<PAGE>
 
                                                                Exhibit 4.4III


                                                              CONFORMED COPY

       =======================================================================


                                DUQUESNE LIGHT COMPANY

                                          TO

                                  MELLON BANK, N.A.

                                               TRUSTEE

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


                            SUPPLEMENTAL INDENTURE NO. 12

                            Dated as of September 1, 1995



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





                    Establishing a series of Securities designated
                           First Mortgage Bonds, Series J,
                limited in aggregate principal amount to $685,000,000


       =======================================================================
<PAGE>
 


            SUPPLEMENTAL  INDENTURE NO.  12,  dated as  of  September 1,  1995,
       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. 12 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.

            The Company has  heretofore executed and delivered to  Mellon Bank,
       N.A.,  as  Trustee, Supplemental  Indentures  for  the purposes  recited
       therein and  for the  purpose of  creating series  of Securities  as set
       forth in Schedule A hereto.

            The  Company  desires to  establish a  series  of Securities  to be
       designated "First Mortgage Bonds,  Series J" to be limited  in aggregate
       principal  amount  (except  as contemplated  in  Section  301(b) of  the
       Original Indenture)  to $685,000,000, such  series of  Securities to  be
       hereinafter sometimes called "Series No. 9."

            The  Company has duly authorized the execution and delivery of this
       Supplemental  Indenture No. 12 to establish the Securities of Series No.
       9 and has duly authorized the issuance of such Securities;  and all acts
       necessary to make this  Supplemental Indenture No. 12 a  valid agreement
       of the  Company,  and to  make  the Securities  of  Series No.  9  valid
       obligations of the Company, have been performed.

            NOW, THEREFORE,  THIS  SUPPLEMENTAL INDENTURE  NO.  12  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 hereby
       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:

                                GRANTING CLAUSE FIRST

                 All  right,  title  and interest  of  the  Company  in and  to
            property (other  than Excepted Property), real,  personal and mixed
<PAGE>
 


            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
            B hereto; (b) all  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;

                                GRANTING CLAUSE SECOND

                 Subject to the applicable exceptions permitted by Section 810,
            Section  1303  and Section  1305  of  the Original  Indenture,  all
            property (other  than  Excepted Property)  of the  kind and  nature
            described in Granting Clause First which may be hereafter  acquired
            by the Company, it being the intention of the Company that all such
            property  acquired by the Company  after the date  of the execution
            and  delivery of  this Supplemental  Indenture No.  12 shall  be as
            fully embraced within and  subjected to the Lien hereof as  if such
            property were  owned by the Company as of the date of the execution
            and delivery of this Supplemental Indenture No. 12;

                                GRANTING CLAUSE FOURTH

                 All other  property of whatever  kind and nature  subjected or
            intended to be subjected to the Lien of the Mortgage by any  of the
            terms and provisions thereof;

                                  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
<PAGE>
 

       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;

            PROVIDED,  HOWEVER, that if, after the right, title and interest of
       the  Trustee in  and  to  the  Mortgaged  Property  shall  have  ceased,
       terminated  and become  void  in accordance  with  Article Nine  of  the
       Original  Indenture, the principal of and premium, if any, and interest,
       if any, on the Securities  shall have been paid to the  Holders thereof,
       or shall have  been paid to the Company  pursuant to Section 603  of the
       Original Indenture, then and  in that case the  Mortgage and the  estate
       and rights thereby  granted shall cease, terminate and be  void, and the
       Trustee  shall cancel and discharge the Mortgage and execute and deliver
       to the Company such instruments as the Company shall require to evidence
       the discharge thereof;  otherwise the  Mortgage shall be  and remain  in
       full force and effect; and

            THE PARTIES HEREBY FURTHER COVENANT AND AGREE as follows:

                                     ARTICLE ONE

                              NINTH SERIES OF SECURITIES

            There  is hereby created  a series of  Securities designated "First
       Mortgage  Bonds, Series J"  and  limited in  aggregate principal  amount
       (except  as contemplated in Section 301(b) of the Original Indenture) to
       $685,000,000.   The form and  terms of  the Securities of  Series No.  9
       shall be established in or pursuant to an Officer's Certificate.

                                     ARTICLE TWO

                               MISCELLANEOUS PROVISIONS

            This Supplemental Indenture No. 12 is a supplement to the Mortgage.
       As supplemented by this  Supplemental Indenture No. 12, the  Mortgage is
       in all respects ratified,  approved and confirmed, and the  Mortgage and
       this Supplemental Indenture No. 12 shall together constitute one and the
       same instrument.
<PAGE>
 

                 IN  WITNESS  WHEREOF,  the  parties hereto  have  caused  this
       Supplemental Indenture  No. 12 to be duly executed, and their respective
       corporate seals to be hereunto  affixed and attested, all as of  the day
       and year first above written.

