<PAGE>

                                                                     EXHIBIT 4.5

     This is a Security Agreement and a Mortgage of Chattels as Well as a
                  Mortgage of Real Estate and Other Property

                         Ninth Supplemental Indenture

                                     from

                       SAN DIEGO GAS & ELECTRIC COMPANY

                                      to

                 THE BANK OF CALIFORNIA, NATIONAL ASSOCIATION

                                 _____________
                          Dated as of August 1, 1968
<PAGE>

                              TABLE OF CONTENTS*


                                                                           Page

Parties.....................................................................  1
Recitals....................................................................  1

                                   ARTICLE I
                            AMENDMENTS OF INDENTURE

Section 1.    Definition of Permanent Additions.............................  3
Section 2.    Amendment of Section 11 of Article II of Original Indenture...  4
Section 3.    Addition of Section 18 to Article II of Original Indenture....  5
Section 4.    Addition of Clause (2) of Section 2 to Article XI.............  5
Section 5.    Addition of Section 9 to Article XVII of Original Indenture...  5

                                  ARTICLE II
                           MISCELLANEOUS PROVISIONS

Section 1.    This Indenture Supplemental to Indenture of July 1, 1940.....   5
Section 2.    Effective Date...............................................   6
Section 3.    Covenants, Stipulations, Promises and Agreements.............   6

                             -------------------
Signatures and Acknowledgments............................................. 6,7








-----------------------
   * For convenience only and not a part of the Ninth Supplemental Indenture.
<PAGE>

        This Ninth Supplemental Indenture Is a Security Agreement and a
                 Mortgage of Chattels as Well as a Mortgage of
                        Real Estate and Other Property

     THIS NINTH SUPPLEMENTAL INDENTURE, made as of the 1st day of August, 1968,
by and between SAN DIEGO GAS & ELECTRIC COMPANY, a corporation duly organized
and existing under and by virtue of the laws of the State of California, having
its principal office in that State in the City of San Diego (the "Company"), the
party of the first part, and THE BANK OF CALIFORNIA, NATIONAL ASSOCIATION, a
corporation duly organized under an act known as the "National Bank Act," of the
United States of America, having its principal office in the City and County of
San Francisco, State of California, as Trustee (the "Trustee"), the party of the
second part.

     WHEREAS, the Company executed and delivered a Mortgage and Deed of Trust
(the "Original Indenture"), dated July 1, 1940, to the Trustee to secure payment
of the principal of and the interest on all bonds of the Company at any time
outstanding thereunder according to their tenor and effect, and to provide the
terms and provisions with respect to its First Mortgage Bonds, 3/8% Series due
July 1, 1970, issued in the aggregate principal amount of $16,000,000 and
presently outstanding; and

     WHEREAS, the Company executed and delivered to the Trustee a First
Supplemental Indenture dated as of December 1, 1946, a Second Supplemental
Indenture dated as of March 1, 1948, a Third Supplemental Indenture dated as of
April 1, 1952, a Fourth Supplemental Indenture dated as of April 1, 1954, a
Fifth Supplemental Indenture dated as of October 1, 1955, a Sixth Supplemental
Indenture dated as of October 1, 1957, a Seventh Supplemental Indenture, dated
as of October 1, 1960, and an Eighth Supplemental Indenture dated as of March 1,
1967, whereby, among other things, the Company set forth certain of the
particulars of the Bonds of series designated "First Mortgage Bonds, 2 3/4%
Series due December 1, 1981" issued in the aggregate principal amount of
$2,800,000, "First Mortgage Bonds, Series C due 1978" issued in the aggregate
principal amount of $10,000,000, "First Mortgage Bonds, Series D due 1982"
issued in the aggregate principal amount of $12,000,000, "First Mortgage Bonds,
Series E due 1984" issued in the aggregate principal amount of $17,000,000,
"First Mortgage Bonds, Series F due 1985" issued in the aggregate principal
amount of $18,000,000, "First Mortgage Bonds, Series G due 1987" issued in the
aggregate principal amount of $12,000,000, "First Mortgage Bonds, Series H due
1990" issued in the aggregate principal amount of $30,000,000, "First Mortgage
Bonds, Series I due 1997," issued in the aggregate principal amount of
$25,000,000, respectively, all of which First Mortgage Bonds except the 2 3/4%
Series due December 1, 1981 in the principal amount of $2,800,000 (which have
heretofore been retired) are presently issued and outstanding; and

