Sizing the equipment ground wire
A wire equipment ground is sized from Table 250.122 by the breaker or fuse protecting the circuit. If the circuit wires are made larger, for example to reduce voltage drop, the ground wire must be enlarged in proportion.
What the code requires
A wire-type equipment grounding conductor must be at least the size in Table 250.122 for the breaker or fuse ahead of it, but it never has to be larger than the circuit conductors. Selected copper sizes, with aluminum or copper-clad aluminum in parentheses: 15 A, 14 AWG (12); 20 A, 12 AWG (10); 60 A, 10 AWG (8); 100 A, 8 AWG (6); 200 A, 6 AWG (4); 300 A, 4 AWG (2); 400 A, 3 AWG (1).
Larger ratings: 500 A, 2 AWG (1/0); 600 A, 1 AWG (2/0); 800 A, 1/0 (3/0); 1000 A, 2/0 (4/0); 1200 A, 3/0 (250 kcmil). Where parallel conductors run in multiple raceways and a wire-type ground is used, each raceway gets one, sized from the breaker or fuse rating for the whole circuit.
If the hot conductors are made larger for any reason other than the temperature and bundling corrections in 310.15(B) and (C), such as voltage drop, the wire ground must be increased in proportion to the increase in circular-mil area.
One ground may serve several circuits in the same raceway, cable, trench or cable tray, sized for the largest breaker or fuse. For feeder taps, the ground is sized from the breaker or fuse ahead of the feeder but need not be larger than the tap conductors.
On the job
- A 20 A circuit upsized from 12 to 10 AWG for voltage drop needs its ground upsized to 10 AWG as well.
- An undersized ground on a 60 A EV charger or subpanel feeder is a common correction. 10 AWG copper is the minimum.
Exceptions that matter
- A qualified person may size the ground to provide an effective ground-fault path under 250.4(A)(5) or (B)(4) instead of increasing it in proportion.
Related sections
- NEC 250.4
- NEC 250.32: Grounding at detached garages, ADUs and outbuildings
- NEC 250.104(B)
- NEC 250.118: What can serve as the equipment ground
- NEC 310.15: Derating wire for heat and crowded conduits
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Kyle Robert Anderson KBC Electric · CSLB #1073066 · C-10 Electrical · Licensed since 2021