("DEPOSITARY"), or its nominee who is hereby designated
as "U.S. Depositary" under the Original Indenture.

SECTION 204.  INTEREST RATES AND INTEREST PAYMENT DATES

         (a) Interest Rate. The Notes shall bear interest at the annual rate set
forth on the face thereof (the "INTEREST RATE") from the Original Issue Date to
the Stated Maturity Date. Interest on the Notes will be payable semi-annually on
June 15 and December 15 of each year (each, an "INTEREST PAYMENT DATE"),
commencing on December 15, 2001. Such interest will be payable to the holder
thereof as of the related Record Date.

         (b) Computation of Interest. The amount of interest payable for any
period will be computed on the basis of a year of 360 days consisting of twelve
30-day months. Except for the effect of any adjustment in the Interest Payment
Date as provided in the following sentence, the amount of interest payable for
any period shorter than a full six-month period for which interest is computed,
will be computed on the basis of the actual number of days elapsed in such a
180-day period. If any Interest Payment Date would otherwise be a day that is
not a Business Day, such Interest Payment Date will be postponed to the next
succeeding Business Day, and no interest will accrue on such payment for the
period from and after such Interest Payment Date to the date of such payment on
the next succeeding Business Day, except that, if such Business Day is in the
next succeeding calendar year, such payment shall be made on the immediately
preceding Business Day, in each case with the same force and effect as if made
on such date.

SECTION 205.  FORM AND OTHER TERMS OF THE NOTES

     (a) Attached hereto as EXHIBIT A is the form of Note, which form is hereby
established as the form in which Notes may be issued.

     (b) Subject to (a) above, any Note may be issued in such other form as may
be provided by, or not inconsistent with, the terms of the Original Indenture
and this Third Supplemental Indenture.

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SECTION 206.  AUTHENTICATION AND DELIVERY

         As provided in and pursuant to Section 303 of the Original Indenture,
each time that the Company delivers Notes to the Trustee or Authenticating Agent
for authentication after the initial issuance of Notes under this Indenture, the
Company shall deliver a Supplemental Company Order in the form of EXHIBIT B to
this Third Supplemental Indenture for the authentication and delivery of such
Notes and the Trustee or such Authenticating Agent shall authenticate and
deliver such Notes.

SECTION 207.  REDEMPTION, NO SINKING FUND

         The Notes are subject to redemption, in whole or in part, at any time,
and at the option of the Company, at a redemption price equal to the greater of:

                 (i)  100% of the principal amount of Notes then outstanding to
         be redeemed, or

                 (ii) the sum of the present values of the remaining scheduled
         payments of principal and interest on the Notes then outstanding to be
         redeemed (not including any portion of such payments of interest
         accrued as of the redemption date) discounted to the redemption date on
         a semiannual basis (computed based on a 360-day year consisting of
         twelve 30-day months) at the Adjusted Treasury Rate, plus 25 basis
         points (0.25%), as calculated by an Independent Investment Banker,

plus, in both of the above cases, accrued and unpaid interest thereon to the
redemption date.

         The Company will mail a notice of redemption at least 30 days but no
more than 60 days before the redemption date to each holder of Notes to be
redeemed. If the Company elects to partially redeem the Notes, the Trustee will
select in a fair and appropriate manner the Notes to be redeemed.

         Unless the Company defaults in payment of the redemption price, on and
after the redemption date, interest will cease to accrue on the Notes or
portions thereof called for redemption.

         The Notes are not entitled to the benefit of any sinking fund or
analogous provision.

         "ADJUSTED TREASURY RATE" means, with respect to any redemption date:

                 (iii) the yield, under the heading which represents the average
         for the immediately preceding week, appearing in the most recently
         published statistical release designated "H.15(519)" or any successor
         publication which is published weekly by the Board of Governors of the
         Federal Reserve System and which establishes yields on actively traded
         United States Treasury securities adjusted to constant maturity under
         the caption "Treasury Constant Maturities," for the maturity
         corresponding to the Comparable Treasury Issue (if no maturity is
         within three months before or after the Remaining Life, as defined
         below, yields for the two published maturities most closely

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<PAGE>   7
         corresponding to the Comparable Treasury Issue will be determined and
         the Adjusted Treasury Rate will be interpolated or extrapolated from
         such yields on a straight line basis, rounding to the nearest month);
         or

                 (iv) if such release (or any successor release) is not
         published during the week preceding the calculation date or does not
         contain such yields, the rate per annum equal to the semi-annual
         equivalent yield to maturity of the Comparable Treasury Issue,
         calculated using a price for the Comparable Treasury Issue (expressed
         as a percentage of its principal amount) equal to the Comparable
         Treasury Price for such redemption date.

