SUPPLEMENTAL INDENTURE NO.    , dated as of           , 20     between PUBLIC SERVICE COMPANY OF COLORADO , a corporation duly organized and existing under the laws of the State of Colorado (hereinafter sometimes called the “Company”), and U.S. BANK TRUST NATIONAL ASSOCIATION (formerly First Trust of New York, National Association) , a national banking association, as successor trustee (hereinafter sometimes called the “Trustee”) to Morgan Guaranty Trust Company of New York under the Indenture, dated as of October 1, 1993 (hereinafter called the “Original Indenture”), as previously supplemented and as further supplemented by this Supplemental Indenture No.        .  The Original Indenture and any and all indentures and all other instruments supplemental thereto are hereinafter sometimes collectively called the “Indenture”.

Recitals of the Company

The Original Indenture was authorized, executed and delivered by the Company to provide for the issuance from time to time of its Securities (such term and all other capitalized terms used herein without definition having the meanings assigned to them in the Original Indenture), to be issued in one or more series as contemplated therein, and to provide security for the payment of the principal of and premium, if any, and interest, if any, on the Securities.  The Original Indenture has been recorded in the office of the Clerk and Recorder of each county in the State of Colorado in which the Company owns real property that is used in or in connection with the Electric Utility Business, as more fully set forth in Schedule A hereto.

The Company has heretofore executed and delivered to the Trustee the Supplemental Indentures referred to in Schedule B hereto for the purpose of establishing various series of bonds and appointing the successor Trustee.

The Company desires to establish a new series of Securities to be designated “       % First Mortgage Bonds, Series No.       due 20     ,” such series of Securities to be hereinafter sometimes called “Series No.        .”

The Company has duly authorized the execution and delivery of this Supplemental Indenture No.         to establish the Securities of Series No.       and has duly authorized the issuance of such Securities; and all acts necessary to make this Supplemental Indenture No.        a valid agreement of the Company, and to make the Securities of Series No.          valid obligations of the Company, have been performed.

Granting Clauses

NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO.         WITNESSETH , that, in consideration of the premises and of the purchase of the Securities by the Holders thereof, and in order to secure the payment of the principal of and premium, if any, and interest, if any, on all Securities from time to time Outstanding and the performance of the covenants contained therein and in the Indenture and to declare the terms and conditions on which such Securities are secured, the Company hereby grants, bargains, sells, releases, conveys, assigns, transfers, mortgages, pledges, sets over and confirms to the Trustee, and grants to the Trustee a security interest in, the following:

Granting Clause First

All right, title and interest of the Company, as of the date of the execution and delivery of this Supplemental Indenture No.       , in and to property (other than Excepted Property), real, personal and mixed and wherever situated, in any case used or to be used in or in connection with the Electric Utility Business (whether or not such use is the sole use of such property), including without limitation (a) all lands and interest in land described or referred to in Schedule C hereto; (b) all other lands, easements, servitudes, licenses, permits, rights of way and other rights and interests in or relating to real property used or to be used in or in connection with the Electric Utility Business or relating to the occupancy or use of such real property, subject however, to the exceptions and exclusions set forth in clause (a) of Granting Clause First of the Original Indenture; (c) all plants, generators, turbines, engines, boilers, fuel handling and transportation facilities, air and water pollution control and sewage and solid waste disposal facilities and other machinery and facilities for the generation of electric energy; (d) all switchyards, lines, towers, substations, transformers and other machinery and facilities for the transmission of electric energy; (e) all lines, poles, conduits, conductors, meters, regulators and other machinery and facilities for the distribution of electric energy; (f) all buildings, offices, warehouses and other structures used or to

be used in or in connection with the Electric Utility Business; (g) all pipes, cables, insulators, ducts, tools, computers and other data processing and/or storage equipment and other equipment, apparatus and facilities used or to be used in or in connection with the Electric Utility Business; (h) any or all of the foregoing properties in the process of construction; and (i) all other property, of whatever kind and nature, ancillary to or otherwise used or to be used in conjunction with any or all of the foregoing or otherwise, directly or indirectly, in furtherance of the Electric Utility Business;

Granting Clause Second

Subject to the applicable exceptions permitted by Section 810(c), Section 1303 and Section 1305 of the Original Indenture, all property (other than Excepted Property) of the kind and nature described in Granting Clause First which may be hereafter acquired by the Company, it being the intention of the Company that all such property acquired by the Company after the date of the execution and delivery of this Supplemental Indenture No.        shall be as fully embraced within and subjected to the Lien hereof as if such property were owned by the Company as of the date of the execution and delivery of this Supplemental Indenture No.        ;

