What size wire for 50 amps?

8 AWG copper with 75 °C terminations (50 A), or 6 AWG copper with 60 °C terminations (55 A). Aluminum: 6 AWG at 75 °C (50 A). Longer runs need a size up for voltage drop; the calculator shows when.

What size wire for 100 amps?

3 AWG copper at 75 °C (100 A) or 1 AWG aluminum at 75 °C (100 A). With 60 °C terminations, 1 AWG copper. For a dwelling service or main feeder 310.12 allows 4 AWG copper or 2 AWG aluminum at 83 %; that rule is not applied here.

What size wire for 200 amps?

3/0 copper at 75 °C (200 A) or 250 kcmil aluminum (205 A). 310.12 allows 2/0 copper or 4/0 aluminum for a dwelling service or main feeder rated 200 A; check the edition in force where you are.

Why does the calculator use the 60 °C column when my wire is THHN?

110.14(C)(1)(a): terminations on equipment rated 100 A or less are assumed to be 60 °C unless the equipment is listed and marked for 75 °C conductors. Most modern breakers and lugs are marked 60/75 °C, so choose 75 °C in that case. The 90 °C column is for derating only, never for the terminal.

Do I count the neutral and the ground?

The equipment grounding conductor is never counted. A neutral that carries only the unbalanced current of a multiwire circuit is not counted; a neutral on a two-wire circuit, or one carrying harmonic current from nonlinear loads, is (310.15(E)).

Wire size calculator

Minimum conductor size

Minimum size · copper8AWGFITS75 °C insulation on 75 °C terminations · needs 50 A at 30 °C ambient with 3 current-carrying conductors · NEC 2023 Table 310.16

Load current

The load, or the breaker rating you must feed

Conductor

Continuous load

Table 310.16 ampacity, 8 AWG at 75 °CNEC 2023 Table 310.16, identical to 2017 Table 310.15(B)(16): 50 A
After correction and adjustment× 1.00 for 30 °C (NEC 2023 Table 310.15(B)(1)(1)) × 1.00 for 3 conductors (NEC 2023 Table 310.15(C)(1)): 50.0 A
Termination cap, 110.14(C) at 75 °CCircuits > 100 A or larger than 1 AWG use the 75 °C column: 50 A
Usable ampacitythe smallest of the three above: 50.0 A

Ampacity from NEC 2023 Table 310.16 (identical to 2017 Table 310.15(B)(16)), corrected for ambient and conductor count, capped by the termination rating (110.14(C)) and the small-conductor limits of 240.4(D). Equipment listings, conductor markings and local amendments govern; a licensed electrician makes the final call.

Insulation rating

Termination rating

Ambient temperature

30 °C is the table base; attics and rooftops run hotter

Current-carrying conductors

In the same raceway or cable; 3 or fewer needs no adjustment

Length, one-way (optional)

Adds a voltage-drop check against the 3 % note

Voltage

For the drop check

Phase

Nothing typed leaves the browser; the inputs live only in the page address.

Type the load, pick the conductor material and the insulation and termination ratings, and the rim marks the smallest size whose corrected ampacity carries it. The tally shows every step: the Table 310.16 value, the temperature and conductor-count factors from 310.15, the termination cap from 110.14(C), and the 240.4(D) limit on small conductors. Give a length and the page also checks the 3 % voltage-drop note and upsizes if needed. This is the table arithmetic; equipment listings and local amendments still govern.

Worked examples

50 A circuit, THHN copper, 75 °C terminations: 8 AWG THHN is 55 A in the 90 °C column, but the 75 °C termination caps it at 50 A, which is exactly the load. Result: 8 AWG copper. With 60 °C terminations 8 AWG only carries 40 A and 6 AWG is needed. Open this run
100 A feeder in aluminum: 1 AWG aluminum at 75 °C is 100 A on the nose; 2 AWG is only 90 A. Copper needs 3 AWG (100 A). Add 150 ft and 240 V for the drop check. Open this run
200 A service, copper vs aluminum: 3/0 copper at 75 °C is 200 A. Aluminum needs 250 kcmil (205 A) by the plain table; 310.12 permits 4/0 aluminum for a dwelling service or main feeder at 83 % of the rating, which this calculator does not apply. Open this run
40 A continuous load, 45 °C attic, six conductors in the raceway: Continuous: 40 × 1.25 = 50 A required. 6 AWG THHN 75 A × 0.87 (45 °C at 90 °C) × 0.80 (4–6 conductors) = 52.2 A, under the 65 A cap. 8 AWG gives 55 × 0.87 × 0.8 = 38.3 A and fails. Result: 6 AWG. Open this run

How it is worked out

Required ampacity = load current, or 125 % of it for a continuous load (210.19(A)(1), 215.2(A)(1)).