The Adjusted Treasury Rate will be calculated on the third Business Day
preceding the redemption date.

         "COMPARABLE TREASURY ISSUE" means the United States Treasury security
selected by an Independent Investment Banker as having a maturity comparable to
the remaining term of the Notes to be redeemed that would be used, at the time
of selection and in accordance with customary financial practice, in pricing new
issues of corporate debt securities of comparable maturity to the remaining term
of such Notes (the "REMAINING LIFE").

         "COMPARABLE TREASURY PRICE" means (1) the average of five Reference
Treasury Dealer Quotations for such redemption date, after excluding the highest
and lowest Reference Treasury Dealer Quotations, or (2) if the Independent
Investment Banker obtains fewer than five such Reference Treasury Dealer
Quotations, the average of all such quotations.

         "INDEPENDENT INVESTMENT BANKER" means Banc of America Securities LLC
and its successors, or if that firm is unwilling or unable to serve as such, an
independent investment and banking institution of national standing appointed by
the Company.

         "REFERENCE TREASURY DEALER" means:

                 (i) Banc of America Securities LLC and its successors; provided
         that, if Banc of America Securities LLC ceases to be a primary U.S.
         Government securities dealer in New York City (a "Primary Treasury
         Dealer"), the Company will substitute another Primary Treasury Dealer;
         and

                 (ii) up to four other Primary Treasury Dealers selected by the
         Company.

         "REFERENCE TREASURY DEALER QUOTATIONS" means, with respect to each
Reference Treasury Dealer and any redemption date, the average, as determined by
the Independent Investment Banker, of the bid and asked prices for the
Comparable Treasury Issue (expressed in each case as a percentage of its
principal amount) quoted in writing to the Independent Investment Banker at 5:00
p.m., New York City time, on the third Business Day preceding such redemption
date.

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                                 ARTICLE THREE
                        AMENDMENTS TO ORIGINAL INDENTURE

SECTION 301.  AMENDMENT TO CORRECT SECTION 610 OF ORIGINAL INDENTURE

         In order to correct a mistaken reference, pursuant to Section 901(9) of
the Original Indenture, Section 610(d)(1) of the Original Indenture is hereby
amended, effective immediately, to read as follows:

             "(d) If at any time:

                  (1) the Trustee shall fail to comply with Section 608 after
         written request therefor by the Company or any Holder who has been a
         bona fide Holder of a Security for at least six months, or"

SECTION 302. ADDITIONAL AMENDMENT TO MODIFY DEFINITION IN THE ORIGINAL INDENTURE

         In order to modify a provision of the Original Indenture in a manner
not adversely affecting the interests of the Holders of Securities of any series
in any material respect, pursuant to Section 901(9) of the Original Indenture,
the definition of "Company Request" or "Company Order" in Section 101 of the
Original Indenture is hereby amended, effective immediately, to read as follows:

                  "'COMPANY REQUEST' or 'COMPANY ORDER' means a written request
         or order signed in the name of the Company by its President, a Vice
         President, its Chief Financial Officer, its Treasurer or an Assistant
         Treasurer and delivered to the Trustee."

