Equipment Grounding Conductor Sizing (250.122)

What the equipment grounding conductor does

The equipment grounding conductor (EGC) is the low-impedance fault-return path. It bonds the non-current-carrying metal parts of an installation — enclosures, raceways, equipment frames — back to the source so that a line-to-ground fault has a deliberate path to clear the overcurrent device quickly. Without an adequate EGC, a fault might leave metal energized instead of tripping the breaker. Sizing it correctly is squarely an Article 250 topic and one of the highest-volume areas on the journeyman exam.

Sizing the EGC from the overcurrent device

The minimum EGC size comes from Table 250.122, and the input is the rating of the overcurrent device ahead of the circuit — not the size of the circuit conductors. A 15-amp circuit needs a 14 AWG copper EGC, a 20-amp circuit needs 12 AWG, a 60-amp circuit needs 10 AWG, a 100-amp circuit needs 8 AWG, and a 200-amp circuit needs 6 AWG copper. Find the breaker or fuse rating, read across, and that is your minimum. The EGC is also never required to be larger than the circuit conductors it runs with, per 250.122(A).

The table value is the minimum only when the ungrounded conductors are no larger than the load and ampacity require. Wire a 20-amp circuit in the 12 AWG the device calls for and the 12 AWG copper EGC from the table stands, with nothing to upsize. But under 250.122(B), the instant those conductors are increased above that required minimum — for any reason, including simply running the heavier wire that happened to be on the truck — the wire-type EGC must be increased in the same proportion. Pull that 20-amp circuit in 10 AWG instead of 12 AWG and the phase conductors grow from about 6,530 to 10,380 circular mils, a factor of roughly 1.59; the 12 AWG EGC (6,530 circular mils) must grow by the same factor, to about 10,380 circular mils — a 10 AWG EGC. The convenience upsize is the textbook case that drags the EGC up right along with it.

The voltage-drop upsize rule

There is one major exception, and it is a classic exam trap. Under 250.122(B), when the ungrounded conductors are increased in size for any reason — most commonly to correct voltage drop on a long run — the equipment grounding conductor must be increased in proportion to the circular-mil area of the ungrounded conductors. In other words, if you upsize the phase conductors, you upsize the EGC by the same ratio.

Worked example: a 40-amp circuit normally takes a 10 AWG copper EGC. To fight voltage drop you increase the phase conductors from 8 AWG to 6 AWG. The circular-mil area grows from about 16,510 to 26,240, a factor of roughly 1.59. Apply that factor to the EGC: 10 AWG is about 10,380 circular mils, and 10,380 × 1.59 ≈ 16,500 circular mils — which rounds up to 8 AWG. The EGC must be upsized to 8 AWG to match the upsized phase conductors.

EGC types and parallel runs

A wire is only one option. Section 250.118 recognizes several wiring methods as equipment grounding conductors — among them rigid metal conduit, intermediate metal conduit, EMT, and the metal armor or sheath of certain cables — so the fault path is not always a green wire. When you do pull a wire-type EGC, it can be copper or aluminum and bare, covered, or insulated.

Parallel circuits are a frequent trap. Where conductors run in parallel in separate raceways, 250.122(F) requires a full-size equipment grounding conductor in each raceway, each sized from the circuit's overcurrent device — not the per-conductor share of the load. Pair that with the rule that an EGC is never required to exceed the circuit conductors, and you have covered the situations the journeyman exam tests most.