Why is the ground sized from the breaker, not the load?

NEC Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device protecting the circuit. The EGC must carry fault current long enough for that device to trip, so its size scales with the protection rather than the normal operating load.

What size ground does a 100 A breaker need?

Per Table 250.122 a circuit protected at 100 amps requires a minimum 8 AWG copper or 6 AWG aluminum equipment grounding conductor. Larger overcurrent ratings step the EGC up accordingly through the table.

How do parallel conductors change the ground size?

NEC 250.122(F) requires that when phase conductors are run in parallel in separate raceways, a full-size equipment grounding conductor sized to the OCPD must be installed in each raceway. You do not divide the ground among the raceways.

Do I upsize the ground if I upsize the conductors?

Yes. NEC 250.122(B) requires that if you increase the phase-conductor size for voltage drop, the equipment grounding conductor must be increased proportionally by the same cross-sectional area ratio so its fault-clearing capacity stays matched.

Can the ground ever be smaller than required by the table?

No, the table gives the minimum, but the EGC also never needs to be larger than the circuit conductors themselves. Use whichever is smaller of the table value and the phase conductor size when the phase conductors are unusually small for the protection.

What is the Equipment Grounding Conductor Sizing (NEC 250.122)?

Looks up the minimum equipment grounding conductor size from the bundled NEC Table 250.122 given the overcurrent device rating for the circuit, for copper or aluminum, and applies the 250.122(F) rule to run a full-size EGC in each parallel raceway. For electricians ensuring code-compliant grounding. It runs free in your browser on Gera Tools, with nothing uploaded.

Equipment Grounding Conductor Sizing (NEC 250.122)

Name: Equipment Grounding Conductor Sizing (NEC 250.122)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The equipment grounding conductor is the safety path that carries fault current back to clear a breaker, and it is sized from the overcurrent device, not the load. This tool reads the minimum size straight from NEC Table 250.122 for copper or aluminum.

Why the EGC is sized from the OCPD, not the load

This surprises many students of the NEC. The answer lies in the purpose of the equipment grounding conductor: its job is not to carry normal operating current, but to carry fault current long enough and effectively enough to operate the overcurrent device. The higher the OCPD rating, the more fault current may flow before the device trips, and the larger the conductor must be to carry that fault current without overheating or fusing open before the breaker clears.

A circuit might run a 15 A load on conductors protected by a 60 A breaker, yet the EGC is sized for the 60 A device. If that circuit develops a ground fault, up to 60 A (and during the initial fault, potentially much more for a short time) must flow through the ground path. A conductor sized only for the 15 A load would not survive long enough for the 60 A breaker to trip.

How it works

The lookup keys on the overcurrent protective device (OCPD) rating and returns the minimum conductor for the chosen material:

find the first table row where OCPD ≤ row maximum
EGC = copper or aluminum size in that row
if parallel sets > 1 → run that full EGC in EVERY raceway (250.122(F))

Selected entries from Table 250.122 for copper:

OCPD rating (A)	Minimum copper EGC	Minimum aluminum EGC
15	14 AWG	12 AWG
30	10 AWG	8 AWG
60	10 AWG	8 AWG
100	8 AWG	6 AWG
200	6 AWG	4 AWG
400	3 AWG	1 AWG
800	1/0 AWG	3/0 AWG

Note that 15 A and 30 A both use 10 AWG copper — the table assigns minimum sizes by bracket, not a unique size per ampere rating.

Parallel conductors and the 250.122(F) rule

When a feeder is run as multiple parallel sets in separate raceways, it might seem logical to split the ground conductor proportionally — if the load needs 2/0 AWG and you have two raceways, run 2 AWG in each. That reasoning is wrong, and the NEC is explicit: each raceway must contain a full-size EGC rated for the OCPD protecting the feeder. If the OCPD requires 6 AWG copper, every conduit gets a 6 AWG copper EGC.

The proportional upsize rule (250.122(B))

If you increase the phase conductors above the minimum to reduce voltage drop, the EGC must be increased in the same proportion. The formula is:

new EGC area = (upsized phase area / minimum phase area) × Table 250.122 EGC area

Then select the next standard AWG at or above that calculated area. Skipping this step after a voltage-drop upsize is a common NEC compliance error.