SECTION 303.  AMENDMENT TO SECTION 801 OF ORIGINAL INDENTURE

         Subparagraph (1) of Section 801 of the Original Indenture is amended,
effective as provided in Section 304 hereof, to read as follows:

                  "(1) the Corporation formed by such consolidation into which
         the Company is merged or the Person which acquires by conveyance or
         transfer the properties and assets of the Company substantially as an
         entirety (a) shall be, if a Corporation, a Corporation organized and
         existing under the laws of (i) the United States of America or any
         State or the District of Columbia or (ii) a foreign jurisdiction and
         which consents to the jurisdiction of the courts of the United States
         of America or of any State, and (b) shall expressly assume, by an
         indenture supplemental hereto, executed and delivered to the Trustee,
         in form satisfactory to the Trustee, the due and punctual payment of
         the principal of (and premium, if any) and interest on all the
         Securities and the performance of every covenant of this Indenture on
         the part of the Company to be performed or observed;"

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SECTION 304.  EFFECTIVENESS OF AMENDMENT UNDER SECTION 303.

     The amendment to the Original Indenture set forth in Section 303 hereof
shall be effective upon the approval of the Holders of Outstanding Securities
under the Indenture as required by Section 901(5) and Section 902 of the
Original Indenture. For this purpose, the Holders of the Notes, by their
acquisition thereof, shall be deemed to have approved such amendment.



                                  ARTICLE FOUR
                                  MISCELLANEOUS

SECTION 401. EFFECT ON ORIGINAL INDENTURE

         The Third Supplemental Indenture is a supplement to the Original
Indenture. As supplemented by this Third Supplemental Indenture, the Original
Indenture is in all respects ratified, approved and confirmed, and the Original
Indenture and this Third Supplemental Indenture shall together constitute one
and the same instrument.

SECTION 402.  COUNTERPARTS

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

SECTION 403.  RECITALS

         The recitals contained herein shall be taken as the statements of the
Company, and the Trustee assumes no responsibility for their correctness. The
Trustee makes no representations as to the validity or sufficiency of this Third
Supplemental Indenture.

SECTION 404.  GOVERNING LAW

         This Third Supplemental Indenture shall be governed by and construed in
accordance with the laws of the jurisdiction that govern the Original Indenture
and its construction.

              [The balance of this page intentionally left blank.]


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



                                       TAMPA ELECTRIC COMPANY



                                        By: /s/ Gordon L. Gillette
                                           -------------------------------------
                                        Name:    Gordon L. Gillette
                                        Title:   Senior Vice President - Finance
                                                 and Chief Financial Officer





                                       THE BANK OF NEW YORK, AS TRUSTEE



                                       By: /s/ James Hall
                                           -------------------------------------
                                           Name: James Hall
                                           Title: Vice President


<PAGE>   11


State of Georgia                    )
                                    ) SS.:
County of Carroll                   )

     On the 20th day of June, 2001 before me personally came Gordon Lindsay
Gillette to me known, who, being by me duly sworn, did depose and say that s/he
is Sr. V.P., Finance of TAMPA ELECTRIC COMPANY, one of the corporations
described in and which executed the foregoing instrument.


                                            Glenda H. Kunz
                                            ------------------------------------
                                            Notary Public





State of New York           )
                            ) SS.:
County of New York          )

         On the 21st day of June, 2001 before me personally came James Hall, to
me known, who, being by me duly sworn, did depose and say that he/she is Vice
President of THE BANK OF NEW YORK, one of the corporations described in and
which executed the foregoing instrument.





                                            Karen Katlan
                                            ------------------------------------
                                            Notary Public


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                                                                       EXHIBIT A

                                  FORM OF NOTE





[Not reproduced here to avoid unnecessary reproduction. See Exhibit 4.3 to this
Current Report on Form 8-K for a copy of the executed Note.]


<PAGE>   13


                                                                       EXHIBIT B

                             TAMPA ELECTRIC COMPANY

                              6.875% NOTES DUE 2012

                           SUPPLEMENTAL COMPANY ORDER



         Pursuant to Section 206 of Article Two of the Third Supplemental
Indenture, dated as of June 15, 2001, to the Indenture, dated as of July 1,
1998, as amended, you are instructed to prepare and authenticate a Note, of the
series identified above, in the principal amount of $______________. The Note is
being delivered in exchange for issued and outstanding Notes of the series
identified above.

         IN WITNESS WHEREOF, I have hereunto set my hand this _________ day
of _________, ____.

                                           TAMPA ELECTRIC COMPANY



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



                                 Exhibit B -- 1