Document:

<PAGE>

                                                                 Exhibit 10(AA)

                     AMENDED AND RESTATED CREDIT AGREEMENT
                     ------------------------------------

                                  ARRANGED BY

                               MELLON BANK, N.A.

          This AMENDED AND RESTATED CREDIT AGREEMENT (this "Amendment and
                                                            -------------
Restatement"), is made and entered into as of August 28, 2000, among NEVADA
-----------
POWER COMPANY (the "Company"), MELLON BANK, N.A. ("Mellon"), in its capacity as
                    -------                        ------
administrative agent under the Credit Agreement described below (the
"Administrative Agent"), the financial institutions listed on the signature
---------------------
pages hereto under the caption "Continuing Lenders" (the "Continuing Lenders")
                                                          ------------------
and the financial institutions, if any, listed on the signature pages hereto
under the caption "New Lenders" (the "New Lenders"; the Continuing Lenders and
                                      -----------
the New Lenders are hereinafter collectively referred to as the "Lenders").
                                                                 -------

                                    RECITALS
                                    --------

          A.  The Company, Mellon, in its capacity as Administrative Agent,
First Union National Bank and Wells Fargo Bank, N.A., in their capacity as
syndication agents, and the other financial institutions listed on the signature
pages thereto have entered into a Credit Agreement, dated as of June 24, 1999
(as amended, modified and supplemented through but excluding the date hereof,
the "Credit Agreement").
     ----------------

          B.  The Company desires, and the Administrative Agent and the Lenders
are willing, upon the terms set forth in this Amendment and Restatement, to
amend the Credit Agreement as set forth herein and to restate the Credit
Agreement in its entirety to read as set forth in the Credit Agreement with the
amendment set forth below.

          NOW, THEREFORE, in consideration of the foregoing, the premises and
mutual covenants contained herein and for other valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
hereby (i) agree that the Credit Agreement shall be amended and restated in its
entirety to read as set forth in the Credit Agreement with the amendments set
forth below and (ii) further agree as follows:

          1.  Defined Terms.  Unless otherwise defined herein, capitalized terms
              -------------
used herein shall have the meanings given thereto in the Credit Agreement.

          2.  Effectiveness of this Amendment and Restatement.  So long as each
              -----------------------------------------------
of the following conditions shall be satisfied or waived by the Administrative
Agent and the Lenders, this Amendment and Restatement shall become effective and
the Credit Agreement shall be amended and restated as provided herein at 9:00
A.M. (Pacific time) (the "Effective Time") on August 28, 2000 (the "Effective
                          --------------                            ---------
Date"):
----

          (a) Execution of Amendment and Restatement.  The Company and the
              --------------------------------------
     Lenders shall have executed a copy of this Amendment and Restatement
     (whether the same or different copies) and shall have delivered the same to
     the Administrative Agent.

                                       1
<PAGE>

          (b) No Litigation.  The Administrative Agent and the Lenders shall be
              -------------
     satisfied that, immediately prior to the Effective Time, no judgment,
     order, injunction or other restraint shall have been issued or filed which
     restrains, and no hearing seeking injunctive relief or other restraint is
     pending or has been noticed which seeks to restrain, the Company from
     consummating the transactions described in the Loan Documents or this
     Amendment and Restatement.

          (c) No Default; Representations and Warranties.  The Administrative
              ------------------------------------------
     Agent and the Lenders shall be satisfied that, immediately prior to the
     Effective Time and after giving effect to this Amendment and Restatement,
     (i) there shall exist no Default or Event of Default and (ii) the
     representations and warranties of the Company contained in the Loan
     Documents to which the Company is a party are true and correct in all
     material respects as of the Effective Time with the same effect as though
     such representations and warranties had been made on and as of the
     Effective Time (except for such representations and warranties made as of a
     specified date, which shall be true and correct in all material respects as
     of such specified date).

