<PAGE>
 
                                                                     EXHIBIT 4.3

 
--------------------------------------------------------------------------------


                             DUQUESNE LIGHT COMPANY

                                       TO

                               MELLON BANK, N.A.

                                            Trustee

                          ---------------------------


                         Supplemental Indenture No. 13

                              Dated March 22, 1996



                   Supplemental to the Indenture of Mortgage
                  and Deed of Trust dated as of April 1, 1992



               Subjecting additional property to the lien of the
                      Indenture dated as of April 1, 1992



--------------------------------------------------------------------------------
<PAGE>
 
     SUPPLEMENTAL INDENTURE No. 13, dated March 22, 1996, between DUQUESNE LIGHT
COMPANY, a corporation duly organized and existing under the laws of the
Commonwealth of Pennsylvania (hereinafter sometimes called the "Company"), and
MELLON BANK, N.A., a national banking association organized and existing under
the laws of the United States of America, trustee (hereinafter sometimes called
the "Trustee"), under the Indenture of Mortgage and Deed of Trust, dated as of
April 1, 1992 (hereinafter called the "Original Indenture"), this Supplemental
Indenture No. 13 being supplemental thereto.  The Original Indenture and any and
all indentures and instruments supplemental thereto are hereinafter sometimes
collectively called the "Mortgage."

                            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 Recorders' Offices of the
various counties of Pennsylvania as follows:

     In Allegheny County in Mortgage Book Vol. 12068, page 8;
     In Beaver County in Mortgage Book Vol. 1208, page 520;
     In Greene County in Mortgage Book Vol. 100, page 174;
     In Washington County in Mortgage Book Vol. 1873, page 1;
     In Westmoreland County in Mortgage Book Vol. 2862, page 221;

and has also been recorded in the Office of the Clerk of County Commission of
Monongalia County, West Virginia, in Deed of Trust Book Vol. 672, page 129, the
Office of the Clerk of County Commission of Hancock County, West Virginia, in
Deed of Trust Book Vol. 293, page 46, the Recorder's Office of Belmont County,
Ohio, in Mortgage Book Vol. 586, page 273, the Recorder's Office of Columbiana
County, Ohio, in Mortgage Book Vol. 318, page 289, the Recorder's Office of
Jefferson County, Ohio, in Mortgage Book Vol. 65, page 675, the Recorder's
Office of Lake County, Ohio, in Mortgage Book Vol. 711, page 217, and the
Recorder's Office of Monroe County, Ohio, in Mortgage Book Vol. 129, page 301.

     Section 1401 of the Original Indenture provides that the Company and the
Trustee may enter into one or more supplemental indentures for the purpose,
among others, of subjecting additional property to the Lien of the Mortgage.

     The Company has acquired additional properties which it desires to subject
to the Lien of the Mortgage by this Supplemental Indenture No. 13.

     NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. 13 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
Mortgage and to declare the terms and conditions on which such Securities are
secured, the Company 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:
<PAGE>
 
                             Granting Clause First

     All right, title and interest of the Company 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 generation, purchase,
transmission, distribution or sale by the Company of electric energy (whether or
not such use is the sole use of such property), including without limitation (a)
all land and interests in land described in Schedule A hereto; (b) all other
lands, easements, servitudes, licenses, permits, rights of way and other rights
and interests in or relating to real property or the occupancy or use of the
same; (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; and (g) all pipes, cables,
insulators, ducts, tools, computers and other data processing and/or storage
equipment and other equipment, apparatus and facilities and 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 generation, purchase, transmission,
distribution or sale by the Company of electric energy;

                               Excepted Property

     Expressly excepting and excluding, however, from the Lien and operation of
the Mortgage 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 forever;

     SUBJECT, HOWEVER, to Permitted Liens and to Liens which have been granted
by the Company to other Persons prior to the date of the execution and delivery
of the Original Indenture, and subject also, as to any property acquired by the
Company after the date of execution and delivery of the Original Indenture, to
vendors' Liens, purchase money mortgages and other Liens thereon at the time of
the acquisition thereof (including, but not limited to, the Lien of any Class
"A" Mortgage), it being understood that with respect to any of such 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 Mortgage shall at all times be junior 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; and

     THE PARTIES HEREBY FURTHER COVENANT AND AGREE that this Supplemental
Indenture No. 13 is a supplement to the Mortgage.  As supplemented by this
Supplemental Indenture No. 13, the Mortgage is in all respects ratified,
approved and confirmed, and the Mortgage and this Supplemental Indenture No. 13
shall together constitute one and the same instrument.
<PAGE>
 
     IN WITNESS WHEREOF, the parties have caused this Supplemental Indenture No.
13 to be duly executed, and their respective corporate seals to be affixed and
attested, all as of the day and year first above written.

