Notes to Episode 32 of Code Talk, Grounding and Bonding - Equipment Grounding
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250-122. Size of Equipment Grounding Conductors
(B) Increase in Size. Where conductors are adjusted in size to compensate for voltage drop, equipment-grounding conductors, where installed, shall be adjusted proportionately according to circular mil area.
From the NEC Commentary in the Handbook:
Equipment grounding conductors on the load side of the service disconnecting means overcurrent devices are sized based on the size of the overcurrent devices ahead of them. Where the ungrounded conductors are increased in size for any reason, the equipment grounding conductors must also be increased proportionately.
A 240-volt, single-phase, 250-ampere load is supplied from a 300-ampere breaker located in a panelboard 500 ft away. The conductors are 250-kcmil copper, installed in rigid nonmetallic conduit with a No. 4 copper equipment-grounding conductor. The conductors are increased to 350 kcmil; therefore, the equipment-grounding conductor must be increased as follows:
250,000 = 1.4
Referring to Chapter 9, Table 8, we find that No. 4 has a cross-sectional area of 41,740 circular mils.
1.4 x 41,740 circular mils = 58,436 circular mils
Again referring to Chapter 9, Table 8, we find that 58,436 circular mils converts to a No. 2 copper equipment grounding conductor.