          (d) Corporate Proceedings.  All corporate and legal proceedings and
              ---------------------
     all instruments and agreements in connection with the transactions
     contemplated in this Amendment and Restatement and the other Loan Documents
     shall be satisfactory in form and substance to the Administrative Agent and
     the Lenders, and the Administrative Agent shall have received all
     information and copies of all documents and papers, including records of
     corporate proceedings and governmental approvals, if any, which any Lender
     reasonably may have requested in connection therewith, such documents and
     papers where appropriate to be certified by proper corporate or
     governmental authorities.

          (e) Amendment and Restatement Fee.  The Administrative Agent shall
              -----------------------------
     have received from the Company, for the account of each Lender that
     executed and delivered its counterpart to this Amendment and Restatement by
     5:00 P.M. (Pacific time) on August 18, 2000, an amendment and restatement
     fee equal to the product of 0.25% and the amount of such Lender's
     Commitment at the Effective Time.

          (f) Arrangement Fee.  Mellon shall have received the arrangement fee
              ---------------
     set forth in the engagement letter dated June 12, 2000 from Mellon to
     Sierra Pacific Resources.

          (g) Other Payments.  The Administrative Agent and each Lender shall
              --------------
     have received all other amounts, if any, owing from the Company to such
     Person pursuant to the Credit Agreement through and including the Effective
     Date.

All the certificates and other documents and papers referred to in this Section
2, unless otherwise specified, shall be delivered to the Administrative Agent's
counsel, White & Case LLP, at 633 West Fifth Street, 19/th/ Floor, Los Angeles,
CA 90071, for the account of each of the Lenders and in sufficient counterparts
for each of the Lenders and shall be satisfactory in form and substance to the
Administrative Agent and the Lenders.

                                       2
<PAGE>

          3.   Amendment.  At the Effective Time,
               ---------

          (a)  The defined term "Revolving Termination Date" set forth in
                                 --------------------------
     Section 1.01 of the Credit Agreement shall be deleted in its entirety and
     the following shall be substituted therefor:

               ""Revolving Termination Date" means the earlier of (i) August 27,
                 --------------------------
          2001, or such date after August 27, 2001 to which the Commitments are
          extended in accordance with Section 2.06(e), and (ii) the date the
          Obligations and Commitments under this Agreement terminate, whether by
          prepayment, cancellation, acceleration or otherwise.".

          (b) Section 6.01(b) of the Credit Agreement shall be deleted in its
     entirety and the following shall be substituted therefor:

               "(b) Fixed Charge Coverage Ratio. The Borrower shall not permit
                    the Fixed Charge Coverage Ratio, determined as of the last
                    day of each fiscal quarter set forth below for the period
                    consisting of the four consecutive fiscal quarters ended on
                    such last day, to be less than the ratio set forth opposite
                    such last day:

               Fiscal Quarter Ended                    Ratio
               --------------------                    -----
               September 30, 2000                      1.10:1
               December 31, 2000                       1.20:1
               March 31, 2001                          1.20:1
               June 30, 2001 and each
               fiscal quarter ended thereafter         1.50:1.".

          4.   Representations and Warranties. The Company makes, as of the
               ------------------------------
Effective Date, each of the representations and warranties set forth in Article
III of the Credit Agreement and such representations and warranties are, by this
reference, incorporated herein as if set forth herein in their entirety,
provided that references to "Loan Documents" shall, for purposes of this
paragraph, be deemed to include this Amendment and Restatement.

          5.   Miscellaneous.
               -------------

          (a)  Except as expressly modified by this Amendment and Restatement,
the Credit Agreement shall continue to be and remain in full force and effect in
accordance with its terms.  Any future reference to the Credit Agreement shall,
from and after the Effective Time, be deemed to be a reference to the Credit
Agreement as amended and restated by this Amendment and Restatement.

          (b)  This Amendment and Restatement may be executed in any number of
counterparts, each of which shall constitute an original, but all of which when
taken together shall constitute but one instrument.