                                       DUQUESNE LIGHT COMPANY



                                       By:    /s/ Gary L. Schwass
                                          ------------------------------
                                              Gary L. Schwass
                                              Senior Vice President and
                                              Chief Financial Officer
Attest:



/s/ Diane S. Eismont
--------------------
        Secretary


                                       MELLON BANK, N.A., Trustee



                                       By:    /s/ E. D. Renn
                                          ------------------------------
                                              E. D. Renn
                                              Vice President


Attest:



/s/ Kent Christman
-------------------------
        Authorized Officer
<PAGE>
 
COMMONWEALTH OF PENNSYLVANIA  )
                              )   ss.:
COUNTY OF ALLEGHENY           )


     On the 21st day of March, 1996, before me personally came Gary L. Schwass,
to me known, who, being by me duly sworn, did depose and say that he is the
Senior Vice President and Chief Financial Officer of Duquesne Light Company, the
corporation described in and which executed the foregoing instrument; that he
knows the seal of said corporation; that the seal affixed to said instrument is
such corporate seal; that it was so affixed by authority of the Board of
Directors of said corporation, and that he signed his name thereto by like
authority.


                              /s/ Joanne E. Kirin
                              --------------------------
                                      Notary Public



COMMONWEALTH OF PENNSYLVANIA  )
                              )   ss.:
COUNTY OF ALLEGHENY           )


     On the 22nd day of March, 1996, before me personally came E. D. Renn, to me
known, who, being by me duly sworn, did depose and say that she is a Vice
President of Mellon Bank, N.A., the national banking association described in
and which executed the foregoing instrument; that she knows the seal of said
national banking association; that the seal affixed to said instrument is the
seal of said national banking association; that it was so affixed by authority
of the Board of Directors of said national banking association, and that she
signed her name thereto by like authority.


                              /s/ Judith A. Hyde
                              --------------------------
                                       Notary Public
<PAGE>
 
                        CERTIFICATE OF PRECISE RESIDENCE


     I hereby certify that the precise residence of Mellon Bank, N.A., is One
Mellon Bank Center, Second Ward, Pittsburgh, Allegheny County, Pennsylvania.


                                     /s/ Kent Christman
                                     -----------------------------------------
                                     Authorized Signatory of Mellon Bank, N.A.


                                                                March 22, 1996
<PAGE>
 
                                   Schedule A


                                       I

     All of the following described property situate in the County of Beaver and
Commonwealth of Pennsylvania, the deeds herein recited being recorded in the
Recorder's Office of said County, and reference being made thereto for a more
particular description of said property, viz:


          Undivided 17.01% interest as tenant in common with Pennsylvania Power
          Company, The Cleveland Electric Illuminating Company, Ohio Edison
          Company and The Toledo Edison Company in a parcel of land situate in
          the Borough of Shippingport.  Conveyed by Thomas R. Allison and Nancy
          D. Allison, husband and wife, to Duquesne Light Company, et al.  Deed
          dated July 12, 1995.  Deed Book Volume 1658, page 398.  Tax Parcel
          I.D. No. 62-180-0111-N00.

          Undivided 17.01% interest as tenant in common with Pennsylvania Power
          Company, The Cleveland Electric Illuminating Company, Ohio Edison
          Company and The Toledo Edison Company in a parcel of land situate in
          the Borough of Shippingport.  Conveyed by Harry N. Shane, widower, to
          Duquesne Light Company, et al.  Deed dated December 28, 1995.  Deed
          Book Volume 1683, page 842.  Tax Parcel I.D. No. 62-180-0111-001.
<PAGE>
 
                             RECORDING INFORMATION


                    Allegheny County, Pennsylvania
                    Office of Recorder of Deeds
                    Recorded March 25, 1996
                    Mortgage Book Volume 15589, page 74

                    Beaver County, Pennsylvania
                    Office of Recorder of Deeds
                    Recorded March 25, 1996
                    Mortgage Book Volume 1414, page 397

                    Greene County, Pennsylvania
                    Office of Recorder of Deeds
                    Recorded March 26, 1996
                    Mortgage Book Volume 153, page 564

                    Washington County, Pennsylvania
                    Office of Recorder of Deeds
                    Recorded March 26, 1996
                    Mortgage Book Volume 2857, page 569

                    Westmoreland County, Pennsylvania
                    Office of Recorder of Deeds
                    Recorded March 26, 1996
                    Mortgage Book Volume 3754, page 190

                    Belmont County, Ohio
                    Office of Recorder
                    Received March 28, 1996
                    Recorded March 29, 1996
                    Mortgage Book Volume 651, page 630

                    Columbiana County, Ohio
                    Office of Recorder
                    Recorded March 26, 1996
                    Official Records Volume 522, page 636

                    Jefferson County, Ohio
                    Office of Recorder
                    Received March 26, 1996
                    Recorded March 27, 1996
                    Official Records Volume 189, page 762

                    Lake County, Ohio
                    Office of Recorder
                    Recorded March 26, 1996
                    Official Records File 960009987
<PAGE>
 
                    Monroe County, Ohio
                    Office of Recorder
                    Received March 28, 1996
                    Recorded March 28, 1996
                    Official Records Volume 20, page 434

                    Hancock County, West Virginia
                    Office of Clerk of County Commission
                    Recorded March 26, 1996
                    Deed of Trust Book 335, page 57

                    Monongalia County, West Virginia
                    Office of Clerk of County Commission
                    Recorded March 26, 1996
                    Deed of Trust Book 806, page 676