If I upsize from 12 AWG to 10 AWG for voltage drop, what size ground do I need?

A 10 AWG ground. NEC 250.122(B) requires the equipment grounding conductor to increase proportionately by circular mil area when ungrounded conductors are upsized, and the 12 to 10 ratio lands exactly on 10 AWG.

Does upsizing for derating also require a bigger ground wire?

No. The NEC 2023 exception to 250.122(B) excludes increases made solely for ambient temperature correction or conductor bundling adjustment under 310.15.

If You Upsize Wire for Voltage Drop, Can You Keep the Smaller Ground?

A user asked the app a classic long run question: going from 12 AWG up to 10 AWG on a 20 amp circuit to handle voltage drop, can the equipment grounding conductor stay a 12? Under NEC 2023 the answer is no. When the ungrounded conductors grow, the wire type equipment grounding conductor has to grow with them.

What 250.122(B) actually says

Table 250.122 sets the minimum equipment grounding conductor by overcurrent device rating, and for a 20 amp breaker that baseline is 12 AWG copper. But 250.122(B) adds that if the ungrounded conductors are increased in size from the minimum that has sufficient ampacity for the intended installation, the equipment grounding conductor must be increased proportionately according to the circular mil area of the ungrounded conductors. The 2023 edition carved out one exception: upsizing that only compensates for ambient temperature correction or bundling adjustment under 310.15 does not trigger the proportional increase. Voltage drop is not in that exception, so a voltage drop upsize drags the ground along with it.

The circular mil math

A 12 AWG conductor is 6,530 circular mils and a 10 AWG is 10,380. The ratio is 10,380 divided by 6,530, or 1.59. Multiply the baseline 12 AWG ground by the same ratio: 6,530 times 1.59 is 10,380 circular mils, which is exactly a 10 AWG. So a 20 amp circuit run in 10 AWG for voltage drop needs a 10 AWG equipment grounding conductor. The pattern holds as runs get longer: jump the hots two sizes and the ground jumps two sizes from its table minimum as well. In practice, matching the ground to the upsized circuit conductors on small branch circuits always satisfies the rule and ends the debate at inspection.

Why the rule exists

The equipment grounding conductor has one job: carry enough fault current to open the breaker fast. On a long run, a small ground adds impedance to the fault loop, which stretches clearing time exactly where the extra length already hurts. The proportional rule keeps the fault path impedance in step with the circuit conductors. Note the rule applies to wire type grounds; if the raceway itself is the equipment grounding conductor, such as EMT under 250.118, there is nothing to upsize.

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