                                       3
<PAGE>

          (c)  THIS AMENDMENTAND RESTATEMENT AND THE RIGHTS AND OBLIGATIONS OF
THE PARTIES HEREUNDER SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF NEW YORK WITHOUT REFERENCE TO CONFLICTS OF LAW RULES.

          (d)  This Amendment and Restatement may be executed by facsimile
signature and each such signature shall be treated in all respects as having the
same effect as an original signature.

          (e)  The rules of construction set forth in Section 1.03 of the Credit
Agreement are, by this reference, incorporated herein as if set forth in their
entirety, provided that references to "this Agreement" in such section shall
mean references to this Amendment and Restatement.

                                     *****

                                       4
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment and
Restatement to be duly executed as of the date first above written.

                                      NEVADA POWER COMPANY

                                      By_______________________________
                                      Name:
                                      Title:

                                      MELLON BANK, N.A., as
                                         Administrative Agent

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      Continuing Lenders
                                      ------------------

                                      MELLON BANK, N.A.

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      FIRST UNION NATIONAL BANK

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      WELLS FARGO BANK, N.A.

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      BANK OF AMERICA, N.A.

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      THE BANK OF NEW YORK

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      BANK ONE, N.A.
                                      (Main Office-Chicago)

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      CREDIT SUISSE FIRST BOSTON

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      BNP PARIBAS

                                      By_______________________________
                                      Name:
                                      Title:

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      UNION BANK OF CALIFORNIA, N.A.

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      BAYERISCHE LANDESBANK GIROZENTRALE

                                      By_______________________________
                                      Name:
                                      Title:

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      FLEET NATIONAL BANK

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      FIRST SECURITY BANK OF NEVADA

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      KBC BANK, N.V.

                                      By_______________________________
                                      Name:
                                      Title:

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      U.S. BANK NATIONAL ASSOCIATION

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]
<PAGE>

                                      New Lenders
                                      -----------

                                      THE INDUSTRIAL BANK OF JAPAN, LIMITED

                                      By_______________________________
                                      Name:
                                      Title:

           [Signature Page to Amended and Restated Credit Agreement]<PAGE>

                                                                   Exhibit 10(N)

                        AMENDMENT AND WAIVER AGREEMENT
                        ------------------------------

          This AMENDMENT AND WAIVER AGREEMENT (this "Agreement"), is made and
                                                     ---------
entered into as of March 1, 2001, among NEVADA POWER COMPANY (the "Company"),
                                                                   -------
MELLON BANK, N.A. ("Mellon"), FIRST UNION NATIONAL BANK ("First Union"), WELLS
                    ------                                -----------
FARGO BANK, N.A. ("Wells Fargo"), and the other parties set forth on the
                   -----------
signatures pages hereto (the "Lenders").
                              -------

                                    RECITALS
                                    --------

          A.  The Company, Mellon, in its capacity as administrative agent (the
"Administrative Agent"), First Union and Wells Fargo, each in their capacity as
 --------------------
syndication agent, and the Lenders have entered into a Credit Agreement, dated
as of June 24, 1999 (as amended, modified and supplemented through the date
hereof, the "Credit Agreement").
             ----------------

          B.  Pursuant to Section 6.01(a) of the Credit Agreement, the Company's
ratio of Total Indebtedness to the sum of Total Indebtedness and Shareholders'
Equity (the "Debt to Equity Ratio"), determined as of the last day of each
             --------------------
fiscal quarter, shall not exceed 0.65 to 1.

          C.  The Company reasonably believes that it will be in breach of its
fixed charge coverage ratio set forth in Section 6.01(b) of the Credit Agreement
for the four fiscal quarter period ended December 31, 2000 when it delivers its
financial statements and compliance certificate for such quarter pursuant to
Sections 5.01(a) and (c) of the Credit Agreement (as such breach relates to the
four fiscal quarter period ended December 31, 2000 only, the "Financial Covenant
                                                              ------------------
Breach").
------

          D.  The effect of the Financial Covenant Breach would, if not waived
by March 31, 2001, result in a Default.

          E.  The Company has requested that the Lenders (a) amend Section
6.01(a) of the Credit Agreement to reduce the Debt to Equity Ratio to 0.58 to 1
and (b) waive any Default or Event of Default resulting from the Financial
Covenant Breach.

