What size ground wire is required for a 30 amp circuit?

A 30 amp circuit requires a 10 AWG copper equipment ground, per NEC Table 250.122. That matches the bare wire already inside 10-2 cable, so dryer and water heater runs carry the right ground by default.

In aluminum the same circuit grounds with 8 AWG.

The table holds 10 AWG copper across 30 through 60 amp protection, which surprises people sizing a 40 or 50 amp circuit; the ground does not grow again until 100 amps, where it becomes 8 copper.

The one modifier to remember is proportional upsizing: stretch the circuit conductors for a long run and 250.122(B) requires the ground to scale with them.

The equipment grounding conductor calculator reads the table for any breaker, both metals.