Does Size Matter for Grounding Conductor?

Dear Swami Candela,

It occurs to me that copper is heavy, and I heard that it's allowed to reduce the size of the grounding conductor compared to the ungrounded conductors in a feeder cable set. Is this true? If it's safe to carry a smaller grounding conductor then not only can it save money and weight, but it seems that it would also be more fuel efficient to transport.

Signed,

Weighed Down in St. Louis

Dear Weighed,

The truth is sometimes elusive and sometimes not. In the United States it is spelled out in Article 520.53(H)(2), which says "The equipment grounding conductor...shall be permitted to be reduced in size as permitted by 250.122," and Article 250.122 shows a chart with permissible sizes of the equipment grounding conductor based on the rating or setting of the automated overcurrent device in the circuit ahead of the equipment. For 400-amp service, the grounding conductor has to be at least #3 AWG or larger. That's because the grounding conductor should only ever conduct in the event of a fault to ground, and then only long enough to trip the circuit breaker or blow the fuse, which should be a very short duration.