Wire Ampacity Calculator (NEC 310.16)

Pick the AWG, metal and temperature column to read the NEC 310.16 base ampacity, then apply the ambient-temperature correction and the adjustment for the number of current-carrying conductors to get the real, derated ampacity you can use.

Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.

Conductor and overcurrent sizing must comply with the NEC edition adopted by your jurisdiction, including all temperature, bundling and termination-rating derating. Values reference NEC 310.16 / Chapter 9 (ed. 2020/2023) — confirm the current code with a licensed electrician.

Calculator

Base ampacity	50 A
Temperature factor	1.00 ×
Bundling factor	0.80 ×
Derated ampacity	40.0 A

Formula

Derated ampacity = base × temperature-factor × bundling-factor. The base comes from NEC Table 310.16 for the AWG, metal and temperature column. The temperature factor (NEC 310.15(B)(1)) corrects for ambient heat above 30°C. The bundling factor (NEC 310.15(C)(1)) reduces ampacity when more than three current-carrying conductors share a raceway: 4–6 conductors ×0.80, 7–9 ×0.70, 10–20 ×0.50.

Worked example

An 8 AWG copper conductor on the 75°C column has a base ampacity of 50 A. At an 86°F (30°C) ambient the temperature factor is 1.00, and with 6 current-carrying conductors in the raceway the bundling factor is 0.80. Derated ampacity = 50 × 1.00 × 0.80 = 40 A — so a conductor good for 50 A in free air is limited to 40 A in that bundle.

Ampacity is the current a conductor can carry continuously without its insulation exceeding the rated temperature, and the table value is only the starting point. Two adjustments pull the real number down: a hot ambient such as an attic or an engine bay, and bundling several current-carrying conductors in one raceway, which traps heat. This tool applies those derating factors so you see the ampacity that actually governs your install rather than the optimistic table figure. Remember that the column you may use is set by the weakest termination in the circuit — usually the 60 or 75 °C rating of the breaker or device — not by the wire's own 90 °C insulation.

Reference table: NEC 310.16 Ampacity Chart (60 / 75 / 90°C, Cu & Al)

Frequently asked questions

What is conductor ampacity?

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. The NEC Table 310.16 value is a starting point that must be adjusted for ambient temperature and for bundling with other current-carrying conductors.

When do I derate for the number of conductors?

When more than three current-carrying conductors run together in a raceway or cable. Grounding conductors and the grounded (neutral) conductor that carries only unbalanced current usually do not count. The factors are 0.80 for 4–6, 0.70 for 7–9 and 0.50 for 10–20.

Which temperature column should I use?

Use the column matching the lowest temperature rating in the circuit — usually 75°C for modern terminations (NEC 110.14(C)). You may correct and adjust from the 90°C column for THHN/THWN-2, but the final answer can never exceed the ampacity allowed by the terminations.

How does ambient temperature affect ampacity?

Higher ambient heat leaves less margin to carry current, so ampacity is reduced. Above 30°C (86°F) the correction factor drops below 1.0; below 30°C it rises above 1.0. This tool converts your °F input and applies the 75°C-column correction.

Source: NEC Table 310.16 plus the 310.15 temperature-correction and conductor-adjustment factors · All sources