NEC 250.122 / 250.66
What Size Ground Wire Do I Need?
Two different grounds, two different tables: the equipment ground by breaker, and the grounding electrode conductor by service, with the ground-rod exception most people miss.
Equipment Ground (EGC) by Breaker, NEC 250.122
The green or bare ground that runs with a circuit or feeder, sized by the overcurrent device protecting it (copper).
| Breaker / OCPD | Copper EGC |
|---|---|
| 15A | #14 |
| 20A | #12 |
| 30A | #10 |
| 40A | #10 |
| 50A | #10 |
| 60A | #10 |
| 100A | #8 |
| 200A | #6 |
| 300A | #4 |
| 400A | #3 |
Grounding Electrode Conductor (GEC) by Service, NEC 250.66
The conductor from the service to the earth electrodes (ground rods, water pipe, Ufer), sized by the service-entrance conductor. Based on the typical NEC 310.12 dwelling service conductors.
| Service | Cu Conductor | GEC (Cu) | GEC (Al) |
|---|---|---|---|
| 100A | #4 | #8 | #6 |
| 150A | #1 | #6 | #4 |
| 200A | #2/0 | #4 | #2 |
| 400A | #400 kcmil | #1/0 | #3/0 |
Frequently Asked Questions
What size ground wire for a 200 amp service?
A 200A service uses a #4 copper (or #2 aluminum) grounding electrode conductor to a water pipe, per NEC Table 250.66, based on the typical 2/0 copper service conductors. The important exception: if that conductor's only job is to connect to a driven ground rod, NEC 250.66(A) says it never has to be larger than #6 copper. So it is #4 Cu to the water pipe and Ufer, but only #6 Cu to the ground rods. That single rule is the one most people get wrong.
What size ground wire for a 100 amp service?
A 100A service uses a #8 copper (or #6 aluminum) grounding electrode conductor to a water pipe per NEC Table 250.66, based on the typical #4 copper service conductors. The connection to a ground rod is capped at #6 copper (250.66(A)), and since #6 is a common, sturdier choice for rods, many electricians simply run #6 to both. Confirm your actual service-entrance conductor size, which is what the table is really based on.
What size ground wire for a 30 amp circuit?
A #10 copper equipment grounding conductor (EGC), per NEC Table 250.122. The circuit ground is sized by the breaker, not the load: a 30A breaker gets a #10 ground. This is the green or bare wire that runs with the circuit conductors, and it is different from the grounding electrode conductor at the service.
What size ground wire for a 20 amp circuit?
A #12 copper equipment grounding conductor, per NEC Table 250.122. A 20A circuit uses #12 conductors and a #12 ground; a 15A circuit uses #14. If you upsize the current-carrying conductors for voltage drop, NEC 250.122(B) requires you to upsize the EGC by the same proportion.
What is the difference between the EGC and the GEC?
The equipment grounding conductor (EGC) is the ground that runs with a circuit or feeder and bonds equipment back to the panel; it is sized by the overcurrent device from NEC Table 250.122 (20A = #12, 30A = #10, 100A = #8). The grounding electrode conductor (GEC) connects the service or system to the earth (ground rods, water pipe, Ufer); it is sized by the service-entrance conductor from NEC Table 250.66 (200A service = #4 Cu). They answer two different questions and use two different tables.
Do I need to upsize the ground wire if I upsize the wire?
Yes, for the equipment ground. NEC 250.122(B) requires that when ungrounded conductors are increased in size (commonly for voltage drop on a long run), the equipment grounding conductor is increased proportionally by circular-mil area. The grounding electrode conductor at the service is not upsized this way; it follows Table 250.66 based on the service conductors.