What size ground wire for a 20 amp circuit?

A 20 amp circuit needs a 12 AWG copper equipment ground, per NEC Table 250.122. At this size the ground matches the circuit conductors, which is why 12-2 Romex ships with a 12 gauge bare wire.

A 14 gauge ground on a 20 amp circuit is not permitted. The table's next step down, 14 AWG, belongs to 15 amp protection only.

Grounds only get to be smaller than the hots at larger sizes: the table gives a 10 AWG ground to a 60 amp breaker, for instance. Small circuits ground full-size.

Aluminum equivalents are one step up; a 20 amp circuit grounded in aluminum uses 10 AWG.

The grounding wire size calculator reads Table 250.122 for any breaker rating instantly.