Time-saving trick for sizing equipment grounding conductors

The table you should have memorized

Equipment grounding conductors get sized from NEC Table 250.122, not from the ampacity tables. The input is the rating of the overcurrent device ahead of the circuit, not the size of the ungrounded conductors. Miss that distinction and you either oversize copper and burn margin, or undersize and fail inspection.

Commit the most common rows to memory and you stop pulling out the book on 90% of residential and light commercial work. For anything above 200A, verify in the book every time because the jumps get expensive fast.

• 15A OCPD: 14 AWG Cu / 12 AWG Al
• 20A OCPD: 12 AWG Cu / 10 AWG Al
• 30A, 40A, 60A OCPD: 10 AWG Cu / 8 AWG Al
• 100A OCPD: 8 AWG Cu / 6 AWG Al
• 200A OCPD: 6 AWG Cu / 4 AWG Al
• 400A OCPD: 3 AWG Cu / 1 AWG Al

The upsize rule nobody remembers until the AHJ calls it

When you increase ungrounded conductors for voltage drop, ampacity correction, or any other reason beyond the minimum required, 250.122(B) requires the EGC to be increased proportionally by area in circular mils. This is the single most common ground-up violation on long feeder runs.

Work the ratio by kcmil, not by AWG step. Take the kcmil of what you actually pulled, divide by the kcmil of what the load would have required, and multiply the minimum EGC kcmil by that factor. Round up to the next standard size.

Rule of thumb: if you upsized phase conductors by one trade size for voltage drop, your EGC almost always goes up one trade size too. Verify with the kcmil math on anything longer than 200 feet or when the inspector is sharp.

The time-saving trick: OCPD-first, then check three exceptions

Here is the workflow that saves the most minutes on a job. Stop thinking about load calcs when sizing EGCs. Look at the breaker, look up the row, done. Then run through a three-item mental checklist to catch the exceptions before you cut wire.

1. Did you upsize the phase conductors for voltage drop or any other reason? If yes, apply 250.122(B) proportionally.
2. Are you running parallel sets? 250.122(F) requires a full-size EGC in each raceway or cable, sized from the OCPD, not divided among the parallels.
3. Is this a feeder to a panel that feeds branch circuits? The EGC still sizes from the feeder OCPD per 250.122, but the branch circuit EGCs downstream size from their own breakers.

That is it. Three questions, under ten seconds, and you have covered the situations where Table 250.122 alone gets you in trouble.

Parallel runs where crews lose money

On a 600A feeder run in two parallel sets of 350 kcmil, the EGC in each raceway is 1 AWG copper, per the 600A row of 250.122. Not half of that. Not split between raceways. A full 1 AWG in each pipe. Crews undersize this constantly because the instinct is to divide like you divide the phase conductors.

Same logic for three or four parallel sets. Each raceway gets a full EGC sized from the OCPD. If the raceways are physically separated and one gets faulted, that single EGC has to carry the fault alone until the breaker opens, which is exactly why the code requires full size in each.

When the EGC is not required to be larger than the phase

250.122(A) has a line people forget: the EGC never has to be larger than the ungrounded conductors of the circuit. This matters on small branch circuits with oversized breakers for motor loads, or on any circuit where the table output would exceed the phase conductor size.

For a 12 AWG motor branch circuit protected at 40A for short-circuit protection per Article 430, the table says 10 AWG EGC. The exception lets you run 12 AWG because the phase conductor is 12 AWG. Saves copper, saves pulling time, fully code compliant.

Keep a note on your phone: if the EGC from the table comes out bigger than your ungrounded conductor, stop and check 250.122(A). On motor circuits this exception applies constantly.

Field verification in under a minute

Before you close up a panel or bury a raceway, run the 60-second check. This has caught more than one expensive mistake on jobs where the EGC was spec'd at design but upsized phase conductors slipped through a change order.

• Read the breaker rating feeding the circuit.
• Look up the EGC minimum in 250.122.
• Compare to the actual phase conductor size pulled. If phases are larger than the minimum required by load, do the kcmil ratio.
• On parallel feeders, confirm each raceway has a full EGC, not a divided one.
• Confirm the EGC is not larger than the phase conductor. If it is, drop it to match.

That sequence covers 250.122 in its entirety for the vast majority of installations. Laminate it, tape it to the inside of your panel schedule, or keep it in Ask BONBON. The goal is no book flip, no callback, no correction notice.