FORM OF SUPPLEMENTAL INDENTURE

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Exhibit 4.06

FORM OF SUPPLEMENTAL INDENTURE

SOUTHWESTERN PUBLIC SERVICE COMPANY

and

THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A.
as Trustee

                         SUPPLEMENTAL INDENTURE

Dated as of                         

Supplementing the Indenture

Dated as of February 1, 1999

        THIS [                        ] SUPPLEMENTAL INDENTURE, dated as of [                        ] is between SOUTHWESTERN PUBLIC SERVICE COMPANY, a New Mexico corporation (hereinafter called the "Issuer" or the "Company"), having its principal office at Tyler at Sixth Street, Amarillo, Texas 79101, and THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A. as successor to THE CHASE MANHATTAN BANK, as Trustee (hereinafter called the "Trustee"), having its office at 700 S. Flower Street, Suite 500, Los Angeles, California 90017.

Recitals of the Issuer

        The Issuer and the Trustee have heretofore entered into an Indenture, dated as of February 1, 1999, a First Supplemental Indenture, dated as of March 1, 1999, a Second Supplemental Indenture dated as of October 1, 2001, a Third Supplemental Indenture dated as of October 1, 2003, a Fourth Supplemental Indenture dated as of October 1, 2006 (such Indenture, as supplemented by the First Supplemental Indenture, the Second Supplemental Indenture, the Third Supplemental Indenture, the Fourth Supplemental Indenture and 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. Pursuant to Section 7.08 of the Indenture, JPMorgan Chase Bank, N.A. succeeded The Chase Manhattan Bank as trustee under the Indenture and The Bank of New York subsequently succeeded JPMorgan Chase Bank, N.A. as trustee under the Indenture. The Bank of New York Mellon (formerly The Bank of New York) subsequently resigned as trustee and The Bank of New York Mellon Trust Company, N.A. was appointed as successor trustee under the Indenture. Terms used and not otherwise defined herein shall (unless the context otherwise clearly requires) have the respective meanings given to them in the Indenture.

        The Indenture provides in Article Two 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 (delivered to the Trustee in the form of a Bond Resolution) 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 Series  [            ] Senior Notes, [            ] % due [                        ] of the Issuer, and to establish the terms applicable to such series, pursuant to Sections 2.01 and 10.01 of the Indenture. The Issuer has duly authorized the execution and delivery of this Supplemental Indenture.

        Article Ten of the 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 10.01 thereof, including purposes set forth in subsection (4) of said Section 10.01.

        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, legal and binding agreement of the Issuer, in accordance with its terms.

        The Issuer has requested that the Trustee execute and deliver this Supplemental Indenture.

        NOW, THEREFORE, THIS [                        ] SUPPLEMENTAL INDENTURE WITNESSETH:

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

ARTICLE I.

ESTABLISHMENT OF SERIES [            ] NOTES, [            ] % DUE [            ]

        Section 1.01.    The title of the series of the Securities established by this Supplemental Indenture shall be Series  [            ] Senior Notes, [            ] % due [            ] of the Issuer (hereinafter called the "Series  [            ] Notes"). The Series [            ] Notes shall be issued in registered form substantially in

the form set forth in Exhibit A hereto (which is hereby incorporated herein and made a part hereof), subject to changes in the form thereof made by the Issuer and acceptable to the Trustee.

        Section 1.02.    The Series  [            ] Notes shall be limited to $ [                  ] aggregate principal amount except as provided in Section 1.06 of this Supplemental Indenture.

        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 Section 2.12 of the Indenture.

        Section 1.04.    The Issuer hereby appoints, or confirms the appointment of, The Bank of New York Mellon Trust Company, N.A., as the Trustee, Transfer Agent 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 1.05.    The terms of the Series  [            ] Notes shall be as set forth in Exhibit A hereto, and shall include the payment and other terms reflected on the Series  [            ] Notes as actually executed, authenticated and delivered under the Indenture.

        Section 1.06.    The Series  [            ] Notes may be reopened and additional Securities of Series  [            ] Notes may be issued in excess of the amount initially authenticated and delivered, provided that such additional Securities of Series  [            ] Notes will contain the same terms (including the stated maturity and interest rate) as the other Series  [            ] Notes. Any such additional Securities of Series  [            ] Notes, together with the other Series  [            ] Notes, shall constitute a single series for purposes of the Indenture.

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 or sufficiency of this Supplemental Indenture. All rights, protections, privileges, indemnities and benefits granted or afforded to the Trustee under the Indenture shall be deemed incorporated herein by this reference and shall be deemed applicable to all actions taken, suffered or omitted by the Trustee under this Supplemental Indenture

        Section 2.02.    The Indenture, as supplemented by this Supplemental Indenture, is in all respects hereby adopted, ratified and confirmed.

        Section 2.03.    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 2.04.    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 2.05.    The laws of the State of New York shall govern this Supplemental Indenture and the Series  [            ] Notes, unless federal law governs.

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        Section 2.06.    The Article headings herein are for convenience only and shall not affect the interpretation hereof.

        IN WITNESS WHEREOF, the parties hereto have caused this [                        ] Supplemental Indenture to be duly executed, and the Company has caused its corporate seal to be hereunto affixed and attested as of the [            ] day of [            ] , [            ] .

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EXHIBIT A

        THIS SECURITY IS A GLOBAL SECURITY REGISTERE