Wire Gauge Ampacity Calculator

Methodology

A size qualifies when its 75 °C copper ampacity (14 AWG 20 A, 12 AWG 25 A, 10 AWG 35 A, 8 AWG 50 A, up to 4/0 AWG 230 A) and, for 14 to 10 AWG, its 240.4(D) limit (15, 20, 30 A) are both at least the load current. The smallest qualifying size is shown. Voltage drop = 2 × 12.9 × L × I / CM (single-phase) or √3 × 12.9 × L × I / CM (three-phase), as a percent of the system voltage.

Assumptions

• Copper conductors, at most three current-carrying conductors and an ambient temperature of 30 °C or below.
• All insulation types use the 75 °C ampacity column, so the insulation is not asked for.

Frequently asked questions

What size wire does this tool pick for 15, 20 and 30 amps?

14 AWG copper for loads up to 15 A, 12 AWG for 16 to 20 A and 10 AWG for 21 to 30 A, because the 240.4(D) limits are 15, 20 and 30 A. Above 30 A the 75 °C ampacity decides, and above 230 A no covered size qualifies and the page shows "No covered size". What is permitted on a real circuit is decided by the adopted NEC edition and your inspector.

Why does the page show 25 A for 12 AWG but a 20 A limit?

25 A is the 75 °C ampacity from Table 310.16; 20 A is the 240.4(D) limit on overcurrent protection for 12 AWG copper. The page requires both to reach the load, and for 14, 12 and 10 AWG the limit is lower, so it sets the size. The limits equal the 60 °C column, which is the column that applies to terminations on equipment rated 100 A or less unless the equipment is marked for 75 °C.

How is voltage drop calculated?

VD = 2 × K × L × I / CM for single-phase, with √3 in place of 2 for three-phase. K is 12.9 for copper, L the one-way length in ft, I the load in amps and CM the conductor area. On a 208 V three-phase circuit, 20 A over 50 ft on 12 AWG is √3 × 12.9 × 50 × 20 / 6530 = 3.42 V, or 1.6%, about half the 3.3% the same run loses at 120 V single-phase. The NEC’s 3% for a branch circuit is a recommendation in an informational note, not a requirement, and the limit here is informational.

What size wire for 40 amps?

8 AWG copper, whose 75 °C ampacity is 50 A; 10 AWG is out because its 240.4(D) limit is 30 A. Over 100 ft at 120 V that 8 AWG run drops 6.25 V, 5.2%, so with a 3% limit the page names 4 AWG (2.47 V, 2.1%). On 240 V over 50 ft the drop is 3.13 V, 1.3%. The ampacity assumes terminations marked for 75 °C; check the breaker and terminations with your inspector.

Does it correct for hot ambient temperatures or bundled conductors?

No. It refuses an ambient temperature above 30 °C with a message and assumes no more than three current-carrying conductors, so no correction factor is applied. At 30 °C or below the ambient correction is 1.00.