     WHEREAS, the Original Indenture and each of said Supplemental Indentures
have been recorded in the Official Records of the Recorders of the Counties of
San Diego, Orange and Riverside, respectively, as follows:
<PAGE>

<TABLE>
<CAPTION>
                            Official                             Counties of
Document                    Records          San Diego             Orange              Riverside
-----------------------------------------------------------------------------------------------------
<S>                       <C>           <C>                  <C>                  <C>
Original Indenture            Book              1087                 1062                 1765
                              Page               1                   300                  364
                              Date         Oct. 10, 1940        Oct. 10, 1940        July 13, 1955
-----------------------------------------------------------------------------------------------------
First Supplemental            Book              2321                 1506                 1765
Indenture                     Page              48                   472                  499
                              Date         Jan. 2, 1947         Jan. 9, 1947         July 13, 1955
-----------------------------------------------------------------------------------------------------
Second Supplemental           Book              2537                 1616                 1765
Indenture                     Page              363                  190                  448
                              Date         Mar. 16, 1948        Mar. 15, 1948        July 13, 1955
-----------------------------------------------------------------------------------------------------
Third Supplemental            Book              4424                 2311                 1765
Indenture                     Page              535                  116                  475
                              Date         Apr. 3, 1952         Apr. 3, 1952         July 13, 1955
-----------------------------------------------------------------------------------------------------
Fourth Supplemental           Book              5193                 2701                 1765
Indenture                     Page              217                  153                  336
                              Date         Apr. 2, 1954         Apr. 2, 1954         July 13, 1955
-----------------------------------------------------------------------------------------------------
Fifth Supplemental            Book              5893                 3304                 1829
Indenture                     Page              291                  205                  3
                              Date         Dec. 5, 1955         Dec. 5, 1955         Dec. 5, 1955
-----------------------------------------------------------------------------------------------------
Sixth Supplemental            Book              6829                 4099                 2175
Indenture                     Page              390                  109                  538
                              Date         Nov. 12, 1957        Nov. 12, 1957        Nov. 12, 1957
-----------------------------------------------------------------------------------------------------
Seventh Supplemental          Book           Series 1
                                             Book 1960               5455                 2780
Indenture                     Page        File No. 202061            385                  3
                              Date         Oct. 10, 1960        Oct. 10, 1960        Oct. 10, 1960
-----------------------------------------------------------------------------------------------------
Eighth Supplemental           Book         1967 Series 8             8197                20925
Indenture                     Page        File No. 33860             129             (Endorsement
                              Date         Mar. 13, 1967        Mar. 13, 1967            No.)
                                                                                    Mar. 13, 1967;
</TABLE>

and

     WHEREAS, the Company desires to amend the original Indenture as heretofore
amended, supplemented and now in effect, as hereinafter set forth; and

     WHEREAS, the execution and delivery of this Ninth Supplemental Indenture
has been duly authorized by resolution of the Board of Directors of the Company,
has been duly authorized and approved by the Public Utilities Commission of the
State of California and has been approved by the holders of more than 66 2/3% in
principal amount of the First Mortgage Bonds of the Company presently
outstanding; and

                                       2
<PAGE>

     WHEREAS, the Corporation has requested the Trustee to join in the execution
and delivery of this Ninth Supplemental Indenture; and

     WHEREAS, all other acts and things necessary to make this Ninth
Supplemental Indenture a valid, binding and legal instrument and a valid,
binding and legal amendment of the original Indenture, have been duly performed
and done;

