NEC Table 250.66 Grounding Electrode Conductor Table Explained

Free online tool • 2026 NEC Compliant

NEC Table 250.66 sets the minimum size for the grounding electrode conductor (GEC) — the conductor that bonds your electrical service to the grounding electrode system (ground rods, water pipe, or concrete-encased electrode). The required GEC size scales with the size of your service-entrance conductors, not your panel's amperage rating directly. The table also caps GEC size for specific electrode types: rod, pipe, or plate electrodes never require larger than 6 AWG copper (250.66(A)), and concrete-encased electrodes never require larger than 4 AWG copper (250.66(B)), regardless of service size.

Select the rating of the circuit breaker or fuse protecting this branch circuit or feeder.

Minimum Required Size

NEC Table 250.122
12 AWG

copper · Equipment Grounding Conductor (EGC)

NEC Table 250.122 Reference Table (Copper)

OCD RatingMin Copper Size
15 A14 AWG
20 A12 AWG
30 A10 AWG
40 A10 AWG
60 A10 AWG
100 A8 AWG
200 A6 AWG
300 A4 AWG
400 A3 AWG
500 A2 AWG
600 A1 AWG
800 A1/0 AWG
1000 A2/0 AWG
1200 A3/0 AWG
1600 A4/0 AWG
2000 A250 kcmil

NEC Reference: The EGC runs with the branch circuit or feeder. For a 20 A overcurrent device, use minimum 12 AWG copper wire.

How to Use This Calculator

1

Identify service-entrance conductor size

Find the AWG or kcmil size of your incoming service conductors (copper or aluminum).

2

Look up the matching GEC size

Use NEC Table 250.66 to find the minimum required GEC size for that conductor size.

3

Apply electrode-type caps

If connecting only to rod/pipe/plate electrodes, cap at 6 AWG copper; for concrete-encased electrodes, cap at 4 AWG copper.

4

Select conductor material

Confirm whether you are sizing in copper or aluminum, since the table lists both.

Frequently Asked Questions

How do I read NEC Table 250.66?

Find your service-entrance conductor size (copper or aluminum) in the table's left columns, then read across to find the minimum required grounding electrode conductor (GEC) size. For example, 2 AWG through 1/0 AWG copper service conductors require a 6 AWG copper GEC; 250 kcmil through 500 kcmil copper service conductors require a 2/0 AWG copper GEC.

Why is my GEC smaller than expected for a large service?

Table 250.66 includes size caps for specific electrode types. A GEC connecting only to rod, pipe, or plate electrodes is never required to be larger than 6 AWG copper, even on very large services, per NEC 250.66(A). A GEC to a concrete-encased electrode is capped at 4 AWG copper per 250.66(B).

Does GEC size depend on panel amperage or wire size?

GEC size is based on the size of the service-entrance (or largest ungrounded) conductors, not the panel's amp rating directly. Two services rated the same amperage can require different GEC sizes if they use different conductor sizes to achieve that rating.

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Specific Sizing Guides & Calculators

Ground Wire Size for a 200 Amp Panel

What size ground wire for a 200 amp panel? GEC is typically 4 AWG copper per Table 250.66. Feeder EGC for 200A sub-panel is 6 AWG copper per Table 250.122.

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