          NOW, THEREFORE, in consideration of the foregoing, the premises and
mutual covenants contained herein and for other valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the parties hereto
hereby agree as follows:

          1.  Defined Terms.  Unless otherwise defined herein, capitalized terms
              -------------
used herein shall have the meanings given thereto in the Credit Agreement.

          2.  Effectiveness of this Agreement.  This Agreement shall become
              -------------------------------
effective and the waiver and amendment described in Section 3 below shall become
effective at the time (the "Effective Time") on the first date (the "Effective
                            --------------                           ---------
Date") on which each of the following conditions shall be satisfied or waived by
----
the Administrative Agent and the Required Lenders:
<PAGE>

          (a) Execution of Agreement.  The Company and the Required Lenders
              ----------------------
     shall have executed a copy of this Agreement (whether the same or different
     copies) and shall have delivered the same to the Administrative Agent.

          (b) Quarterly Compliance Certificate.  The Company shall have
              --------------------------------
     delivered to the Administrative Agent a Quarterly Compliance Certificate
     for the four fiscal quarter period ended December 31, 2000, which Quarterly
     Compliance Certificate shall certify that the Company is in compliance with
     its financial covenants with respect to such quarter, except that the Fixed
     Charge Coverage Ratio shall be not less than 0.76 to 1.

          (c) No Litigation.  The Administrative Agent and the Required Lenders
              -------------
     shall be satisfied that, immediately prior to the Effective Time, no
     judgment, order, injunction or other restraint shall have been issued or
     filed which restrains, and no hearing seeking injunctive relief or other
     restraint is pending or has been noticed which seeks to restrain, the
     Company from consummating the transactions described in the Loan Documents
     or this Agreement.

          (d) No Default; Representations and Warranties.  The Administrative
              ------------------------------------------
     Agent and the Required Lenders shall be satisfied that, immediately prior
     to the Effective Time and after giving effect to this Agreement, (i) there
     shall exist no Default or Event of Default and (ii) the representations and
     warranties of the Company contained in the Loan Documents to which the
     Company is a party are true and correct in all material respects as of the
     Effective Time with the same effect as though such representations and
     warranties had been made on and as of the Effective Time (except for such
     representations and warranties made as of a specified date, which shall be
     true and correct in all material respects as of such specified date).

          (e) Corporate Proceedings.  All corporate and legal proceedings and
              ---------------------
     all instruments and agreements in connection with the transactions
     contemplated in this Agreement and the other Loan Documents shall be
     satisfactory in form and substance to the Administrative Agent and the
     Required Lenders, and the Administrative Agent shall have received all
     information and copies of all documents and papers, including records of
     corporate proceedings and governmental approvals, if any, which any Lender
     reasonably may have requested in connection therewith, such documents and
     papers where appropriate to be certified by proper corporate or
     governmental authorities.

          (f) Fee.  For consenting to the amendment and waiver contained in this
              ---
     Agreement, each Lender that executes and delivers this Agreement prior to
     5:00 p.m., Pacific Time, on March 1, 2001 shall have received on or before
     the Effective Date a fee in immediately available funds equal to the
     product of (i) 0.05% and (ii) the aggregate amount of such Lender's
     Commitments.

          (g) Other Payments.  The Administrative Agent and each Lender shall
              --------------
     have received all amounts, if any, owing from the Company to such Person
     through and including the Effective Date.

                                      -2-
<PAGE>

All the certificates and other documents and papers referred to in this Section
2, unless otherwise specified, shall be delivered to the Administrative Agent's
counsel, White & Case LLP, at 633 West Fifth Street, 19/th/ Floor, Los Angeles,
CA 90071, facsimile (213) 687-0758, for the account of each of the Lenders and
in sufficient counterparts for each of the Lenders and shall be satisfactory in
form and substance to the Administrative Agent and the Required Lenders.

