Voltkit US · NEC 2023
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NEC 2023NEC 250.122 · 250.66

Grounding Conductor Calculator

Size the equipment grounding conductor from the overcurrent device, including proportional upsizing when the phase conductors are increased, or size the grounding electrode conductor for a service.

Equipment ground · Cu EGC
12AWG
Aluminum
10 AWG
Table row
20 A

NEC Table 250.122 · NEC 2023

Values checked against the printed NEC 2023 tables, using the same engine as the Voltkit app.

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  1. Table 250.122 rowOCPD 20 A → "not exceeding" 20 A12 AWG Cu
  2. EGC, aluminumsame row10 AWG

NEC 250.122(A) & (B), Table 250.66

Conductor type
Phase conductors upsized? (250.122(B))

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Two different ground wires

Equipment grounding conductor (EGC)

Table 250.122 sizes the EGC by the rating of the overcurrent device ahead of the circuit, using the first row whose "not exceeding" value covers it. A 20 A circuit gets 12 AWG copper; a 60 A circuit 10 AWG; a 200 A feeder 6 AWG copper or 4 AWG aluminum.

Grounding electrode conductor (GEC)

Table 250.66 sizes the GEC by the largest ungrounded service-entrance conductor. A 200 A dwelling service on 4/0 aluminum (the size 310.12's 83% rule lands on) needs a 4 AWG copper GEC (2 AWG aluminum); on 2/0 copper, 4 AWG copper. Those are the figures for the conductor to a water pipe; the run to the rods can stop at 6 AWG copper (250.66(A)).

Worked example: upsizing the EGC (250.122(B))

A 100 A feeder needs 3 AWG copper for ampacity, but it's pulled in 1 AWG to control voltage drop. Table 250.122 gives an 8 AWG copper EGC for 100 A. The phase conductors grew from 52,620 to 83,690 circular mils, a ratio of 1.590. The EGC must grow by the same ratio, and the next standard size that's large enough is 4 AWG copper (4 AWG aluminum).

Not sure the phase conductors were upsized? Find the size ampacity alone requires with the Wire Size calculator; if you're installing something bigger, 250.122(B) applies.

Common mistakes

  • Sizing the EGC from the conductor size instead of the breaker size.
  • Using Table 250.66 for an equipment ground, or Table 250.122 for an electrode conductor.
  • Upsizing the hots for voltage drop and leaving the EGC at the table minimum.
  • Splitting one EGC between parallel raceways instead of a full-size EGC in each (250.122(F)).

This page cites the code but doesn't reproduce NEC text or tables. Use your copy of the NEC 2023 for the exact wording.

Worked examples on this page are computed by the same engine as the Voltkit app. The page cites NEC 2023 sections but doesn't reproduce NFPA text or tables; use your copy of the code for the exact wording.

Worked examples

Frequently asked questions

Do service-entrance conductors need an EGC?

No. Service conductors run two hots and the grounded (neutral) conductor, which does the fault-return job back to the utility transformer. EGCs, and Table 250.122, start on the load side of the service disconnect. On the service side, metal raceways and enclosures are bonded with supply-side bonding jumpers sized under 250.102(C), and the grounding electrode conductor comes from Table 250.66.

When do I have to upsize the ground wire?

When the ungrounded conductors are larger than the minimum size with enough ampacity, usually for voltage drop. 250.122(B) then makes a wire-type EGC grow by the same circular-mil ratio. Enter the size ampacity requires and the size you installed; the calculator applies the ratio and rounds up to the next standard size.

Each parallel set is in its own conduit. Does each need a full-size EGC?

Yes. Under 250.122(F), where conductors are run in parallel in multiple raceways, each raceway gets its own EGC sized from Table 250.122 by the overcurrent device protecting the whole circuit, not divided between the raceways.

Why can a GEC to ground rods be 6 AWG when the table says bigger?

Because 250.66(A) caps the part of the GEC that is the only connection to rod, pipe or plate electrodes at 6 AWG copper (4 AWG aluminum), whatever Table 250.66 says. A GEC that only reaches a concrete-encased electrode is capped at 4 AWG copper (250.66(B)). The full table size still applies to the conductor that runs to a water pipe electrode.

Does a service with parallel conductors change the GEC size?

Use the equivalent area: add the circular mils of the parallel conductors in one phase and look up that total as the largest service-entrance conductor in Table 250.66.

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