Ground Wire Size Follows the Breaker
The equipment grounding conductor is sized by the rating of the circuit breaker protecting the circuit, using NEC Table 250.122. This is different from how the hot and neutral conductors are sized. The ground only has to carry fault current long enough to trip the breaker, so it is keyed to the overcurrent device rather than to the load or to the size of the other conductors.
NEC Table 250.122 Ground Wire Chart
| Breaker Rating | Copper Ground | Aluminum Ground |
|---|---|---|
| 15 amps | 14 AWG | 12 AWG |
| 20 amps | 12 AWG | 10 AWG |
| 30 to 60 amps | 10 AWG | 8 AWG |
| 100 amps | 8 AWG | 6 AWG |
| 200 amps | 6 AWG | 4 AWG |
| 300 amps | 4 AWG | 2 AWG |
| 400 amps | 3 AWG | 1 AWG |
Reading the Table
Find the rating of the breaker in the left column and read across. If your exact breaker rating is not listed, the code says to use the next higher rating shown. For example, an 80-amp breaker falls between the 60 and 100 rows, so you use the 100-amp row, which is 8 AWG copper.
The Upsizing Rule
When you increase the size of the hot and neutral conductors beyond the minimum, such as upsizing for voltage drop on a long run, NEC 250.122(B) requires the ground wire to be increased in the same proportion. A bigger circuit conductor means a bigger ground, even though the breaker rating has not changed.
Bare or Green
The equipment ground is run as bare copper or as a green or green-with-yellow-stripe insulated conductor. In NM-B Romex the ground is the bare conductor inside the cable, already sized correctly for the cable. When pulling individual conductors in conduit, add a green or bare ground of the size shown in the table.
Code and Safety Note
Grounding is what makes a fault trip the breaker instead of energizing metal parts, so correct sizing is a safety essential. Follow the National Electrical Code and local amendments, confirm with your authority having jurisdiction, and use a licensed electrician for circuit and panel work.
IB Lighting stocks bare copper ground wire and green Southwire THHN in our electrical wire and cable collection, with free freight plus free freight insurance on every order. Call (800) 674-9019 for the right ground conductor for your circuit.