<PAGE>
                                                                 EXHIBIT 4(c)(3)

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                             PUBLIC SERVICE COMPANY
                                   OF COLORADO

                                       TO

                              THE BANK OF NEW YORK,

                                   as Trustee

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

                                   SUPPLEMENTAL INDENTURE
                       -----------

                        Dated as of
                                    --------------------

                           Supplementing the Indenture
                            dated as of July 1, 1999

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

                   Establishing the Securities of Series ____
                designated _____% Senior Notes due _____________

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<PAGE>


                  THIS __________ SUPPLEMENTAL INDENTURE, dated as of
____________, is between PUBLIC SERVICE COMPANY OF COLORADO, a Colorado
corporation (hereinafter called the "Issuer" or the "Company"), having its
principal office at 1225 17th Street, Denver, Colorado 80202, and THE BANK OF
NEW YORK, as Trustee (hereinafter called the "Trustee"), having its principal
corporate trust office at 101 Barclay Street, New York, New York 10286.

                             Recitals of the Issuer

                  The Issuer has heretofore executed and delivered an Indenture,
dated as of July 1, 1999 (the "Original Indenture", the Original Indenture,
previously supplemented and as further supplemented by this supplemental
indenture being hereinafter referred to as the "Indenture"), relating to the
issuance at any time or from time to time of its Securities on terms to be
specified at the time of issuance. Terms used and not otherwise defined herein
shall (unless the context otherwise clearly requires) have the respective
meanings given to them in the Original Indenture.

                  The Original Indenture provides in Article Three thereof that,
prior to the issuance of Securities of any series, the form of such Securities
and the terms applicable to such series shall be established in, or pursuant to,
the authority granted in a resolution of the Board of Directors or established
in one or more indentures supplemental thereto.

                  The Issuer desires by this supplemental indenture, among other
things, to establish the form of the Securities of a series, to be titled ____%
Notes due ________________ of the Issuer, and to establish the terms applicable
to such series, pursuant to Sections 201, 301 and 901 of the Original Indenture.
The Issuer has duly authorized the execution and delivery of this supplemental
indenture.

                  Article Nine of the Original Indenture provides that the
Issuer, when authorized by a resolution of its Board of Directors, and the
Trustee may from time to time and at any time amend the Indenture without the
consent of Securityholders for certain purposes enumerated in Section 901
thereof, including the purposes set forth in subsection (7) of said Section 901.

                  The execution and delivery of this supplemental indenture by
the parties hereto are in all respects authorized by the provisions of the
Indenture.

                  All things necessary have been done to make this supplemental
indenture a valid agreement of the Issuer, in accordance with its terms.

                  NOW, THEREFORE, THIS ______ SUPPLEMENTAL INDENTURE WITNESSETH:

                  For and in consideration of the premises, it is mutually
covenanted and agreed, as follows:


<PAGE>


                                    ARTICLE I

                          Establishment of  % Notes due

         Section 1.01. The title of the series of the Securities established by
this supplemental indenture shall be ____% Notes due __________________ of the
Issuer (hereinafter called the "Series ___ Notes").

         Section 1.02. The Series ___ Notes shall be limited to
[$______________] in aggregate principal amount.

         Section 1.03. The Series ___ Notes may be issued in whole or in part as
one or more Global Securities and The Depository Trust Company, or a nominee
thereof, shall be the Depository for such Global Security or Global Securities.
The Depository for such Global Security or Global Securities representing Series
___ Notes may surrender one or more Global Securities representing Series ___
Notes in exchange in whole or in part for individual Series ___ Notes on such
terms as are acceptable to the Issuer and such Depository and otherwise subject
to the terms of the Indenture.

         Section 1.04. The principal of the Series ___ Notes shall be payable on
_________________.

         Section 1.05. The Series ___ Notes shall bear interest at the rate of
___% per annum and shall accrue from _____________. The Interest Payment Dates
shall be ____________ and _____________ in each year, commencing
_______________. The Regular Record Dates in respect of such Interest Payment
Dates shall be ________________ and _______________ in each year, respectively.

         Section 1.06. The Corporate Trust Office of The Bank of New York shall
be the place at which the principal of the Series ___ Notes shall be payable.
Any interest thereon shall be paid as specified in Section 307 of the Original
Indenture.

         Section 1.07. [Interest Payment Deferral Provisions, if any.]

         Section 1.08. [Optional Redemption Provisions, if any.]

         Section 1.09. [Mandatory Redemption Provisions, if any.]

         Section 1.10. The Series ___ Notes shall be issued in denominations of
[$_______] and any integral multiple thereof.

         Section 1.11. [Original Issue Discount Provisions, if any.]

         Section 1.12. [Additional Defaults, if any.]

         Section 1.13. Sections 1301 and 1302 of the Indenture shall be
applicable to the Series ___ Notes.