     NOW, THEREFORE, in consideration of the premises IT IS HEREBY AGREED AND
PROVIDED:

                                   ARTICLE I
                            AMENDMENTS OF INDENTURE

     Section 1.  The definition of "Permanent Additions" contained in Section 4
of Article I of the Indenture is hereby amended to read as follows:

          "Section 4.  The term "Permanent Additions" shall mean all property,
     real, personal or mixed (including therein, without in anywise limiting or
     impairing by the enumeration of the same, the scope and intent of the
     foregoing except as hereinafter specifically limited, all lands, buildings,
     plants, power houses, dams, reservoirs, stations, lines, gas plants,
     holders, pipes, mains, conduits, cables, machinery, pumps, transmission and
     distribution lines, pipe lines, rights-of-way, distribution systems,
     substations, transformers, meters, service systems and supply systems,
     wires, poles, cross-arms, apparatus, and improvements, extensions and
     additions, including operating public utility properties acquired as an
     entirety whether by purchase, consolidation, merger or otherwise) which
     shall have been made, acquired, constructed or erected by the Company or by
     the Company and one or more other public utilities jointly subsequent to
     June 30, 1940, or in the process of construction or erection in so far as
     actually constructed or erected subsequent to June 30, 1940, and used or to
     be used in the business of generating, manufacturing, storing,
     transporting, transmitting, distributing or supplying electricity or gas
     for light, heat, power, refrigeration or other purposes.  Permanent
     Additions shall also include betterments, improvements, extensions and
     additions to the steam properties owned by the Company on June 30, 1940, if
     made, acquired, constructed or erected by the Company subsequent to June
     30, 1940.  The term "Permanent Additions" shall not include

          (1)  the undivided interest or interests of the one or more other
     public utilities in any such property made, acquired, constructed or
     erected by the Company and one or more other public utilities jointly;

          (2)  any property (other than paving, grading and other improvements
     to public properties) not subject to the lien of this Indenture as a first
     lien except for "permitted liens" as that term is hereinafter defined and
     except for Non-callable Liens as that term is hereinafter defined;

                                       3
<PAGE>

          (3)  any plant used or intended for use in the manufacture of
     artificial gas if acquired in place as an entirety, or substantially as an
     entirety, except a plant built or constructed for the account of the
     Company; or any property used or intended for use in the transmission of
     natural or artificial gas if located outside of San Diego County,
     California, or the counties immediately adjacent thereto;

          (4)  any property acquired by the Company for the purpose of producing
     natural gas, oil or coal, or natural gas or oil rights owned in fee or
     under lease or gas wells or oil wells or equipment therefor, or coal mines
     or equipment therefor;

          (5) any franchises or governmental permits or licenses granted to or
     acquired by the Company as such, separate and distinct from the property
     operated thereunder or in connection therewith;

          (6)  any steam properties except to the extent hereinabove expressly
     permitted;

          (7)  any item of property acquired or constructed to replace a similar
     item of property whose retirement has not been credited to a capital
     account; or any property whose cost has been charged, or is properly
     chargeable, to repairs or maintenance or other operating expense account,
     or whose cost has not been charged, or is not properly chargeable, to a
     capital account; or

          (8)  any leased plant or system or any plant or system located on any
     leased property, easement or right of way terminable after a fixed period
     of time, or any undivided interest therein, or any betterments, extensions,
     improvements or additions of, upon, or to, any such plant or system, to the
     extent that such plant or system or betterments, extensions, improvements
     or additions thereto, may not be capitalized by the Company in accordance
     with the Uniform System of Accounts of the Public Utilities Commission or
     such other commission as may have jurisdiction thereof, or, in the absence
     of any such applicable Uniform System of Accounts, in accordance with
     generally accepted principles of accounting.

