Equipment Grounding Conductor (EGC) Size Calculator | WireStandard

Enter the upstream overcurrent device (breaker or fuse) rating to find the minimum Equipment Grounding Conductor (EGC) size per NEC Table 250.122 — for both copper and aluminum. The EGC provides the low-impedance fault path that allows the OCPD to trip quickly on a ground fault. Required by code on virtually every branch circuit and feeder.

NEC 250.122 Lookup Table

NEC 250.122 is a lookup, not a formula. Key values: 15–60 A → 10 AWG Cu / 8 AWG Al; 100 A → 8 AWG Cu / 6 AWG Al; 200 A → 6 AWG Cu / 4 AWG Al; 400 A → 3 AWG Cu / 1 AWG Al; 1000 A → 2/0 AWG Cu / 4/0 AWG Al. When phase conductors are upsized for voltage drop, the EGC must be scaled proportionally per NEC 250.122(B): multiply EGC area by (new phase area / minimum required phase area).

Worked Example

200 A feeder protected by a 200 A breaker. NEC Table 250.122 → minimum 6 AWG copper EGC. Phase conductors are upsized from 3/0 AWG (167.8 mm²) to 250 kcmil for voltage drop. Per NEC 250.122(B), scale the EGC: 250 / 167.8 = 1.49×. 6 AWG copper = 26.7 mm² × 1.49 = 39.8 mm² → next standard size is 4 AWG. Reusing the bare-minimum 6 AWG with upsized phase conductors is a NEC 250.122(B) violation.

Frequently Asked Questions

Can the EGC be smaller than the neutral conductor?

Yes. The EGC is sized from NEC Table 250.122 based on the OCPD rating; the neutral is sized for the maximum unbalanced load current. They serve different functions and are sized by different rules, so the EGC is often smaller than the neutral.

Is a bare copper wire acceptable for an EGC?

Yes. NEC 250.119 permits the EGC to be bare, covered, or insulated. For sizes 4 AWG and larger, or when installed in a raceway, bare copper is permitted. If insulated, the EGC must be solid green or green with one or more yellow stripes.

When must the EGC be run in parallel?

When phase conductors are run in parallel per NEC 310.10(H), each conduit or raceway must contain its own EGC sized per the upstream OCPD. The EGC cannot be split across raceways — each path needs a complete, full-sized grounding conductor.