Ground Wire (EGC) Size Calculator
Size the equipment grounding conductor from the breaker or fuse rating using NEC Table 250.122, copper or aluminum, with the 250.122(B) upsize for enlarged conductors. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Wire Size & Ampacity Calculator
Find the minimum copper or aluminum wire size for a load using NEC Table 310.16, with 310.15 ambient and conductor-count derating and the 110.14(C) terminal limit. Free, no sign-up.
Voltage Drop Calculator
Calculate voltage drop % and the minimum copper or aluminum wire size to stay within the NEC 3% / 5% recommendation, single- or three-phase. Free, no sign-up.
Conduit Fill Calculator
Size conduit per NEC Chapter 9. Add THHN, XHHW, or other conductors and get the minimum trade size and fill % for EMT, IMC, RMC, PVC, ENT, FMC, and LFMC. Free, no sign-up.
Explanation
The equipment grounding conductor (EGC) is the wire that bonds metal enclosures, raceways, and equipment back to the system ground so a fault clears the breaker instead of energizing the metal. Under NEC 250.122, its minimum size comes from one number: the rating of the overcurrent device protecting the circuit — not the load current and not the size of the phase conductors. This calculator reads Table 250.122 for copper and aluminum, then applies the 250.122(B) upsize rule when the conductors are enlarged.
How EGC sizing works
Start with the breaker or fuse rating and read the minimum EGC straight from Table 250.122 — a 100 A circuit takes 8 AWG copper or 6 AWG aluminum, a 200 A circuit takes 6 AWG copper or 4 AWG aluminum. Two adjustments can change that base value: the EGC is increased in proportion when the ungrounded conductors are upsized (250.122(B)), and it never has to be larger than those ungrounded conductors (250.122(A)).
| Term | Meaning |
|---|---|
| OCPD | Overcurrent device rating ahead of the equipment (A) |
| cmil | Conductor cross-sectional area in circular mils |
| 250.122(B) | Proportional EGC increase when conductors are upsized |
| 250.122(A) | EGC need not exceed the ungrounded conductor size |
To size the ungrounded (phase) conductors themselves, which sets the OCPD you start from, use the wire size & ampacity calculator.
NEC Table 250.122
The full table maps the overcurrent device rating — “not exceeding” the value shown — to the minimum copper and aluminum EGC. A 30 A or 40 A breaker, for instance, falls under the 60 A row and takes 10 AWG copper or 8 AWG aluminum.
Upsizing the EGC (250.122(B))
When you enlarge the ungrounded conductors beyond the minimum the load requires — most often to control voltage drop on a long run — the EGC must grow with them. NEC 250.122(B) requires the wire-type EGC to be increased in proportion to the circular-mil-area increase of the ungrounded conductors. Enter the required and installed conductor sizes above and the calculator scales the base EGC up to the next standard size that meets the proportioned area.
A few limits to keep in mind: the EGC never has to be larger than the ungrounded conductors (250.122(A)); conductors run in parallel are sized for the full circuit rating in each raceway (250.122(F)); and the EGC counts as one of the conductors when you check conduit fill. Always confirm the final design against the current NEC and the authority having jurisdiction.