     The term Permanent Additions may include properties otherwise conforming to
the foregoing provisions of this Section acquired by the Company by means of or
in connection with the merger into the Company of any other corporation or the
consolidation of the Company with any other corporation or as a part of the
assets of any other corporation acquired as an entirety or substantially as an
entirety."

     Section 2.  Section 11 of Article II is hereby amended to read as follows:

          "Section 11.  In all cases of exchanges of bonds contemplated by the
     next preceding section hereof, the bonds to be exchanged shall be
     surrendered at the office or agency of the Company in such place or places
     as shall be designated for the purpose in such bonds or in this Indenture
     or any supplement thereto, with all unmatured coupons thereto appertaining
     (in the case of coupon bonds) and the Company shall execute and the Trustee
     shall authenticate and deliver in exchange therefor the bond or bonds which
     the
<PAGE>

     bondholder making the exchange shall be entitled to receive. All bonds so
     surrendered for exchange and the coupons appertaining thereto shall be
     cancelled by the Trustee. Notwithstanding any legend endorsed on any
     outstanding registered bond that such bond has been issued in lieu of or in
     exchange for coupon bond(s) of the same issue and series with specified
     numbers and that coupon bond(s) of said issue and series bearing the same
     numbers will be issued in exchange for said registered bond, the Company
     may execute and the Trustee may authenticate and deliver coupon bond(s) of
     the same issue and series bearing different numbers than those specified in
     such legend in exchange for any such registered bond surrendered for
     exchange for coupon bonds, provided that such coupon bonds so issued in
     exchange shall be in the same aggregate principal amount as such registered
     bond surrendered for exchange. Upon every exchange of bonds (including
     transfers of registered bonds without coupons under the provisions of
     Section 12 of this Article), the Company (subject to the provisions of
     Section 14 of this Article) may make a charge therefor sufficient to
     reimburse it for any tax or taxes or other governmental charge required to
     be paid by the Company by reason of such exchange and in addition may
     charge a sum not exceeding Two Dollars ($2) for each bond issued upon any
     such exchange, which shall be paid by the party requesting such exchange as
     a condition precedent to the exercise of the privilege of making such
     exchange. The Company shall not be obligated to make exchanges of bonds for
     a period of ten (10) days next preceding any interest payment date."

     Section 3.  A new section is hereby added to Article II of the original
Indenture to be designated as Section 18 and to read as follows:

          "Section 18.  Notwithstanding any of the terms or provisions of the
     Indenture, or any of the terms or provisions of any of the bonds now or
     hereafter outstanding, whenever in the Indenture or in any bond of any
     series (including series heretofore issued) hereafter authenticated and
     delivered under any provision of the Indenture reference is made to the
     execution, issue or signing of such bond by the Company or an officer
     thereof, or to the attestation of its corporate seal affixed thereto, the
     signature of the proper officer of the Company acting for any such purpose
     may be either a manual signature of such officer or a facsimile thereof."

     Section 4.  Clause (2) of Section 2 of Article XI is hereby amended to read
as follows:

          "(2) cancel, or make changes or alterations in, or substitutions of,
     any and all contracts, leases and easements; or waive the right to
     partition property additions owned as a tenant-in-common with one or more
     other public utilities;"

     Section 5.  A new section is hereby added to Article XVII of the original
Indenture to be designated as Section 9 and to read as follows:

          "Section 9.  Anything in this Article XVII contained to the contrary
     notwithstanding, the Trustee shall receive the written consent or consents
     of the holders of not less than 66 2/3% in principal amount of bonds then
     outstanding and entitled to
<PAGE>

     consent in lieu of the holding of a meeting pursuant to this Article XVII
     and in lieu of all action at such a meeting."

                                  ARTICLE II
                           MISCELLANEOUS PROVISIONS

     Section 1.  This instrument is executed and shall be construed as an
indenture supplemental to the Original Indenture and shall form a part thereof
and, as amended by this Ninth Supplemental Indenture, the Original Indenture, as
heretofore amended and supplemented, is hereby confirmed.

