What Size Ground Wire Do I Need?

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.

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.

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.

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 the ungrounded conductors are larger than the circuit needs, NEC 250.122(B) requires the equipment grounding conductor to be increased by the same proportion of circular-mil area. That applies whatever the reason for the increase, not only voltage drop; the only reasons it excludes are the ambient-temperature correction of 310.15(B) and the conductor-count adjustment of 310.15(C). Our ground wire size calculator computes the increase from the conductor actually in the pipe.

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.

Yes, for the equipment ground. NEC 250.122(B) requires that when the ungrounded conductors are increased in size, the equipment grounding conductor is increased proportionally by circular-mil area. Voltage drop on a long run is the common reason, but the rule turns on the increase, not on why you made it: the only increases it excludes are those required by 310.15(B) for ambient temperature and 310.15(C) for more than three current-carrying conductors, so a conductor upsized purely for heat or conduit fill does not trigger it. The grounding electrode conductor at the service is not upsized this way; it follows Table 250.66 based on the service conductors. The ground wire size calculator does this arithmetic against the printed Chapter 9 Table 8 circular mils.