For each size, corrected ampacity = Table 310.16 value in the insulation’s column × the ambient factor from Table 310.15(B)(1)(1) × the adjustment from Table 310.15(C)(1) when more than three current-carrying conductors share the raceway or cable.

The usable ampacity is the smaller of the corrected value and the termination column’s uncorrected value (110.14(C): 60 °C for circuits up to 100 A or 14–1 AWG unless the equipment is marked 75 °C; 75 °C above that), and, for 14, 12 and 10 AWG, no more than the 240.4(D) limit.

The answer is the first size whose usable ampacity is at least the required ampacity. If a length is given, the K-factor drop is computed for that size and the smallest size within 3 % is substituted when the drop exceeds the note.

The figures behind it

NEC Table 310.16 (2023) = Table 310.15(B)(16) (2017): Allowable ampacities at 60, 75 and 90 °C for copper and aluminum, 18 AWG to 2000 kcmil, 30 °C ambient, not more than three current-carrying conductors. Read from a manufacturer PDF headed NEC 2023 and cross-checked against the NFPA-typeset 2017 table: all 180 cells identical. Checked 2026-09-05. Source
Tables 310.15(B)(1)(1) and 310.15(C)(1): Ambient temperature correction (30 °C base) and adjustment for more than three current-carrying conductors (80 / 70 / 50 / 45 / 40 / 35 %). 2023 text, cross-checked with 2017 and 2011 reproductions. Source
240.4(D) and 110.14(C): Small-conductor overcurrent limits (14 AWG Cu 15 A, 12 AWG Cu 20 A, 10 AWG Cu 30 A; 12 AWG Al 15 A, 10 AWG Al 25 A) and the termination temperature rule (≤ 100 A or 14–1 AWG: 60 °C column unless equipment is marked 75 °C; larger: 75 °C). 2017 text verbatim; the copper limits are also confirmed by the 2023 table’s asterisk note. The aluminum limits are confirmed only by a manufacturer summary of unstated edition. Source
K-factor 12.9 (copper) and 21.2 (aluminum): Ohm·cmil/ft at 75 °C, the Table 8 resistance per foot multiplied by the area; VD = 2·K·I·L/CM single-phase, √3·K·I·L/CM three-phase. IAEI Magazine, “Voltage Drop Formulas”, March 2017. Source

Questions people ask

What size wire for 50 amps?: 8 AWG copper with 75 °C terminations (50 A), or 6 AWG copper with 60 °C terminations (55 A). Aluminum: 6 AWG at 75 °C (50 A). Longer runs need a size up for voltage drop; the calculator shows when.
What size wire for 100 amps?: 3 AWG copper at 75 °C (100 A) or 1 AWG aluminum at 75 °C (100 A). With 60 °C terminations, 1 AWG copper. For a dwelling service or main feeder 310.12 allows 4 AWG copper or 2 AWG aluminum at 83 %; that rule is not applied here.
What size wire for 200 amps?: 3/0 copper at 75 °C (200 A) or 250 kcmil aluminum (205 A). 310.12 allows 2/0 copper or 4/0 aluminum for a dwelling service or main feeder rated 200 A; check the edition in force where you are.
Why does the calculator use the 60 °C column when my wire is THHN?: 110.14(C)(1)(a): terminations on equipment rated 100 A or less are assumed to be 60 °C unless the equipment is listed and marked for 75 °C conductors. Most modern breakers and lugs are marked 60/75 °C, so choose 75 °C in that case. The 90 °C column is for derating only, never for the terminal.
Do I count the neutral and the ground?: The equipment grounding conductor is never counted. A neutral that carries only the unbalanced current of a multiwire circuit is not counted; a neutral on a two-wire circuit, or one carrying harmonic current from nonlinear loads, is (310.15(E)).

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