what are the minimum clearances required by the penna dept of transportation
for overhead utility wires that cross a public right of way ?

Hello Bill

Thank-you for your question.

The Pennsylvania Code says 18 feet is the minimum clearance for
utility wires over a public right of way.

"Location of wires, cables or conductors. A wire, cable or conductor
which overhangs a portion of the right-of-way shall be placed to
provide a minimum vertical clearance of 18 feet over the pavement and
shoulder, except where the National Electrical Safety Code requires
vertical clearances in excess of 18 feet due to voltage or span
lengths."

These provisions have been made in accordance with the State Highway
Law.
"The provisions of this #459.9 amended under sections 411, 420 and 702
of the State Highway Law (36 P. S. # # 670-411, 670-420 and 670-702)."

Pennsylvania Code: Chapter 459 OCCUPANCY OF HIGHWAYS BY UTILITIES
Section 9  Special conditions, aboveground facilities 
Paragraph (c)
http://www.pacode.com/secure/data/067/chapter459/s459.9.html

The list of contents for the entire chapter can be found at:

Pennsylvania Code: Chapter 459 OCCUPANCY OF HIGHWAYS BY UTILITIES
http://www.pacode.com/secure/data/067/chapter459/chap459toc.html

I hope this answers your question. 
If you need to pursue the exceptions in cases of extra high voltage or
unusual span lengths, some of the many documents relating to the
National Electrical Safety Code can be downloaded from the website of
the Institute of Electrical and Electronics Engineers:

IEEE: National Electrical Safety Code Zone
http://standards.ieee.org/nesc/interpretations.html

They also sell a handbook on the code for $115:

IEEE: NESC handbook
http://shop.ieee.org/store/product.asp?prodno=SP1125

This page from the PPL (Pennsylvania Power and Light) Corporation
suggests that in general the Pennsylvania 18 feet rule may be slightly
tougher than the NESC rules:

"Per the National Electrical Safety Code (NESC), cabled service drop,
600 Volts and less, shall have a minimum clearance above:
(1)
Roads, Streets, Alleys, Parking Lots subject to truck traffic - 16
feet.
Pennsylvania State highways - 18 feet. (Note - Service drops shall not
cross limited access highways or the Pa. Turnpike).
(2)
Lands traversed by agricultural vehicles such as cultivated, grazing,
forest, orchard, etc. - 16 feet."

PPL: clearances over areas and buildings
http://www.pplweb.com/delivering_energy/a/reference/emsi/rule4.htm#code

I hope this is helpful. Please let me know if it requires any
clarification.

Regards - Leli

searches used:
Pennsylvania "Department of Transportation"
overhead utility wires cables clearance "minimum clearance"
"National Electrical Safety Code" NESC IEEE

://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22public+rights+OR+right+of+way%22+overhead+cables+pennsylvania+&btnG=Google+Search

://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=+%22national+electrical+safety+code%22+&btnG=Google+Search

timely and specific but could have included a more comprehensive
comment about cable tv and telecom wires which share  the same utility
right-of-ways and poles and fixtures, my question addressed public
utility wire clearances and not electric transmission wires
exclusively as cited in the answers references.