Granting Clause Fourth

All other property of whatever kind and nature subjected or required to be subjected to the Lien of the Indenture by any of the provisions thereof;

This Instrument shall constitute a financing statement under the Colorado Uniform Commercial Code (the “UCC”) to be filed in the real estate records, and is filed as a fixture filing under the UCC covering goods which are, or are to become, fixtures on the real property described herein, in the Original Indenture and all supplements to the Original Indenture;

Excepted Property

Expressly excepting and excluding, however, from the Lien and operation of the Indenture all Excepted Property of the Company, whether now owned or hereafter acquired;

TO HAVE AND TO HOLD all such property, real, personal and mixed, unto the Trustee, its successors in trust and their assigns forever;

SUBJECT, HOWEVER, to (a) Liens existing at the date of the execution and delivery of the Original Indenture, (b) as to property acquired by the Company after the date of the execution and delivery of the Original Indenture, Liens existing or placed thereon at the time of the acquisition thereof (including, but not limited to, the Lien of any Class A Mortgage and purchase money Liens), (c) Retained Interests and (d) any other Permitted Liens, it being understood that, with respect to any property which was at the date of execution and delivery of the Original Indenture or thereafter became or hereafter becomes subject to the Lien of any Class A Mortgage, the Lien of the Indenture shall at all times be junior, subject and subordinate to the Lien of such Class A Mortgage;

IN TRUST, NEVERTHELESS, for the equal and proportionate benefit and security of the Holders from time to time of all Outstanding Securities without any priority of any such Security over any other such Security;

PROVIDED, HOWEVER, that the right, title and interest of the Trustee in and to the Mortgaged Property shall cease, terminate and become void in accordance with, and subject to the conditions set forth in, Article Nine of the Original Indenture, and if, thereafter, the principal of and premium, if any, and interest, if any, on the Securities shall have been paid to the Holders thereof, or shall have been paid to the Company pursuant to Section 603 of the Original Indenture, then and in that case the Indenture shall terminate, and the Trustee shall execute and deliver to the Company such instruments as the Company shall require to evidence such termination; otherwise the Indenture, and the estate and rights thereby granted shall be and remain in full force and effect; and

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THE PARTIES HEREBY FURTHER COVENANT AND AGREE as follows:

ARTICLE ONE

Securities of Series No.

There are hereby established the Securities of Series No.      , which shall have the terms and characteristics set forth below (the lettered subdivisions set forth below corresponding to the lettered subdivisions of Section 301 of the Original Indenture):

(a)           the title of the Securities of Series No.       shall be “       % First Mortgage Bonds, Series No.       due 20     ”;

(b)           [the Securities of Series No.        shall initially be authenticated and delivered in the aggregate principal amount of $            . The Securities of Series No.      may be reopened and additional Securities of Series No.        may be issued in excess of the amount initially authenticated and delivered, provided that such additional Securities of Series No.        will contain the same terms (including the Stated Maturity and interest rate) as the other Securities of Series No.       . Any such additional Securities of Series No.       , together with the Securities of Series No.       initially authenticated, shall constitute a single series for purposes of the Indenture and shall be limited to an aggregate principal amount of $             ][the Securities of Series No.            shall be limited to the aggregate principal amount of $                 ];

(c)           interest on the Securities of Series No.      shall be payable to the Persons in whose names such Securities are registered at the close of business on the Regular Record Date for such interest, except as otherwise expressly provided in the form of such Securities attached as Exhibit A hereto;

(d)           the principal of the Securities of Series No.       shall be payable on          , 20   , the Stated Maturity for Series No.    ;

(e)           the Securities of Series No.     shall bear interest at a rate of      % per annum; interest shall accrue on the Securities of Series No.     from       , 20   or the most recent date to which interest has been paid or duly provided for; the Interest Payment Dates for such Securities shall be                 and                  in each year, commencing               , 20    and the Regular Record Dates with respect to the Interest Payment Dates for such Securities shall be              and                in each year, respectively (whether or not a Business Day);

(f)            the Corporate Trust Office of U.S. Bank Trust National Association in New York, New York shall be the place at which (i) the principal of, premium, if any, and interest, if any, on the Securities of Series No.       shall be payable, (ii) registration of transfer of such Securities may be effected, (iii) exchanges of such Securities may be effected and (iv) notices and demands to or upon the Company in respect of such Securities and the Indenture may be served; and U.S. Bank Trust National Association shall be the Security Registrar for such Securities; provided, however, that the Company reserves the right to change, by one or more Officer’s Certificates, any such place or the Security Registrar; and provided, further, that the Company reserves the right to designate, by one or more Officer’s Certificates, its principal office in Denver, Colorado as any such place or itself as the Security Registrar;

(g)           the Securities of Series No.      [shall not redeemable prior to maturity] [shall be redeemable at the option of the Company at any time prior to their maturity on and after                , in whole or in part, at a redemption price of                    ].

