National Electrical Installation Standards

Standards as High as Your Own

Tuesday, May 31, 2022


Hello CQD,

We have had a discussion and debate on the correct code section that covers the equipment grounding conductor on the load side of a transformer. The section covering separately derived systems covers sizing for grounding electrode conductors, and equipment bonding jumper but fails to mention sizing of the equipment grounding conductor. In the sections referencing the equipment grounding conductor, it calls for sizing to be for the switch protecting the conductors. So would that dictate sizing for the load side switch fed from the transformer? Or do you relate back to sizing based on the ungrounded conductor size or even the switch size on the line side of the transformer? Any guidance is always appreciated.

Lenny Simpson



Hi Lenny,

Thanks for the question. Section 250.122 requires that equipment grounding conductors be sized based on the rating of the overcurrent protective device protecting the circuit using Table 250.122. This means it should be based on the load side switch fed from the transformer, but only if there is an overcurrent device. If the secondary conductors from a transformer have no overcurrent protection, then there is no equipment grounding conductor, it is a supply-side bonding jumper that must be installed between the transformer enclosure and the first system overcurrent device enclosure. Supply-side bonding jumpers must be sized using 250.102(C). See Section 250.30(A)(2) for sizing supply-side bonding jumpers. Equipment grounding conductors are sized using Table 250.122 based on the rating of the overcurrent device protecting the circuit conductors and equipment.

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