Change List for this Rule


General Order 95


Section V


Detailed Construction Requirements for Supply Lines
(Class H, L and T Circuits)

58.2    Transformers


A.    Grounding or Bonding


(1)    Grounding of Windings: Transformer windings, providing service not exceeding 300 volts (except those used exclusively for energizing street lighting systems or used exclusively for energizing signal and traffic circuits) shall be effectively grounded. Banked transformers are not required to have each transformer winding grounded, however, at least one winding shall be effectively grounded.  Where the secondary system is grounded at any point, the grounded conductor shall be run to each service.


(2)    Location of Transformer Winding Grounds: Transformer ground connections shall be provided at one of the following locations:


At the transformer pole or


At a pole adjacent to the transformer pole.


Transformer ground connections other than those occurring on common primary and secondary grounded neutral systems shall have an ampacity not less than that of No. 6 AWG copper wire.


Where a common primary and secondary grounded neutral system is used, ground connections shall conform to the requirements of Rule 59.4A .


(3)    Transformer Case Grounding or Bonding: (See Rule 54.4G for Grounded Equipment Clearances)

Transformers shall not be supported on metal poles or metal supports in contact with the ground unless the cases are securely bonded to the metal poles or parts of structures in contact with the ground and such poles or structures are effectively grounded.

Where transformer cases are bonded, the case bonding system shall not be electrically connected to any unassociated hardware or to other bonds.


B.    Connections between Windings

Any metallic connection between the primary and secondary windings of a distribution transformer (as in common neutral systems) shall be made externally and not within the transformer case.


Note:    Added March 29, 1966 by Decision No. 70489 and revised August 9, 1966 by Decision No. 71094. Revised January 19, 1994 by Resolution SU25. (Formerly Rule 58.3) and Revised January 13, 2006 by Decision No. 05-01-030