(h)           not applicable;

(i)            [the Securities of Series No.         shall be issuable in denominations of $          and multiples of $           in excess thereof;]

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(j)            not applicable;

(k)           not applicable;

(l)            not applicable;

(m)          not applicable;

(n)           not applicable;

(o)           not applicable;

(p)           not applicable;

(q)           the Securities of Series No.       are to be initially registered in the name of Cede & Co., as nominee for The Depository Trust Company (the “Depositary”).  Such Securities shall not be transferable or exchangeable, nor shall any purported transfer be registered, except as follows:

(i)            such Securities may be transferred in whole, and appropriate registration of transfer effected, if such transfer is by such nominee to the Depositary, or by the Depositary to another nominee thereof, or by any nominee of the Depositary to any other nominee thereof, or by the Depositary or any nominee thereof to any successor securities depositary or any nominee thereof; and

(ii)           such Securities may be exchanged for definitive Securities registered in the respective names of the beneficial holders thereof, and thereafter shall be transferable without restriction, if:

(A)          the Depositary, or any successor securities depositary, shall have notified the Company and the Trustee that it is unwilling or unable to continue to act as securities depositary with respect to such Securities or the Depositary has ceased to be a clearing agency registered under the Securities Exchange Act of 1934, as amended, and the Trustee shall not have been notified by the Company within ninety (90) days of the identity of a successor securities depositary with respect to such Securities; or

(B)           the Company shall have delivered to the Trustee a Company Order to the effect that such Securities shall be so exchangeable on and after a date specified therein; or

(C)  (1)  an Event of Default shall have occurred and be continuing, (2) the Trustee shall have given notice of such Event of Default pursuant to Section 1102 of the Original Indenture and (3) there shall have been delivered to the Company and the Trustee an Opinion of Counsel to the effect that the interests of the beneficial owners of such Securities in respect thereof will be materially impaired unless such owners become Holders of definitive Securities;

(r)            not applicable;

(s)           no service charge shall be made for the registration of transfer or exchange of the Securities of Series No.      ; provided, however, that the Company may require payment of a sum sufficient to cover any tax or other governmental charge payable in connection with the exchange or transfer;

(t)            not applicable;

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(u)                                  (i)            If the Company shall have caused the Company’s indebtedness in respect of any Securities of Series No.        to have been satisfied and discharged prior to the Maturity of such Securities, as provided in Section 901 of the Original Indenture, the Company shall, promptly after the date of such satisfaction and discharge, give a notice to each Person who was a Holder of any of such Securities on such date stating (A)(1) the aggregate principal amount of such Securities and (2) the aggregate amount of any money (other than amounts, if any, deposited in respect of accrued interest on such Securities) and the aggregate principal amount of, the rate or rates of interest on, and the aggregate fair market value of, any Eligible Obligations deposited pursuant to Section 901 of the Original Indenture with respect to such Securities and (B) that the Company will provide (and the Company shall promptly so provide) to such Person, or any beneficial owner of such Securities holding through such Person (upon written request to the Company sent to an address specified in such notice), such other information as such Person or beneficial owner, as the case may be, reasonably may request in order to enable it to determine the federal income tax consequences to it resulting from the satisfaction and discharge of the Company’s indebtedness in respect of such Securities.  Thereafter, the Company shall, within forty-five (45) days after the end of each calendar year, give to each Person who at any time during such calendar year was a Holder of such Securities a notice containing (X) such information as may be necessary to enable such Person to report its income, gain or loss for federal income tax purposes with respect to such Securities or the assets held on deposit in respect thereof during such calendar year or the portion thereof during which such Person was a Holder of such Securities, as the case may be (such information to be set forth for such calendar year as a whole and for each month during such year) and (Y) a statement to the effect that the Company will provide (and the Company shall promptly so provide) to such Person, or any beneficial owner of such Securities holding through such Person (upon written request to the Company sent to an address specified in such notice), such other information as such Person or beneficial owner, as the case may be, reasonably may request in order to enable it to determine its income, gain or loss for federal