Grounding electrode conductor vs equipment grounding conductor: which table sizes which?
The grounding electrode conductor sizes from Table 250.66, based on the largest ungrounded service conductor. The equipment grounding conductor sizes from Table 250.122, based on the overcurrent device ahead of the circuit. Different tables, different inputs. The exam swaps them on purpose, so anchor each one to its table.
Last reviewed June 2026 · Answered by JourneymanIQ staff
GEC: Table 250.66, sized by service conductor
The grounding electrode conductor connects the system to earth. You size it off the largest ungrounded service-entrance conductor using Table 250.66. Bigger service conductors, bigger GEC.
EGC: Table 250.122, sized by the OCPD
The equipment grounding conductor rides with the circuit and clears faults. You size it off the rating of the overcurrent device protecting that circuit using Table 250.122. The breaker or fuse sets the size, not the load.
Catch the upsize rule
If you upsize the phase conductors for voltage drop, you upsize the equipment grounding conductor proportionally under 250.122(B). That is a favorite exam trap because most people forget the ground grows too.
GEC from 250.66 by service conductor. EGC from 250.122 by the OCPD. Upsize the EGC when the phases grow.
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