                                          DUQUESNE LIGHT COMPANY


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


       /s/ Joan S. Senchyshyn
       ----------------------------
             Secretary



                                          MELLON BANK, N.A., Trustee


                                          By:  /s/ J. H. McAnulty
                                               ----------------------------
                                               Vice President
       Attest:


       /s/ Kent Christman
       ---------------------------
          Authorized Officer
<PAGE>
 



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


                 On  the 11th day of September, 1995, 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 11th day of September, 1995, before me personally came
       J. H. McAnulty, to me known, who, being by me duly sworn, did depose and
       say  that he  is a  Vice President  of Mellon  Bank, N.A.,  the national
       banking  association  described  in  and which  executed  the  foregoing
       instrument; that he 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 he  signed his
       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.

                                                            September 11, 1995 
<PAGE>
 

                                                           SCHEDULE A


   Supplemental                      Securities of     Series
   Indenture No.     Dated as of     Series No.        Designation
   ------------      -----------     ------------      ------------
        1           April 1, 1992         1            Secured Medium-
                                                       Term Notes, Series B

        2           October 1, 1992       2            First Collateral
                                                       Trust Bonds, Series C

        3           December 1, 1992      3            First Collateral
                                                       Trust Bonds,
                                                       Pollution Control
                                                       Series D

        4           March 30, 1993      None           None

        5           June 1, 1993          4            First Collateral
                                                       Trust Bonds, Series E

        6           June 1, 1993          5            First Collateral
                                                       Trust Bonds,
                                                       Pollution Control
                                                       Series F

        7           August 1, 1993        6            First Collateral 
                                                       Trust Bonds,
                                                       Pollution Control
                                                       Series G

        8           March 21, 1994      None           None

        9           October 1, 1994       7            First Collateral
                                                       Trust Bonds,
                                                       Pollution Control
                                                       Series H

        10          March 22, 1995      None           None

        11          June 1, 1995          8            First Collateral
                                                       Trust Bonds,
                                                       Series I

     (table con't)

                                       Princial Amount
     Supplemental
     Indenture No.       Authorized       Issued(1)         Outstanding(1)
     -------------       ----------       ---------         --------------
          1             $400,000,000     $400,000,000        $400,000,000

          2             $400,000,000     $360,000,000        $360,000,000

          3              $47,925,000      $47,925,000         $47,925,000

          4                 None             None                None

          5             $300,000,000     $200,000,000        $200,000,000

          6              $25,000,000      $25,000,000         $25,000,000

          7              $20,500,000      $20,500,000         $20,500,000

          8                  None             None                None

          9              $75,500,000      $75,500,000         $75,500,000

          10                 None             None                None

          11            $923,000,000     $923,000,000         $90,000,000

     --------------------------
     (1) As of June 1, 1995
<PAGE>
 

                                                           SCHEDULE B


                                         NONE
<PAGE>
 


                                RECORDING INFORMATION


                          Allegheny County, Pennsylvania
                          Office of Recorder of Deeds
                          Recorded September 11, 1995
                          Mortgage Book Volume 15174, page 234

                          Beaver County, Pennsylvania
                          Office of Recorder of Deeds
                          Recorded September 11, 1995
                          Mortgage Book Volume 1386, page 419

                          Greene County, Pennsylvania
                          Office of Recorder of Deeds
                          Recorded September 12, 1995
                          Mortgage Book Volume 146, page 131

                          Washington County, Pennsylvania
                          Office of Recorder of Deeds
                          Recorded September 12, 1995
                          Mortgage Book Volume 2754, page 76

                          Westmoreland County, Pennsylvania
                          Office of Recorder of Deeds
                          Recorded September 12, 1995
                          Mortgage Book Volume 3641, page 314

                          Belmont County, Ohio
                          Office of Recorder
                          Received September 11, 1995
                          Recorded September 12, 1995
                          Mortgage Book Volume 643, page 172

                          Columbiana County, Ohio
                          Office of Recorder
                          Recorded September 12, 1995
                          Official Records Volume 494, page 287

                          Jefferson County, Ohio
                          Office of Recorder
                          Received September 11, 1995
                          Recorded September 12, 1995
                          Official Records Volume 173, page 814

                          Lake County, Ohio
                          Office of Recorder
                          Recorded September 12, 1995
                          Official Records Volume 1156, page 547

                          Monroe County, Ohio
                          Office of Recorder
                          Received September 11, 1995
                          Recorded September 11, 1995
                          Official Records Volume 15, page 759
<PAGE>
 

                          Hancock County, West Virginia
                          Office of Clerk of County Commission
                          Recorded September 11, 1995
                          Deed of Trust Book 330, page 112

                          Monongalia County, West Virginia
                          Office of Clerk of County Commission
                          Recorded September 12, 1995
                          Deed of Trust Book 790, page 345