SUPPLEMENTAL INDENTURE NO. 19 , dated as of May 1, 2009 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. 19.  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 Securities and appointing the successor Trustee.

The Company desires to establish a new series of Securities to be designated “5.125% First Mortgage Bonds, Series No. 20 due 2019,” such series of Securities to be hereinafter sometimes called “Series No. 20.”

The Company has duly authorized the execution and delivery of this Supplemental Indenture No. 19 to establish the Securities of Series No. 20 and has duly authorized the issuance of such Securities; and all acts necessary to make this Supplemental Indenture No. 19 a valid agreement of the Company, and to make the Securities of Series No. 20 valid obligations of the Company, have been performed.

Granting Clauses

NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. 19 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. 19, 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. 19 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. 19;

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

2

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

THE PARTIES HEREBY FURTHER COVENANT AND AGREE as follows:

ARTICLE ONE

Securities of Series No. 20

There are hereby established the Securities of Series No. 20, 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. 20 shall be “5.125% First Mortgage Bonds, Series No. 20 due 2019”;

(b)           the Securities of Series No. 20 shall initially be authenticated and delivered in the aggregate principal amount of $400,000,000.  The Securities of Series No. 20 may be reopened and additional Securities of Series No. 20 may be issued in excess of the amount initially authenticated and delivered, provided that such additional Securities of Series No. 20 will contain the same terms (including the Stated Maturity and interest rate), except for the public offering price and issue date, as the other Securities of Series No. 20.  Any such additional Securities of Series No. 20, together with the Securities of Series No. 20 initially authenticated, shall constitute a single series for purposes of the Indenture and shall be limited to an aggregate principal amount of $800,000,000;

(c)           interest on the Securities of Series No. 20 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. 20 shall be payable on June 1, 2019, the Stated Maturity for Series No. 20;

3

(e)           the Securities of Series No. 20 shall bear interest at a rate of 5.125% per annum; interest shall accrue on the Securities of Series No. 20 from June 4, 2009 or the most recent date to which interest has been paid or duly provided for; the Interest Payment Dates for such Securities shall be June 1 and December 1 in each year, commencing December 1, 2009 and the Regular Record Dates with respect to the Interest Payment Dates for such Securities shall be May 15 and November 15 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. 20 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. 20 shall be redeemable at the option of the Company at any time prior to their Maturity, in whole or in part, at a redemption price equal to the greater of (i) 100% of the principal amount thereof to be redeemed, or (ii) the sum of the present values of the remaining scheduled payments of principal and interest on such Securities to be redeemed (excluding the portion of any such interest accrued to the Redemption Date), discounted to the Redemption Date on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Treasury Yield plus 30 basis points, plus, in each case, accrued and unpaid interest to the Redemption Date.  For purposes hereof, the following defined terms shall have the meaning ascribed to them:

“Treasury Yield” means, for any Redemption Date (1) 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 U.S. 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 term, yields for the two published maturities most closely corresponding to the Comparable Treasury Issue will be determined and the Treasury Yield will be interpolated or extrapolated from such yields on a straight line basis, rounding to the nearest month); or (2) 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 semiannual 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 Treasury Yield for any Redemption Date shall be calculated on the third Business Day preceding such Redemption Date.

“Comparable Treasury Issue” means the United States Treasury security selected by the Independent Investment Banker as having a maturity comparable to the remaining term of the Securities of Series No. 20 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 of the Securities of Series No. 20.

4

“Comparable Treasury Price” means (i) the average of the Reference Treasury Dealer Quotations for the Redemption Date, after excluding the highest and lowest Reference Treasury Dealer Quotations for such Redemption Date, or (ii) if the Trustee obtains fewer than four Reference Treasury Dealer Quotations for the Redemption Date, the average of all of the Reference Treasury Dealer Quotations for such Redemption Date.

“Independent Investment Banker” means Credit Suisse Securities (USA) LLC, BNP Paribas Securities Corp. or Scotia Capital (USA) Inc. or their respective successors or, if such firms or their successors are unwilling or unable to select the Comparable Treasury Issue, an independent investment banking institution of national standing appointed by the Trustee after consultation with the Company.

“Reference Treasury Dealer” means (1) each of Credit Suisse Securities (USA) LLC and BNP Paribas Securities Corp. and any other Primary Treasury Dealer designated by, and not affiliated with, Credit Suisse Securities (USA) LLC or BNP Paribas Securities Corp. or their respective successors, provided, however, that if Credit Suisse Securities (USA) LLC or BNP Paribas Securities Corp. or any of their respective designees ceases to be a Primary Treasury Dealer, the Company will appoint another Primary Treasury Dealer as a substitute and (2) any other Primary Treasury Dealer selected by the Company after consultation with an Independent Investment Banker.

“Primary Treasury Dealer” means any primary U.S. Government securities dealer in the United States.

“Reference Treasury Dealer Quotations” means, for 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 by the Reference Treasury Dealer at 5:00 p.m. New York City time on the third Business Day preceding the Redemption Date.

(h)           not applicable;

(i)            the Securities of Series No. 20 shall be issuable in denominations of $1,000 and multiples thereof;

(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. 20 are to be initially registered in the name of Cede & Co., as nominee for The Depository Trust Company (the “Depositary”).  Such

5

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. 20; 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;

(u)                                  (i)            If the Company shall have caused the Company’s indebtedness in respect of any Securities of Series No. 20 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

6

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 thro