How it works
EGC: Table 250.122 by OCPD amps. 250.122(B) upsize: EGC_cmil × (installed phase cmil ÷ minimum phase cmil), rounded up. GEC: Table 250.66 by service conductor — read the copper or aluminum input column to match your service metal — capped at 6 AWG Cu (rod/pipe, 250.66(A)) or 4 AWG Cu (Ufer, 250.66(B)).
The EGC is a safety conductor that carries fault current back to the source so the breaker trips — so it is sized to the overcurrent device (Table 250.122), never to the load current. When you upsize the ungrounded (hot) conductors for any reason other than ampacity adjustment — most often voltage drop on a long run — NEC 250.122(B) requires the EGC to be increased in the same proportion of circular-mil area. We compute the ratio of the installed phase area to the minimum required phase area, multiply the base EGC area by it, and round up to the next standard size. For parallel sets in separate raceways, 250.122(F)(1) requires a full-sized EGC in each raceway — you cannot divide one conductor between them. The GEC is a different animal: it is sized from Table 250.66 by the largest service-entrance conductor, but the table has two input columns — one for copper service conductors and a separate one for aluminum/copper-clad-aluminum service conductors, with different size breakpoints — so an aluminum service must be read from the aluminum column (a 4/0 aluminum service lands in the same row as 2/0–3/0 copper). The code then caps the GEC where the electrode itself limits the fault path — 6 AWG copper to a ground rod or pipe (250.66(A)) and 4 AWG copper to a concrete-encased electrode (250.66(B)) — so a big service does not force an oversized rod conductor.
Code references
- Equipment grounding conductor sizing by OCPD NEC 2023, Table 250.122 — Minimum Size EGC for Grounding Raceway and Equipment
- Proportional EGC increase when phase conductors are upsized NEC 2023, 250.122(B)
- Full-sized EGC in each parallel raceway NEC 2023, 250.122(F)(1)
- Grounding electrode conductor sizing NEC 2023, Table 250.66 — Grounding Electrode Conductor for AC Systems
- GEC cap — ground rod or pipe electrode NEC 2023, 250.66(A) — sole connection to rod/pipe/plate, 6 AWG Cu max
- GEC cap — concrete-encased (Ufer) electrode NEC 2023, 250.66(B) — sole connection to concrete-encased electrode, 4 AWG Cu max
- Main and supply-side bonding jumper sizing NEC 2023, Table 250.102(C)(1)
- Types of equipment grounding conductors NEC 2023, 250.118
FAQ
What is the difference between a ground wire (EGC) and a grounding electrode conductor (GEC)?
They do two different jobs and are sized from two different tables. The equipment grounding conductor (EGC) runs inside the cable or raceway with every branch circuit and feeder; its job is to carry fault current back to the source so the breaker trips, so it is sized by the breaker (OCPD) rating from Table 250.122 — see also NEC 250.118 for the permitted EGC types (wire, the conduit itself, cable armor, etc.). The grounding electrode conductor (GEC) connects the service neutral/ground bus to the earth electrode (ground rod, Ufer, water pipe); it is sized from Table 250.66 by the largest service-entrance conductor. Using Table 250.66 to size a branch-circuit ground — or Table 250.122 to size the rod conductor — is the most common grounding mistake.
Do I need to upsize my ground wire for a long run?
Often, yes. If you increased the ungrounded (hot) conductors above the minimum required — for example, going from #6 to #4 AWG copper on a 60 A circuit to fight voltage drop over a 100 ft run — NEC 250.122(B) requires you to increase the EGC in the same proportion of circular-mil area. In that example the hots grew from 26,240 to 41,740 cmil (a ×1.59 ratio), so the base #10 EGC (10,380 cmil) must grow to at least 16,510 cmil, which rounds up to #8 AWG copper. The calculator shows this ratio and the adjusted size automatically. If you upsized only for ampacity/temperature adjustment under 310.15(C), the proportional increase does not apply.
Do parallel circuits need a separate ground wire in each conduit?
Yes. When you run conductors in parallel across multiple raceways, NEC 250.122(F)(1) requires a full-sized equipment grounding conductor — sized to the circuit OCPD from Table 250.122 — in each raceway. You cannot run one EGC and share it, and you cannot divide the required size between raceways. Set the "Parallel raceways" field above 1 and the calculator spells out the per-raceway requirement.
Why is the ground wire a different size than the hot wires?
Because they are sized for different purposes. The hot (and neutral) conductors are sized to carry the load current continuously without overheating, while the equipment grounding conductor only has to survive the brief, high fault current long enough to trip the breaker. Table 250.122 lets the EGC be smaller than the hots for most circuits — a 60 A circuit uses #6 copper hots but only a #10 copper EGC — which is perfectly legal and intentional.
Is a #6 ground rod conductor really enough for a 200 A service?
Yes, when the rod is the only electrode in that path. Table 250.66 would point a 200 A service (2/0 copper) at a #4 copper GEC, but NEC 250.66(A) caps the conductor to a ground rod or pipe electrode at #6 copper, because the rod itself limits how much current can flow into the earth. A larger GEC would be wasted copper. The same logic caps the conductor to a concrete-encased (Ufer) electrode at #4 copper under 250.66(B).
This calculator is provided for estimation purposes. Grounding and bonding (NEC Article 250) has many case-specific rules and local amendments. Always verify the result against the current NEC edition and your local authority having jurisdiction with a licensed electrician before installing.