This free wire ampacity calculator looks up the base allowable ampacity of copper and aluminum conductors from NEC Table 310.16, then applies the two adjustment factors the Code requires — ambient temperature correction and conduit-fill (more-than-three-conductors) derating — to give the final adjusted ampacity. Use it to sanity-check conductor sizing for branch circuits and feeders. Always verify against the current adopted NEC edition and a licensed engineer’s judgment.
Table 310.16 gives the allowable ampacity of insulated conductors rated 0–2000 V installed in raceway, cable, or earth, based on conductor size (AWG/kcmil), material (copper or aluminum), and the insulation temperature rating (60°C, 75°C, or 90°C column). For example, 12 AWG copper is 25 A in the 90°C column, but terminations commonly limit it to the 75°C value. The 90°C column is generally used only as the starting point for derating, not as the final usable ampacity.
Table 310.16 ampacities assume a 30°C (86°F) ambient. When the surrounding air is hotter, conductors cannot shed heat as easily, so ampacity is reduced by a correction factor from Table 310.15(B)(1). At higher ambients (e.g. rooftops, boiler rooms) the factor can drop the ampacity substantially. The calculator applies the factor for the conductor’s temperature column.
When more than three current-carrying conductors share a raceway or cable, mutual heating requires an adjustment: 4–6 conductors = 80%, 7–9 = 70%, 10–20 = 50%, and so on. The final adjusted ampacity is the base value multiplied by both the temperature correction and the fill adjustment. The result must then be protected by an overcurrent device per 240.4 and respect the 75°C termination limits of 110.14(C).
Ampacity is the maximum current, in amperes, a conductor can carry continuously without exceeding its insulation temperature rating. It depends on conductor size and material, insulation rating, ambient temperature, how many conductors are bundled together, and the installation method.
Table 310.16 values assume ideal conditions: a 30°C ambient and no more than three current-carrying conductors together. Real installations are often hotter or more crowded, which traps heat. The NEC requires you to multiply the base ampacity by a temperature correction factor and a conductor-count adjustment factor so the conductor never overheats.
Aluminum has higher resistance than copper, so for the same size it carries less current. As a rough guide, aluminum ampacity is about 78% of copper for the same AWG. Aluminum conductors are therefore typically upsized one or two AWG to match a copper circuit, and require terminations and antioxidant compound rated for aluminum.
Use the 90°C column only as the starting point for applying derating factors. The final ampacity used for overcurrent protection and termination sizing is generally limited to the 75°C column (or 60°C for some small conductors), because most equipment terminations are rated 75°C per NEC 110.14(C). Always check the listed temperature rating of your terminals.