Document:

<PAGE>

                                                                    EXHIBIT 4(D)

______________________________________________________________________________

                              NEVADA POWER COMPANY
    (formerly DESERT Merger Sub, Inc., as successor to Nevada Power company)

                                       to

                      IBJ WHITEHALL BANK & TRUST COMPANY,

                                   as Trustee

                          SUPPLEMENTAL INDENTURE NO. 2
                            AND ASSUMPTION AGREEMENT

                            Dated as of June 1, 1999

                                  $122,547,950
            8.20% Junior Subordinated Deferrable Interest Debentures
                                    Series A

______________________________________________________________________________

                                       1
<PAGE>

                              NEVADA POWER COMPANY
                                  $122,547,950
            8.20% Junior Subordinated Deferrable Interest Debentures
                                    Series A

                          SUPPLEMENTAL INDENTURE NO. 2
                            AND ASSUMPTION AGREEMENT

          SUPPLEMENTAL INDENTURE No. 2 AND ASSUMPTION AGREEMENT, dated as of
July 1, 1999,  between Nevada Power Company, a Nevada corporation formerly known
as Desert Merger Sub, Inc., as successor to Nevada Power Company (the
"Company"), and IBJ Whitehall Bank & Trust Company, as successor to IBJ Schroder
Bank & Trust Company, a New York banking corporation, as Trustee (the
"Trustee").

                                    RECITALS
                                    --------

          WHEREAS, Nevada Power Company (the "Merged Company") has heretofore
executed and delivered to the Trustee a Junior Subordinated Indenture, dated as
of March 1, 1997 (the "Indenture"), providing for the issuance from time to time
of series of the Company's Securities; and

          WHEREAS, in connection with the issuance of the Securities by the
Merged Company, the Merged Company also entered into the following agreements
and executed the following instruments:

          (1)       the Indenture;

          (2)       Supplemental Indenture No. 1 dated as of March 1, 1997 from
                    the Merged Company to the Trustee;

          (3)       the Securities;

          (4)       Amended and Restated Trust Agreement dated as of March 1,
                    1997, among the Merged Company, the Trustee, Delaware Trust
                    Capital Management, Inc. and the Administrative Trustees
                    named therein;

          (5)       Guarantee Agreement dated as of March 1, 1997 between the
                    Merged Company and the Trustee; and

          (6)       Agreement as to Expenses and Liabilities dated as of March
                    1, 1997 between the Merged Company and NVP Capital I, a
                    Delaware business trust.

Each of the foregoing agreements or instruments being referred to herein
collectively as the "Nevada Power Obligations".

                                       2
<PAGE>

     WHEREAS, on April 29, 1998, the Merged Company entered into a Merger
Agreement with Sierra Pacific Resources, a Nevada utility holding company,
pursuant to which DESERT Merger Sub, Inc., a wholly owned subsidiary of Sierra
Pacific Resources, merged with the Merged Company with  DESERT Merger Sub, Inc.
being the surviving corporation which surviving corporation then changed its
name to Nevada Power Company; and

     WHEREAS, Section 801 of the Indenture provides that the Merged Company
shall not merge into any other Person (as defined in the Indenture) and that no
Person shall consolidate with or merge into the Merged Company unless:

     (1)     the Person formed by such consolidation or into which the company
             is merged shall be a corporation, partnership or trust, shall be
             organized and existing under the laws of the United States of
             America or any State or the District of Columbia, and shall
             expressly assume, by a supplemental indenture, 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 (including any Additional Interest) on all the
             Securities and the performance of every covenant of the Indenture
             on the part of the Company to be performed or observed;

     (2)     immediately after giving effet to such transaction, no Event of
             Default, and no event which, after notice or lapse of time, or
             both, would become an Event of Default, shall have happened and be
             continuing;

     (3)     in the case of the Securities of a series issued by an NVP Trust,
             such merger is permitted under the related Trust Agreement and
             Nevada Power Guarantee and does not give rise to any breach or
             violation of the related Trust Agreement or Nevada Power Guarantee;
             and

     (4)     the Merged Company delivers to the Trustee an Officer's Certificate
             and an Opinion of Independent Counsel each stating that such merger
             and supplemental indenture complies with Section 801 of the
             Indenture and that all conditions precedent therein provided for
             relating to such transaction have been complied with; and the
             Trustee, subject to Section 601, may rely upon such Officer's
             Certificate and opinion of Independent Counsel as conclusive
             evidence that such transaction complies with Section 801.

     WHEREAS, Section 901(2) of the Indenture provides for the Company and the
Trustee to enter into an indenture supplemental to the Indenture to evidence the
succession of another Person to the Company, and the assumption by any such
successor of the covenants of the Company in the Indenture and in the Securities
contained; and

     WHEREAS, the Company, as the surviving entity of the Merger, has determined
to enter into this Agreement for purposes of complying with said provision of
Section 801 of the Indenture in accordance with the provisions of Article IX of
the Indenture.

