Wire Size / Ampacity Calculator — NEC Table 310.16 for Copper & Aluminum

This page is two tools in one: a wire size calculator that finds the smallest legal conductor for your load and conditions, and an ampacity calculator that reports the true current-carrying capacity after every NEC correction. It works as a wire gauge calculator for copper and aluminum, from #14 AWG branch circuits to 750 kcmil feeders, applying ambient correction, bundling derating, terminal-temperature caps, breaker limits, and a voltage-drop check in a single pass. Every result names the table and rule behind it, so the answer is traceable to the code — not a black box.

How wire sizing works under NEC Table 310.16

NEC Table 310.16 lists base ampacity by conductor size, material, and insulation temperature rating — the starting point, never the final answer. Derating always begins at the insulation column: THHN at 90°C starts higher than TW at 60°C, which is why insulation choice matters before any math. Next, ambient temperature correction per Table 310.15(B)(1) scales ampacity down in hot spaces and up in cool ones. Then 310.15(C)(1) derating applies when more than three current-carrying conductors share a raceway: 4–6 conductors ×0.80, 7–9 ×0.70, 10–20 ×0.50, 21–30 ×0.45, 31–40 ×0.40, and 41+ ×0.35. The derated result is then capped by the terminal-temperature column per 110.14(C) — usually 75°C — and the lower of the two governs. Small conductors face one more ceiling under 240.4(D): copper #14 → 15A, #12 → 20A, #10 → 30A, regardless of what the table allows. Continuous loads running 3+ hours must additionally be sized at 125% of load current per 210.19(A)(1). Finally, long runs get a voltage-drop check at 3% branch / 5% total, and the calculator reports the smallest size satisfying both ampacity and drop.

Supported conductors, insulation and sizes

Copper wire ampacity and aluminum wire ampacity both start from NEC Table 310.16, but aluminum needs one to two sizes larger for equal current. Supported insulation includes THHN / THWN-2 at 90°C, XHHW-2 at 90°C, THW / THWN at 75°C, and TW / UF at 60°C, across copper and aluminum from #14 AWG through 4/0 AWG and kcmil sizes to 750 kcmil. Parallel runs follow 310.10(H): only 1/0 AWG and larger may be paralleled, with identical length, material, size, insulation, and termination in each set. For a quick lookup before running the full calculation, see the NEC wire size chart for common loads and the Table 310.16 reference for raw values.

Derating calculator: temperature and bundling, worked

Use the conditions panel as a derating calculator whenever the installation is hotter or more crowded than the table basis of 30°C and three or fewer conductors. The classic attic example: #10 THHN copper starts at 40A in the 90°C column; at 104°F ambient the correction is ×0.91, and with six current-carrying conductors the 310.15(C)(1) derating factor is ×0.80 — so 40 × 0.91 × 0.80 = 29.1A before the 75°C terminal cap of 35A applies. Grounding conductors never count toward the bundling total, but neutrals sometimes do depending on harmonic and unbalanced load. Because 310.15(C)(1) derating compounds with temperature correction, a wire that looks generous on the wire ampacity chart can end up undersized in a packed, sun-baked raceway — which is exactly the mistake this tool prevents.

Terminal limits and breaker caps

The most common sizing error is stopping at the 90°C column. Per 110.14(C), the final ampacity cannot exceed the lowest-rated termination in the circuit — and most breakers, panels, and equipment lugs are rated 75°C (or 60°C for older gear). A THHN conductor derated from 90°C must still fit under its 75°C column value, and the lower governs. Then the breaker must protect the wire: standard ratings per 240.6 apply, but 240.4(D) caps small copper at 15A for #14, 20A for #12, and 30A for #10. That is why #14 copper with 20A of table ampacity still cannot serve a 20A load — the breaker ceiling, not the wire, decides.

Copper versus aluminum

Aluminum wire ampacity trails copper by roughly one to two AWG sizes: a 20A circuit needs #12 copper but #10 aluminum, and a 30A dryer needs #10 copper but #8 aluminum. Aluminum wins on cost for large feeders — service entrances, subpanels, and long runs — where the price-per-amp crossover typically lands around #2 AWG and up. The trade-offs are termination discipline: always use AL-rated lugs, antioxidant compound, and torque to spec. For side-by-side equivalents, drop parity, and cost break-even math, see copper vs aluminum equivalents.

When voltage drop governs the answer

Ampacity answers “will it overheat”; voltage drop answers “will the load see enough volts.” On long runs, drop governs: a 20A 120V load on #12 copper passes ampacity easily yet exceeds 3% drop past about 60 ft one-way, forcing #10 or #8 for performance even though the wire is thermally fine. This page runs both checks and reports which constraint drove the size. For dedicated distance analysis, use the voltage drop calculator and the ampacity vs voltage drop explainer.

Related wire sizing resources

For fast answers, use the NEC wire size chart and worked sizing examples. For raceway planning that triggers bundling derating, use the conduit fill calculator. For the full collection, browse all electrical calculators.