                                       2
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         Section 1.14. [Currency, if other than U.S. Dollars.]

         Section 1.15. [Additional Covenants, if any.]

         Section 1.16. The Issuer hereby appoints, or confirms the appointment
of, The Bank of New York as the initial Trustee, Securities Registrar and Paying
Agent, subject to the provisions of the Indenture with respect to resignation,
removal and succession, and subject, further, to the right of the Issuer to
appoint additional agents (including Paying Agents).

         Section 1.17. [Exceptions to Business Day Definition, if any.]

         Section 1.18. [Any Additional Terms.]

         Section 1.19. The Series ___ Notes shall be substantially in the form
set forth in Exhibit A hereto, and shall have such further terms as are
reflected in such form, subject to changes in the form thereof made by the
Issuer and acceptable to the Trustee.

                                   ARTICLE II

                                  Miscellaneous

         Section 2.01. The recitals contained herein shall be taken as the
statements of the Issuer, and the Trustee assumes no responsibility for the
correctness of the same. The Trustee makes no representation as to the validity
of this supplemental indenture. The Indenture, as supplemented by this
supplemental indenture, is in all respects hereby adopted, ratified and
confirmed.

         Section 2.02. This supplemental indenture may be executed in any number
of counterparts, and on separate counterparts, each of which shall be an
original; but such counterparts shall together constitute but one and the same
instrument.

         Section 2.03. If any provision of this supplemental indenture limits,
qualifies or conflicts with the duties imposed by any of Sections 310 to 317,
inclusive, of the Trust Indenture Act of 1939, as amended by the Trust Indenture
Reform Act of 1990, through operation of Section 318(c), such imposed duties
shall control.

         Section 2.04. The Article headings herein are for convenience only and
shall not affect the interpretation hereof.


                                       3
<PAGE>


         IN WITNESS WHEREOF, the parties hereto have caused this Second
Supplemental Indenture to be duly executed as of the ____ day of
_________________.

                                            PUBLIC SERVICE COMPANY OF
                                            COLORADO



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


                                            THE BANK OF NEW YORK



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


                                       4
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                                    EXHIBIT A

                       Form of ___% Note due ____________

                  [THIS SECURITY IS A GLOBAL SECURITY REGISTERED IN THE NAME OF
THE DEPOSITORY (REFERRED TO HEREIN) OR A NOMINEE THEREOF AND UNLESS AND UNTIL IT
IS EXCHANGED IN WHOLE OR IN PART FOR THE INDIVIDUAL SECURITIES REPRESENTED
HEREBY, THIS GLOBAL 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.

                  UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED
REPRESENTATIVE OF THE DEPOSITARY TRUST COMPANY, A NEW YORK CORPORATION (55 WATER
STREET, NEW YORK, NEW YORK), TO THE TRUSTEE 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 DEPOSITARY TRUST COMPANY AND ANY PAYMENT IS MADE TO CEDE & CO., ANY
TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON
IS WRONGFUL INASMUCH AS THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST
HEREIN.]

                       PUBLIC SERVICE COMPANY OF COLORADO
                       ___% Note due ____________________



                          Interest Rate:
                                          -----------------
                 Interest Payment Dates:
                                          -----------------
                   Regular Record Dates:
                                          -----------------
         Original Interest Accrual Date:
                                          -----------------
                        Stated Maturity:
                                          -----------------

Registered No.                            Principal Amount
               ---------------------
CUSIP                                     $
      ------------------------------       ------------------------

                  PUBLIC SERVICE COMPANY OF COLORADO, a corporation duly
organized and existing under the laws of the State of Colorado (herein called
the "Company", which term includes any successor corporation under the Indenture
referred to below) promises to pay to _______________ or registered assigns the
principal sum of _______________________ Dollars on _____________________ the
Stated Maturity specified above.


                                      A-1
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1.       Interest.

                  The Company promises to pay interest on the principal amount
                  hereof at the Interest Rate per annum shown above from the
                  Original Interest Accrual Date specified above, or from the
                  most recent Interest Payment Date to which interest has been
                  paid, semiannually in arrears, on the Interest Payment Dates
                  specified above, in each year, commencing with the Interest
                  Payment Date next succeeding the Original Interest Accrual
                  Date specified above, until the principal hereof is paid or
                  duly provided for. Interest will be computed on the basis of a
                  360-day year of twelve 30-day months.

2.       Method of Payment.

                  The Company will pay interest so payable to the person who is
                  the registered holder hereof at the close of business on the
                  Regular Record Date for the next Interest Payment Date, except
                  as otherwise provided in the Indenture and except that
                  interest payable at Maturity will be paid to the person to
                  whom principal is paid at Maturity. Payment of principal shall
                  be made upon presentation hereof at the office of this Paying
                  Agent. The Company will pay principal and interest in money of
                  the United States that at the time of payment is legal tender
                  for payment of public and private debts. The Company may pay
                  principal and interest by check payable in such money. It may
                  mail an interest check to the holder's registered address.

