Calculator 12 / NEC 250.122 · 250.66

Grounding & bonding

Two table lookups on one page: size the equipment grounding conductor (EGC) from the overcurrent device rating, and the grounding electrode conductor (GEC) from the largest ungrounded service or feeder conductor.

(a) Equipment grounding conductor — Table 250.122

Overcurrent device rating (A)

The EGC is sized from the rating of the overcurrent device protecting the circuit (NEC 250.122), not from the load current.

Copper EGC

8 AWG

For a 100 A overcurrent device, the minimum copper equipment grounding conductor is 8 AWG.

NEC Table 250.122

Aluminum EGC

6 AWG

For a 100 A overcurrent device, the minimum aluminum/copper-clad aluminum EGC is 6 AWG.

NEC Table 250.122

250.122(B): if the circuit's ungrounded conductors are increased in size for voltage drop, the EGC must be increased proportionally — EGC′ = Table 250.122 size × (CM of installed ungrounded conductor ÷ CM of the minimum-size ungrounded conductor).

How this was calculated: read across NEC Table 250.122 at the 100 A row — copper column gives 8 AWG, aluminum/copper-clad column gives 6 AWG. Where the device rating falls between table rows, use the next higher row.

(b) Grounding electrode conductor — Table 250.66

Largest ungrounded service / feeder conductor

For parallel conductors, use the equivalent total circular-mil area per phase (sum the sizes, then use the row covering that equivalent size).

Copper GEC

2 AWG

For a largest ungrounded conductor of 250 kcmil (4/0 AWG or 250 kcmil), the minimum copper grounding electrode conductor is 2 AWG.

NEC Table 250.66

Aluminum GEC

1 AWG

For a largest ungrounded conductor of 250 kcmil (4/0 AWG or 250 kcmil), the minimum aluminum/copper-clad aluminum GEC is 1 AWG.

NEC Table 250.66

How this was calculated: 250 kcmil falls in the "4/0 AWG or 250 kcmil" row of NEC Table 250.66 — copper column 2 AWG, aluminum column 1 AWG.

250.66(A): the GEC to a single rod, pipe, or plate electrode need not be larger than 6 AWG copper or 4 AWG aluminum. This exception does not apply to concrete-encased, ground-ring, or other electrodes.

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Inputs

EGC — overcurrent device rating	100 A
EGC — copper / aluminum	8 AWG / 6 AWG
GEC — largest ungrounded conductor	250 kcmil
GEC — copper / aluminum	2 AWG / 1 AWG

Results

Equipment grounding conductor

8 AWG Cu · 6 AWG Al

OCPD rating 100 A → Table 250.122 row: copper 8 AWG, aluminum/copper-clad 6 AWG. Increase proportionally if ungrounded conductors are upsized for voltage drop (250.122(B)).

NEC Table 250.122 · 250.122(B)

Grounding electrode conductor

2 AWG Cu · 1 AWG Al

Largest ungrounded conductor 250 kcmil (4/0 AWG or 250 kcmil) → Table 250.66 row: copper 2 AWG, aluminum 1 AWG. Rod/plate electrode: 250.66(A) caps the GEC at 6 AWG Cu / 4 AWG Al.

NEC Table 250.66 · 250.66(A)

Engineering disclaimer: This design aid is not a substitute for engineering judgment. Confirm final design with a licensed professional engineer and the authority having jurisdiction.

Equipment grounding conductor (EGC)

8 AWG Cu · 6 AWG Al

OCPD rating 100 A → Table 250.122: copper 8 AWG, aluminum/copper-clad 6 AWG. If the ungrounded conductors are upsized for voltage drop, increase the EGC proportionally per 250.122(B).

NEC 250.122 · Table 250.122

Grounding electrode conductor (GEC)

2 AWG Cu · 1 AWG Al

Largest ungrounded conductor 250 kcmil falls in the 4/0 AWG or 250 kcmil row → copper 2 AWG, aluminum/copper-clad 1 AWG.

NEC 250.66 · Table 250.66

How this was calculated

EGC: select the overcurrent device rating — 100 A — and read the matching row of NEC Table 250.122. Minimum copper 8 AWG; minimum aluminum/copper-clad 6 AWG.
250.122(B): when ungrounded conductors are increased for voltage drop, the EGC must be increased in proportion to the circular-mil ratio of the installed vs minimum conductors.
GEC: identify the largest ungrounded service-entrance or feeder conductor — 250 kcmil — and read the matching row of NEC Table 250.66 (4/0 AWG or 250 kcmil). Minimum copper 2 AWG; minimum aluminum/copper-clad 1 AWG.
250.66(A): a GEC run to a single rod, pipe, or plate electrode is not required to be larger than 6 AWG copper / 4 AWG aluminum.

Engineering disclaimer: These lookups size the minimum conductor per the tables. They do not cover every grounding rule — bonding jumpers (250.102), separate EGCs for multiple circuits, service requirements (Article 230), and equipment-specific rules (250.114–250.122 exceptions) still apply. Confirm the installation with the authority having jurisdiction. Rule data is transcribed from NEC Tables 250.122 and 250.66 for reference — always verify against the current licensed copy of the code.