                                       3
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                                   ARTICLE 1

                       RELATION TO INDENTURE; DEFINITIONS

     Section 1.1.  This Supplemental Indenture No. 2 constitutes an integral
part of the Indenture.

     Section 1.2.  For all purposes of this Supplemental Indenture No. 2:

          (1)  Capitalized terms used herein without definition shall have the
meanings specified in the Indenture or in the Amended and Restated Trust
Agreement, dated as of March 1, 1997, among Nevada Power Company, as Depositor,
IBJ Schroder Bank & Trust Company, as Property Trustee, Delaware Trust Capital
Management, Inc., as Delaware Trustee, and the Administrative Trustees named
therein, as the case may be;

          (2)  All references herein to Articles and Sections, unless otherwise
specified, refer to the corresponding Articles and Sections of this Supplemental
Indenture No. 2; and

          (3)  The terms "herein", "hereof", "hereunder" and other words of
similar import refer to this Supplemental Indenture No. 2.

                                   ARTICLE 2

                 REPRESENTATIONS AND ASSUMPTION OF OBLIGATIONS

     Section 2.1.  The Company hereby represents that: (a) it is a corporation
duly organized and existing under the laws of the State of Nevada; (b) there has
not been an Event of Default, and no event which, after notice or lapse of time,
or both, would become an Event of Default, and (3) the merger as described in
the recitals hereof is permitted under the Trust Agreement and the Nevada Power
Guarantee and does not give rise to any breach or violation of the related Trust
Agreement or Nevada Power Guarantee.

     Section 2.2.  The Company hereby expressly assumes the due punctual payment
of the principal of (and premium, if any) and interest (including any Additional
Interest) on all the Securities and the performance of every covenant of the
Indenture and all other Nevada Power Obligations.

                                       4
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                                   ARTICLE 3

                            Miscellaneous Provisions

     Section 3.1.   The Indenture, as supplemented and amended by this
Supplemental Indenture No. 2, is in all respects hereby adopted, ratified and
confirmed.

     Section 3.2.   This Supplemental Indenture No. 2 may be executed in any
number of counterparts, each of which shall be an original, but such
counterparts shall together constitute but one and the same instrument.

     Section 3.3.  THIS SUPPLEMENTAL INDENTURE NO. 2 AND EACH SECURITY SHALL BE
DEEMED TO BE A CONTRACT MADE UNDER THE LAWS OF THE STATE OF NEW YORK AND SHALL
BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEVADA
WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES THEREOF.

     IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture No. 2 to be duly executed, as of the day and year first written above.

                                        NEVADA POWER COMPANY

                                        By:

                                            William E. Peterson
                                            Senior Vice President
                                            and General Counsel
Attest: _______________________
        Secretary

                              IBJ WHITEHALL BANK & TRUST COMPANY, as
                              Trustee

                              By: ___________________________________
                              Name:
                              Title:

                                       5<PAGE>

                                                                    EXHIBIT 4(E)

______________________________________________________________________________

                              NEVADA POWER COMPANY
    (formerly DESERT Merger Sub, Inc., as successor to Nevada Power Company)

                                       to

                      IBJ WHITEHALL BANK & TRUST COMPANY,

                                   as Trustee

                          SUPPLEMENTAL INDENTURE NO. 1
                            AND ASSUMPTION AGREEMENT

                            Dated as of June 1, 1999

                                  $72,164,950
       7 3/4% Junior Subordinated Deferrable Interest Debentures Due 2038

______________________________________________________________________________

                                       1
<PAGE>

                          SUPPLEMENTAL INDENTURE NO. 1
                            AND ASSUMPTION AGREEMENT

          SUPPLEMENTAL INDENTURE No. 1 AND ASSUMPTION AGREEMENT, dated as of
June 1, 1999, between Nevada Power Company, a Nevada corporation formerly known
as Desert Merger Sub, Inc., as successor to Nevada Power Company (the
"Company"), and IBJ Whitehall Bank & Trust Company, as successor to IBJ Schroder
Bank & Trust Company, a New York banking corporation, as Trustee (the
"Trustee").

                                    RECITALS
                                    --------

          WHEREAS, Nevada Power Company (the "Merged Company") has heretofore
executed and delivered to the Trustee an Indenture, dated as of October 1, 1998
(the "Indenture"), providing for the issuance from time to time of series of the
Company's Debentures; and

          WHEREAS, in connection with the issuance of the Debentures by the
Merged Company, the Merged Company also entered into the following agreements
and executed the following instruments:

          (1)   the Indenture;

          (2)   the Debentures;

          (3)   Amended and Restated Declaration of Trust dated as of October 1,
                1998, among the Merged Company, the Trustee, Delaware Trust
                Capital Management, Inc., the Administrative Trustees named
                therein and the holders of undivided beneficial interests in NVP
                Capital III;

          (4)   Preferred Securities Guarantee Agreement dated as of October 1,
                1998 between the Merged Company and the Trustee; and

          (5)   Common Securities Agreement dated as of October 1, 1998 by the
                Merged Company.