3.       Agents.

                  Initially, The Bank of New York, 101 Barclay Street, Floor 21
                  West, New York, New York 10286, Attention: Corporate Trust
                  Administration, will act as Paying Agent, and Securities
                  Registrar. The Company may change the Paying Agent to provide
                  for more than one such agent. The Company may appoint one or
                  more Security Registers. The Company or any Affiliate may act
                  in any such capacity. The Trustee may appoint one or more
                  Authenticating Agents to authenticate the Securities.

4.       Indenture.

                  The securities of this series (the "Securities") have been
                  issued under an Indenture dated as July 1, 1999 (the
                  "Indenture") between the Company and The Bank of New York (the
                  "Trustee," which term includes any successor trustee under the
                  Indenture). The terms of the Securities include those stated
                  in the Indenture and in the Supplemental Indenture creating
                  the Securities and those made part of the Indenture by the
                  Trust Indenture Act of 1939 (15 U.S. Code Sections
                  77aaa-77bbbb). Securityholders are referred to the Indenture,
                  the Supplemental Indenture and the Act for a statement of such
                  terms.


                                      A-2
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5.       Redemption.

                  [This Security is not redeemable prior to maturity.] or [This
                  Security is subject to redemption upon not less than 30 days'
                  notice by first class mail, in whole at any time or in part
                  from time to time at the option of the Company at a redemption
                  price equal to the greater of (i) 100% of the principal amount
                  hereof to be redeemed or (ii) the sum of the present values of
                  the remaining scheduled payments of principal and interest
                  thereon discounted to the redemption date on a semiannual
                  basis (assuming a 360 day year consisting of twelve 30-day
                  months) at the Treasury Yield plus [______ percent (_____%)],
                  plus in each case accrued and unpaid interest to the
                  redemption date.

                           "Treasury Yield" means, with respect to any
                  redemption date, the rate per annum equal to the semiannual
                  equivalent yield to maturity of the Comparable Treasury Issue,
                  assuming 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.

                           "Comparable Treasury Issue" means the United States
                  Treasury security selected by an Independent Investment Banker
                  as having a maturity comparable to the remaining term hereof
                  that would be utilized, 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 hereof. "Independent Investment Banker"
                  means [_________________] or, if such firm is unwilling or
                  unable to select the Comparable Treasury Issue, one of the
                  remaining Reference Treasury Dealers appointed by the Trustee
                  after consultation with the Company.

                           "Comparable Treasury Price" means, with respect to
                  any redemption date, (i) the average of the bid and asked
                  prices for the Comparable Treasury Issue (expressed in each
                  case as a percentage of its principal amount) on the third
                  business day preceding such Redemption date, as set forth in
                  the daily statistical release (or any successor release)
                  published by the Federal Reserve Bank of New York and
                  designated "Composite 3:30 p.m. Quotations for U.S. Government
                  Securities" or (ii) if such release (or any successor release)
                  is not published or does not contain such prices on such
                  business day, (A) the average of the Reference Treasury Dealer
                  Quotations for such redemption date, after excluding the
                  highest and lowest such Reference Treasury Dealer Quotations
                  for such redemption date, or (B) if the Trustee obtains fewer
                  than four such Reference Treasury Dealer Quotations, the
                  average of all such Quotations. "Reference Treasury Dealer
                  Quotations" means, with respect to each Reference Treasury
                  Dealer and any Redemption date, the average, as determined by
                  the Company, 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 Company by such
                  Reference Treasury Dealer at 5:00 p.m. on the third business
                  day preceding such redemption date.


                                      A-3
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                           "Reference Treasury Dealer" means (i) any primary
                  U.S. Government securities dealer in New York City (a "Primary
                  Treasury Dealer") designated by the Company.

                           In the event of redemption of this Security in part
                  only, a new Security or Securities of this series for the
                  unredeemed portion hereof will be issued in the name of the
                  Holder hereof upon the cancellation hereof.]

6.       [Notice of Redemption.

                  Notice of redemption will be mailed at least 30 days before
                  the redemption date to the holder hereof at his address
                  appearing on the Security Register.

                  A notice of redemption shall provide that it is subject to the
                  occurrence of any event before the date fixed for such
                  redemption as described in such notice ("Conditional
                  Redemption") and such notice of Conditional Redemption shall
                  be of no effect unless all such conditions to the redemption
                  have occurred before such date or have been waived by the
                  Company.]

