Sizing Grounding Conductors: Table 250.66 vs Table 250.122
Two tables, two different jobs, and exam writers love to hand you the wrong one. Get clear on which conductor you're sizing, and you'll get the right answer.
Values below are copper, from Tables 250.66 and 250.122 as they've read for the last few editions. Check the edition your exam uses, and the aluminum columns if the question calls for aluminum.
Table 250.66: the grounding electrode conductor (GEC)
The GEC runs from the service or a separately derived system to the grounding electrode. You size it by the largest ungrounded service-entrance conductor (or the equivalent area for parallel sets).
| Largest copper service conductor | Copper GEC |
|---|---|
| 2 AWG or smaller | 8 AWG |
| 1 or 1/0 | 6 AWG |
| 2/0 or 3/0 | 4 AWG |
| Over 3/0 through 350 kcmil | 2 AWG |
| Over 350 through 600 kcmil | 1/0 |
| Over 600 through 1100 kcmil | 2/0 |
| Over 1100 kcmil | 3/0 |
The caps in 250.66(A) and (B) cover a lot of exam questions. The part of a GEC that's the sole connection to a rod, pipe or plate electrode doesn't have to be larger than 6 AWG copper. The part that's the sole connection to a concrete-encased electrode doesn't have to be larger than 4 AWG copper.
Table 250.122: the equipment grounding conductor (EGC)
The EGC runs with the circuit and clears faults. You size it by the rating of the overcurrent device ahead of the circuit, not by the conductor size.
| OCPD rating (A) | Copper EGC |
|---|---|
| 15 | 14 AWG |
| 20 | 12 AWG |
| 60 | 10 AWG |
| 100 | 8 AWG |
| 200 | 6 AWG |
| 300 | 4 AWG |
| 400 | 3 AWG |
| 500 | 2 AWG |
| 600 | 1 AWG |
| 800 | 1/0 |
| 1000 | 2/0 |
If you upsize the ungrounded conductors (for voltage drop, for example), 250.122(B) requires you to upsize the EGC in proportion, by circular mil area.
Quick way to tell them apart
- Question mentions the service, the electrode, a ground rod or a ufer: 250.66, sized by service conductor.
- Question mentions a breaker, a fuse, a feeder or branch circuit: 250.122, sized by OCPD.
- Main bonding jumpers and supply-side bonding jumpers have their own table, Table 250.102(C)(1). Don't use 250.66 for those.
Worked examples
A 200 A service with 3/0 copper service conductors gets a 4 AWG copper GEC from Table 250.66. If that GEC only goes to a pair of ground rods, 6 AWG copper is all that's required.
A 60 A feeder gets a 10 AWG copper EGC from Table 250.122, whatever size the feeder conductors are, unless they were upsized.
Related: conductor ampacity and derating and voltage drop calculations.
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More guides: How to navigate the NEC · 2026 NEC changes for exam takers · Massachusetts exam: 527 CMR and the NEC · How to tab and mark your NEC · 30-day journeyman study plan · Box fill calculations (314.16) · Conductor ampacity and derating · Grounding conductor sizing (250.66 vs 250.122) · Voltage drop calculations