          3.   Amendment and Waiver.  At and from the Effective Time:
               --------------------

               (a) Section 6.01(a) of the Credit Agreement shall be amended by
          deleting the text "0.65 to 1" and inserting the text "0.58 to 1" in
          its place.

               (b) The Administrative Agent and the Required Lenders agree to
          waive the Financial Covenant Breach to the extent (and only to the
          extent) the Company has complied with Section 2(b) of this Agreement.

          4.  Representations and Warranties. The Company makes, as of the
              ------------------------------
Effective Date, each of the representations and warranties set forth in Article
V of the Credit Agreement and such representations and warranties are, by this
reference, incorporated herein as if set forth herein in their entirety;
provided that references to "Loan Documents" shall, for purposes of this
--------
paragraph, be deemed to include this Agreement.

          5.  Miscellaneous.
              -------------

          (a) The waiver contained in this Agreement is a one-time waiver only,
is made only with respect to the specific provisions of the Credit Agreement
referenced above and is made only to the extent and for the limited purposes
described herein.  Such waiver shall not be construed as a waiver for any
purpose other than as expressly set forth herein and shall not constitute an
agreement or obligation on the part of the Administrative Agent or any Lender to
grant any other or future waiver, or prevent any of them from enforcing any
right or remedy under the Credit Agreement or otherwise with respect thereto.

          (b) Except as expressly modified by this Agreement, the Credit
Agreement shall continue to be and remain in full force and effect in accordance
with its terms.  Any future reference to the Credit Agreement shall, from and
after the Effective Time, be deemed to be a reference to the Credit Agreement as
amended by Section 3(a) of this Agreement.

          (c) This Agreement may be executed in any number of counterparts, each
of which shall constitute an original, but all of which when taken together
shall constitute but one instrument.

          (d) THIS AGREEMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES
HEREUNDER SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE
STATE OF NEW YORK WITHOUT REFERENCE TO CONFLICTS OF LAW RULES.

                                      -3-
<PAGE>

          (e) This Agreement may be executed by facsimile signature and each
such signature shall be treated in all respects as having the same effect as an
original signature.

          (f) The rules of construction set forth in Section 1.03 of the Credit
Agreement are, by this reference, incorporated herein as if set forth in their
entirety, provided that references to "this Agreement" in such section shall
mean references to this Agreement.

                                  *    *    *

                                      -4-
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Agreement to
be duly executed as of the date first above written.

                                             NEVADA POWER COMPANY

                                             By: _______________________________
                                                 Name:  Mark A. Ruelle
                                                 Title:  Sr. Vice President, CFO

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                           MELLON BANK, N.A., as Administrative
                                              Agent and as a Lender

                                           By: _______________________________
                                               Name:
                                               Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        FIRST UNION NATIONAL BANK, as
                                             Syndication Agent and as a Lender

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        WELLS FARGO BANK, N.A., as Syndication
                                             Agent and as a Lender

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        BANK OF AMERICA, N.A.

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        THE BANK OF NEW YORK

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        BANK ONE, N.A. (Main Office-Chicago)

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        CREDIT SUISSE FIRST BOSTON

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        BNP PARIBAS

                                        By: _______________________________
                                            Name:
                                            Title:

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        UNION BANK OF CALIFORNIA, N.A.

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        THE INDUSTRIAL BANK OF JAPAN, LTD.

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        BAYERISCHE LANDESBANK GIROZENTRALE

                                        By: _______________________________
                                            Name:
                                            Title:

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        FLEETBOSTON FINANCIAL CORP.

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        FIRST SECURITY BANK OF NEVADA

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        KBC BANK, N.V.

                                        By: _______________________________
                                            Name:
                                            Title:

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement
<PAGE>

                                        U.S. BANK NATIONAL ASSOCIATION

                                        By: _______________________________
                                            Name:
                                            Title:

                               -Signature Page-
                        Amendment and Waiver Agreement

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