Equipment Grounding Conductor Calculator
Choose the breaker or fuse rating to get the minimum equipment grounding conductor for copper or aluminum.
Step-by-step
- Enter valid values to begin.
Table 250.122 lists minimum EGC sizes by overcurrent device rating. When conductors are upsized, 250.122(B) says the EGC grows in the same proportion of circular mils. Circular-mil values: 14 AWG 4,110 • 12 AWG 6,530 • 10 AWG 10,380 • 8 AWG 16,510 • 6 AWG 26,240 • 4 AWG 41,740.
Equipment Grounding Conductor: The Wire That Makes the Breaker Trip
Core Engineering Principles
When a hot wire touches a metal enclosure, the enclosure becomes live. Unless there is a low-impedance path back to the source, the breaker never sees enough current to trip, and the next person to touch the box becomes the path. The equipment grounding conductor is that path. It bonds every enclosure and frame to the system neutral at the source. A ground fault then becomes a short circuit with hundreds or thousands of amps and the breaker clears it in a fraction of a second. That’s why the EGC is sized from the breaker or fuse protecting the circuit, not from the load.
It also has to survive. During those few cycles before the breaker clears, the wire carries a very high current and can melt if it’s too small. Table 250.122 gives the minimum size for each overcurrent device rating. Bigger breaker, bigger EGC.
Upsized conductors: AEGC = Atable × (CMinstalled / CMoriginal) • Never larger than the circuit conductors
NEC & Standard References
Table 250.122 reads “not exceeding” each rating: 15 A takes 14 AWG Cu, 20 A takes 12 AWG, 60 A takes 10 AWG, 100 A takes 8 AWG, 200 A takes 6 AWG, 400 A takes 3 AWG, and 800 A takes 1/0 AWG. 250.122(B) says if you upsize ungrounded conductors for any reason (voltage drop is the usual one) the EGC has to be increased in the same proportion of circular mils. 250.122(A) says you never need an EGC bigger than the circuit conductors. 250.122(F) requires a full-size EGC in each raceway when you parallel conductors, and 250.118 lists which raceways and cable armor are acceptable as the path. An EGC may be bare, covered or insulated, and insulated ones must be green or green with yellow stripes.1. Table 250.122: 20 A → 12 AWG Cu (6,530 CM).
2. Upsize factor = 16,510 / 6,530 = 2.53.
3. EGC area = 6,530 × 2.53 = 16,510 CM → 8 AWG Cu.
4. 250.122(A) says it never has to be larger than the circuit conductors (8 AWG), so 8 AWG is the final answer.
- Don’t forget 250.122(B). It’s the one inspectors catch on long runs where wire was bumped up two sizes.
- Metal conduit is not always a ground. Flexible conduit longer than six feet needs a wire, and flex on vibrating equipment is best always run with a wire.
- Same raceway. Run the EGC with the circuit conductors, not separately, or the impedance goes up and the breaker might not trip.
- Isolated grounds are the exception. An insulated ground for sensitive loads still must connect back to the source and the enclosure per 250.146(D).
- Never use the neutral as a ground downstream of the service. That puts current on the metal.