Calculator
Wire Size Calculator
ReadyApproximate ampacity and a conservative size for the load you enter.
Field aid only. This is an estimate, not the NEC, not a design, and not a permit. Verify it against the current code adopted in your area and the local AHJ before you build. Read the limits.
How it works
You enter a load, a metal, and a temperature column. The page walks the sizes from small to large and stops at the first one that clears two checks: the approximate column figure is at least the load, and, for 14, 12, and 10 AWG, the usual small-conductor overcurrent ceiling is also at least the load.
The column figures are orientation numbers for not more than three current-carrying conductors at 30°C ambient. They are here so you can screen a size in the field. They are not an official table, and they do not include adjustment or correction factors.
The small-conductor ceilings used here are the familiar ones: 15 A on 14 AWG copper, 20 A on 12 AWG copper, 30 A on 10 AWG copper, 15 A on 12 AWG aluminum, and 25 A on 10 AWG aluminum. The overcurrent rules have exceptions. This tool does not apply them.
Worked example
The form opens on 30 A, copper, 75°C. Twelve AWG copper at 75°C is listed here at 25 A, and the usual overcurrent ceiling for 12 AWG copper is 20 A. Neither covers 30 A. Ten AWG copper at 75°C is listed at 35 A, and the usual ceiling is 30 A. Both cover the load, so the page recommends 10 AWG.
10 AWG is the smallest copper size in this table that covers 30 A at 75°C (35 A). Its usual overcurrent ceiling is 30 A.
Limits
- No ambient-temperature correction and no adjustment for more than three current-carrying conductors.
- No continuous-load factor. A continuous load often needs extra room on both the wire and the breaker. Do that check yourself.
- Termination ratings can force you onto the 60°C or 75°C column even when the insulation is marked 90°C.
- 14 AWG aluminum is not in the table. Parallel conductors are not sized here.
- Voltage drop is a separate problem. A wire that carries the amps can still drop too many volts on a long run. Check voltage drop after you pick a size.
- Confirm every figure in the NEC edition adopted on the job before you buy wire.
FAQ
Why does 25 A of copper at 75°C skip 12 AWG?
The 75°C column figure for 12 AWG copper in this table is 25 A, so ampacity alone would stop there. The usual small-conductor overcurrent ceiling for 12 AWG copper is 20 A, so the page steps up to 10 AWG. Exceptions in the overcurrent article are not applied.
Can I use the 90°C column for the load?
Only when the terminations and the rest of the rules allow it. Many lugs are rated 75°C. A 90°C conductor is often still limited to the 75°C figure for the load. The page shows the column you pick. It does not decide whether you are allowed to use it.
Are these official NEC values?
No. They are approximate orientation figures for the common temperature columns. JobWireCalc is not NFPA and this is not a reproduction of the code book. Look up the adopted table before you build.
What about a hot attic or a stuffed raceway?
Both can cut the allowed current. This tool assumes 30°C and not more than three current-carrying conductors. If the job is hotter or more crowded, the size it recommends can be too small.