Wire Size & Ampacity Calculator
Find the minimum copper or aluminum wire size for a load using NEC Table 310.16, with 310.15 ambient and conductor-count derating and the 110.14(C) terminal limit. Free, no sign-up.
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Explanation
Sizing a conductor by ampacity means finding the smallest wire that can carry the load current without overheating. Under the NEC, that takes two checks: the allowable ampacity from Table 310.16, and the terminal temperature limit of 110.14(C). Ambient heat and bundling more than three conductors reduce the usable ampacity, so the table value is rarely the final answer. This calculator applies both checks and the derating factors to return the minimum size.
The two checks and the 75 °C terminal rule
Most breakers and lugs are listed for 75 °C terminations, so per NEC 110.14(C) you size to the 75 °C column even when the conductor insulation is rated 90 °C. The 90 °C rating is still useful: it is the starting value for derating. A conductor must satisfy both of these:
| Symbol | Meaning |
|---|---|
| CF | Ambient temperature correction factor (Table 310.15(B)(1)) |
| AF | Conductor-count adjustment factor (Table 310.15(C)(1)) |
| 1.25 × | Continuous-load factor for the terminal check (210.19/215.2) |
For the base ampacity values themselves — copper and aluminum at 60, 75, and 90 °C — see the AWG wire size chart, which lists the full NEC Table 310.16 ampacities along with conductor area and diameter.
Derating for ambient heat and bundling
Table 310.16 assumes a 30 °C (86 °F) ambient and no more than three current-carrying conductors. Hotter surroundings (310.15(B)(1)) and more than three conductors in the same raceway or cable (310.15(C)(1)) both lower the ampacity, and the two factors multiply together. The conductor-count adjustment is shown below; the full ambient table is in the accordion.
| Current-carrying conductors | Adjustment (310.15(C)(1)) |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
Sizing the overcurrent device (240.4)
Once the conductor is sized, the breaker or fuse that protects it is governed by NEC 240.4. This calculator reports the maximum standard overcurrent device for the conductor you land on, using its usable ampacity:
| Rule | Effect on the breaker |
|---|---|
| 240.4(B) | If the ampacity is not a standard size, the next standard size up is allowed (≤ 800 A, not for multi-receptacle branch circuits) |
| 240.4(C) | Above 800 A, the device must not exceed the conductor ampacity (round down) |
| 240.4(D) | Small conductors are capped regardless of ampacity: 14/12/10 AWG copper at 15/20/30 A, 12/10 AWG aluminum at 15/25 A |
Because of 240.4(D), the breaker for a 12 or 10 AWG conductor is often smaller than its raw ampacity would suggest. Motor, tap, and a few other circuits follow separate rules (240.4(E)/(G)) and are outside this tool.
Notes and limitations
This tool sizes conductors by ampacity for general 600 V wiring under NEC 310.16 and the 310.15 derating factors. It does not check voltage drop, which often governs long runs and is a separate calculation, and it does not cover motor circuits (430.22), conductors in free air, or rooftop-conduit ambient adders. Continuous load is applied at 125% for the terminal check only. The breaker shown is the maximum standard device for the conductor under 240.4 — confirm it also suits the actual load and equipment. Always verify the final design against the current NEC and the authority having jurisdiction.