<PAGE>
 
                                                                    EXHIBIT 4.4I

                                                                  CONFORMED COPY
                                                                  --------------



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                             DUQUESNE LIGHT COMPANY

                                       TO

                               MELLON BANK, N.A.

                                               Trustee

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


                         Supplemental Indenture No. 10

                              Dated March 22, 1995



                   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. 10, dated March 22, 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. 10 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.
<PAGE>
 
  The Company has acquired additional properties which it desires to subject to
the Lien of the Mortgage by this Supplemental Indenture No. 10.

  NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. 10 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:

                             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;

                             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
                                     - 2 -
<PAGE>
 
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. 10 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. 10;

                             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 (including, but not limited to, the Lien of the DLC 1947
Mortgage), 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;

  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


                                     - 3 -
<PAGE>
 
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 that this Supplemental Indenture
No. 10 is a supplement to the Mortgage.  As supplemented by this Supplemental
Indenture No. 10, the Mortgage is in all respects ratified, approved and
confirmed, and the Mortgage and this Supplemental Indenture No. 10 shall
together constitute one and the same instrument.

  IN WITNESS WHEREOF, the parties have caused this Supplemental Indenture No. 10
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/ J. H. McAnulty
                                           ------------------------------
                                            J. H. McAnulty
                                            Vice President


Attest:



/s/ Kent Christman
--------------------------------
  Authorized Officer


                                     - 4 -
<PAGE>
 
COMMONWEALTH OF PENNSYLVANIA    )
                                ) ss.:
COUNTY OF ALLEGHENY             )


       On the 22nd day of March, 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 22nd day of March, 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




                                     - 5 -
<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, 1995








                                     - 6 -
<PAGE>
 
                             RECORDING INFORMATION


Allegheny County, Pennsylvania
Office of Recorder of Deeds
Recorded March 24, 1995
Mortgage Book Volume 14817, page 129

Beaver County, Pennsylvania
Office of Recorder of Deeds
Recorded March 24, 1995
Mortgage Book Volume 1360, page 654

Greene County, Pennsylvania
Office of Recorder of Deeds
Recorded March 24, 1995
Mortgage Book Volume 139, page 795

Washington County, Pennsylvania
Office of Recorder of Deeds
Recorded March 24, 1995
Mortgage Book Volume 2665, page 189

Westmoreland County, Pennsylvania
Office of Recorder of Deeds
Recorded March 24, 1995
Mortgage Book Volume 3545, page 622

Belmont County, Ohio
Office of Recorder
Received March 30, 1995
Recorded March 31, 1995
Mortgage Book Volume 632, page 823

Columbiana County, Ohio
Office of Recorder
Recorded March 30, 1995
Official Records Volume 470, page 924

Jefferson County, Ohio
Office of Recorder
Received March 30, 1995
Recorded March 31, 1995
Official Records Volume 160, page 453



                                     - 7 -
<PAGE>
 
Lake County, Ohio
Office of Recorder
Recorded March 31, 1995
Official Records Volume 1102 page 333

Monroe County, Ohio
Office of Recorder
Received March 30, 1995
Recorded March 30, 1995
Official Records Volume 12, page 103

Hancock County, West Virginia
Office of Clerk of County Commission
Recorded March 30, 1995
Deed of Trust Book 325, page 407

Monongalia County, West Virginia
Office of Clerk of County Commission
Recorded March 24, 1995
Deed of Trust Book 774, page 10



                                     - 8 -
<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 Phillip Assini and Olga Assini, husband and
       wife, to Duquesne Light Company, et al.  Deed dated October 6, 1994.
       Deed Book Volume 1618, page 0578.  Tax parcel I.D. No. 50-181-0217.
       (Bruce Mansfield Power Station)

       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 Township
       of Greene.  Conveyed by Frank Ambrose and Norma J. Ambrose, husband and
       wife, to Duquesne Light Company, et al.  Deed dated December 20, 1994.
       Deed Book Volume 1628, page 0582.  Tax parcel I.D. No. 62-190-0120.
       (Bruce Mansfield Power Station)



(009)
                                      A-1