     In order to facilitate the filing of this Ninth Supplemental Indenture the
same may be executed in several counterparts each of which, when so executed,
shall be deemed to be an original, but such counterparts shall constitute but
one and the same instrument.

     Section 2.  This Ninth Supplemental Indenture is dated for convenience
August 1, 1968, although executed and delivered on the date of the
acknowledgment hereof by the Trustee, and the same shall be effective from the
date on which it is so executed and delivered.

     Section 3.  All of the covenants, stipulations, promises and agreements in
this Ninth Supplemental Indenture contained by or on behalf of the Company shall
bind its successors and assigns, whether so expressed or not.

     IN WITNESS WHEREOF, SAN DIEGO GAS & ELECTRIC COMPANY has caused this Ninth
Supplemental Indenture to be signed in its name and behalf by its President or
one of its Vice Presidents and its corporate seal to be hereunto affixed duly
attested by its Secretary or one of its Assistant Secretaries, and THE BANK OF
CALIFORNIA, NATIONAL ASSOCIATION, to evidence its acceptance of the trusts
hereby created, has caused this Ninth Supplemental Indenture to be signed in its
name and behalf by its President or one of its Vice Presidents and its corporate
seal to be hereunto affixed duly attested by its Secretary or one of its
Assistant Secretaries as of the day and year first above written.

                              SAN DIEGO GAS & ELECTRIC COMPANY

                              By  SHERMAN CHICKERING
                                              Vice President
Attest:

C. HAYDEN AMES
             Assistant Secretary
[Corporate Seal]

                              THE BANK OF CALIFORNIA, NATIONAL ASSOCIATION

                              By  R. G. FUNSTON
                                              Vice President
Attest:

W. E. SHEEHAN
             Assistant Secretary
[Corporate Seal]
<PAGE>

State of California
City and County of San Francisco--ss.

     On this 1st day of August, 1968, before me, ELLEN SMITH, a Notary Public in
and for said City and County and State, personally appeared SHERMAN CHICKERING,
known to me to be Vice President and C. HAYDEN AMES, known to me to be Assistant
Secretary of SAN DIEGO GAS & ELECTRIC COMPANY, a California corporation, one of
the corporations that executed the within instrument, and known to me to be the
persons who executed the within instrument on behalf of the corporation therein
named, and acknowledged to me that such corporation executed the same.

     IN WITNESS WHEREOF, I have hereunto set my hand and affixed my official
seal the day and year in this certificate first above written.

                                                    ELLEN SMITH
                                   Notary Public in and for the City and County
                                       of San Francisco, State of California

My Commission expires Feb. 14, 1972

[Notarial Seal]

State of California
City and County of San Francisco--ss.

     On this 8 day of August, 1968, before me, SELMA R. CONLAN, a Notary Public
in and for said City and County and State, personally appeared R. G. FUNSTON,
known to me to be Vice President, and W. E. SHEEHAN, known to me to be Assistant
Secretary of THE BANK OF CALIFORNIA, NATIONAL ASSOCIATION, a corporation duly
organized under an act known as the "National Bank Act," of the United States of
America, one of the corporations that executed the within instrument, and known
to me to be the persons who executed the within instrument on behalf of the
corporation therein named, and acknowledged to me that such corporation executed
the same as trustee.

     IN WITNESS WHEREOF, I have hereunto set my hand and affixed my official
seal the day and year in this certificate first above written.

                                                  SELMA R. CONLAN
                                  Notary Public in and for the City and County
                                       of San Francisco, State of California

My Commission expires July 5, 1969

[Notarial Seal]

     Executed counterparts of the foregoing Ninth Supplemental Indenture were
recorded on August 14, 1968, Series 9 Book 1968 of Official Records of the
County Recorder of San Diego County, California File/Page No. 138926; in
Official Record Book 8691, at page 69 of the County Recorder of Orange County,
California; and under Recorder's Endorsement No. 78781 of the County Recorder of
Riverside County, California.