7.       Denominations, Transfer, Exchange.

                  The Securities of this series are in registered form without
                  coupons in denominations of ____________ and whole multiples
                  of __________. The transfer of this Security may be registered
                  and this Security may be exchanged as provided in the
                  Indenture. The Securities Registrar may require a holder,
                  among other things, to furnish appropriate endorsements and
                  transfer documents and to pay any taxes and fees required by
                  law or the Indenture. The Securities Registrar need not
                  exchange or register the transfer of any Security of this
                  series or portion thereof selected for redemption. Also, it
                  need not exchange or register the transfer of any Security for
                  a period of 15 days before a selection of Securities of this
                  series to be redeemed.

8.       Persons Deemed Owners.

                  The registered holder of a Security may be treated as its
                  owner for all purposes.

9.       Amendments and Waivers.

                  Subject to certain exceptions, the Indenture may be amended
                  with the consent of the holders of a majority in outstanding
                  principal amount of the Securities. Subject to certain
                  exceptions, a default under the Indenture may be waived with
                  the consent of the holders of a majority in outstanding
                  principal amount of the Securities.

                  Without the consent of any Securityholder, the Indenture may
                  be amended, among other things, to cure any ambiguity,
                  omission, defect or inconsistency; to provide for assumption
                  of Company obligations to Securityholders; or to make


                                      A-4
<PAGE>


                  any change that does not materially adversely affect the
                  rights of any Securityholder.

10.      Restrictive Covenants.

                  The Securities are unsecured general obligations of the
                  Company limited to [$_____________] principal amount. The
                  Indenture does not limit the amount of debt the Company may
                  issue thereunder or otherwise.

11.      Successors.

                  When a successor assumes all the obligations of the Company
                  under the Securities and the Indenture, the Company will be
                  released from those obligations.

12.      Defeasance Prior to Redemption or Maturity.

                  Subject to certain conditions, the Company at any time may
                  terminate some or all of its obligations hereunder and the
                  Indenture if the Company deposits with the Trustee money or
                  U.S. Government Obligations for the payment of principal and
                  interest hereon to redemption or maturity. U.S. Government
                  Obligations are securities backed by the full faith and credit
                  of the United States of America or certificates representing
                  an ownership interest in such Obligations.

13.      Defaults and Remedies.

                  If an Event of Default occurs and is continuing, the Trustee
                  or the holders of at least 25% in principal amount of the
                  outstanding Securities may declare the principal of all such
                  Securities to be due and payable immediately.

                  Securityholders may not enforce the Indenture or the
                  Securities except as provided in the Indenture. The Trustee
                  may require indemnity satisfactory to it before it enforces
                  the Indenture or the Securities. Subject to certain
                  limitations, holders of a majority in principal amount of the
                  Securities may direct the Trustee in its exercise of any trust
                  or power. The Trustee may withhold from Securityholders notice
                  of any continuing default (except a default in payment of
                  principal or interest or any sinking fund installment) if it
                  determines that withholding notice is in their interests. The
                  Company must furnish annual compliance certificates to the
                  Trustee.

14.      Trustee Dealings with Company.

                  The Bank of New York, the Trustee under the Indenture, in its
                  individual or any other capacity, may make loans to, accept
                  deposits from, and perform services for the Company or its
                  Affiliates, and may otherwise deal with the Company or its
                  Affiliates, as if it were not Trustee.


                                      A-5
<PAGE>


15.      No Recourse Against Others.

                  A director, officer, employee or stockholder, as such, of the
                  Company shall not have any liability for any obligations of
                  the Company under the Securities or the Indenture or for any
                  claim based on, in respect of or by reason of such obligations
                  or their creation. Each Securityholder by accepting a Security
                  waives and releases all such liability. The waiver and release
                  are part of the consideration for the issue of the Securities.

16.      Authentication.

                  This Security shall not be valid for any purpose and shall not
                  be entitled to any benefit under the Indenture until
                  authenticated by a manual signature of the Trustee or any
                  Authenticating Agent.

17.      Abbreviations.

                  Customary abbreviations may be used in the name of a
                  Securityholder 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).


                                      A-6
<PAGE>


                  The Company will furnish to any Securityholder upon written
request and without charge a copy of the Indenture, including the Supplemental
Indenture which contains the text of this Security in larger type. Requests may
be made to: Public Service Company of Colorado, 1225 17th Street, Denver,
Colorado, 80202 attention: Corporate Secretary.

                                            PUBLIC SERVICE COMPANY OF
                                            COLORADO



                                            By:
                                               ---------------------------------
                                               Name:
               [corporate seal]                Title:

Attest:
       ---------------------------------
       Name:
       Title:

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

                                            THE BANK OF NEW YORK,
                                            as Trustee


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


                                      A-7
<PAGE>


                                 ASSIGNMENT FORM

         To assign this Security, fill in the form below:


                  I or we assign and transfer this Security to:

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

              -----------------------------------------------------
               (Insert assignee's social security or tax I.D. no.)

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

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

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

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

and irrevocably appoint ____________________ agent 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 other side of this Security)


                                      A-8