Each of the foregoing agreements or instruments being referred to herein
collectively as the "Nevada Power Obligations".

     WHEREAS, on April 29, 1998, the Merged Company entered into a Merger
Agreement with Sierra Pacific Resources, a Nevada utility holding company,
pursuant to which DESERT Merger Sub, Inc., a wholly owned subsidiary of Sierra
Pacific Resources, merged with the Merged Company with  DESERT Merger Sub, Inc.
being the surviving corporation which surviving corporation then changed its
name to Nevada Power Company; and

                                       2
<PAGE>

     WHEREAS, Section 701 of the Indenture provides that the Merged Company
shall not merge into any other Person (as defined in the Indenture) and that no
Person shall consolidate with or merge into the Merged Company unless:

          (1)    the Person formed by such consolidation or into which the
                 Company is merged is a corporation, partnership, limited
                 liability company or trust, is be organized and validly
                 existing under the laws of the United States of America or any
                 State or the District of Columbia, and shall expressly assume,
                 by a supplemental indenture, executed and delivered to the
                 Trustee, in form satisfactory to the Trustee, the due and
                 punctual payment of the principal of and interest on all the
                 Debentures and the performance or observance of every covenant
                 of the Indenture on the part of the Company to be performed or
                 observed;

          (2)    immediately after giving effect to such transaction and
                 treating any indebtedness which becomes an obligation of the
                 Company as a result of such transaction as having been incurred
                 by the Company at the time of such transaction, no Event of
                 Default, and no event which, after notice or lapse of time or
                 both, would become an Event of Default, shall have happened and
                 be continuing;

          (3)    the Company has delivered to the Trustee an Officer's
                 Certificate and an Opinion of Counsel each stating that such
                 consolidation, merger, conveyance, transfer or lease and, if a
                 supplemental indenture is required in connection with such
                 transaction, such supplemental indenture comply with this
                 Article and that all conditions precedent herein provided for
                 relating to such transaction have been complied with.

     WHEREAS, Section 801(2) of the Indenture provides for the Company and the
Trustee to enter into an indenture supplemental to the Indenture to evidence the
succession of another Person to the Company, and the assumption by any such
successor of the covenants of the Company in the Indenture and in the Debentures
contained; and

     WHEREAS, the Company, as the surviving entity of the merger, has determined
to enter into this Agreement for purposes of complying with said provision of
Section 801 of the Indenture in accordance with the provisions of Article Eight
of the Indenture.

                                   ARTICLE 1

                       RELATION TO INDENTURE; DEFINITIONS

     Section 1.1.  This Supplemental Indenture No. 1 constitutes an integral
part of the Indenture.

                                       3
<PAGE>

     Section 1.2.  For all purposes of this Supplemental Indenture No. 1:

          (1) Capitalized terms used herein without definition shall have the
meanings specified in the Indenture;

          (2) All references herein to Articles and Sections, unless otherwise
specified, refer to the corresponding Articles and Sections of this Supplemental
Indenture No. 1; and

          (3) The terms "herein", "hereof", "hereunder" and other words of
similar import refer to this Supplemental Indenture No. 1.

                                   ARTICLE 2

                 REPRESENTATIONS AND ASSUMPTION OF OBLIGATIONS

     Section 2.1.  The Company hereby represents that: (a) it is a corporation
duly organized and existing under the laws of the State of Nevada; and (b) there
has not been an Event of Default, and no event which, after notice or lapse of
time, or both, would become an Event of Default.

     Section 2.2.  The Company hereby expressly assumes the due punctual payment
of the principal of (and premium, if any) and interest (including any Additional
Interest) on all the Debentures and the performance of every covenant of the
Indenture and all other Nevada Power Obligations.

                                   ARTICLE 3

                            Miscellaneous Provisions

     Section 3.1.  The Indenture, as supplemented and amended by this
Supplemental Indenture No. 1, is in all respects hereby adopted, ratified and
confirmed.

     Section 3.2.  This Supplemental Indenture No. 1 may be executed in any
number of counterparts, each of which shall be an original, but such
counterparts shall together constitute but one and the same instrument.

                                       4
<PAGE>

     Section 3.3.  THIS SUPPLEMENTAL INDENTURE NO. 1 AND EACH SECURITY SHALL BE
DEEMED TO BE A CONTRACT MADE UNDER THE LAWS OF THE STATE OF NEW YORK AND SHALL
BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEVADA
WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES THEREOF.

     IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture No. 1 to be duly executed, as of the day and year first written above.

                                        NEVADA POWER COMPANY

                                        By:
                                           William E. Peterson
                                           Senior Vice President
                                           and General Counsel
Attest: _______________________
        Secretary

                                        IBJ WHITEHALL BANK & TRUST COMPANY, as
                                        Trustee

                                        By: ___________________________________
                                        Name:
                                        Title:

                                       5

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