Find the minimum wire size for your load.
Enter your load current, conductor material and insulation temperature to get the smallest NEC Table 310.16 wire size that legally carries the load, with the 240.4(D) small-conductor rule applied.
Your circuit
Terminations are usually limited to the 75 C column even with 90 C wire.
What this means
Ampacity by wire size
| Wire size | Allowed ampacity | Carries your load? |
|---|
How wire sizing works
How to Size Wire with NEC Table 310.16
The minimum conductor size for a circuit is the smallest wire whose allowable ampacity meets or exceeds the load. This calculator looks up the ampacity from NEC Table 310.16 for the chosen material and insulation temperature column, then returns the smallest size that carries your load.
The 60, 75, and 90 degree columns reflect the insulation rating. Most equipment terminations are listed for 75 degrees, so even with 90 degree wire you often size to the 75 degree column.
The 240.4(D) Small-Conductor Rule
NEC 240.4(D) caps overcurrent protection for small conductors regardless of the ampacity table: 15 amps for 14 AWG copper, 20 amps for 12 AWG copper, and 30 amps for 10 AWG copper, with lower values for aluminum. This calculator applies those caps so it never suggests a size the code would not allow on a standard breaker.
Do Not Forget Derating
Table 310.16 values assume not more than three current-carrying conductors and a 30 degree ambient. More conductors in a raceway or a hotter location reduces the usable ampacity under NEC 310.15. Size up when derating applies.
Common questions
What wire size do I need for 50 amps?
For copper at the 75 degree column, 8 AWG carries 50 amps. For aluminum you would step up to 6 AWG. Always confirm the termination temperature rating and any derating.
Why does this cap 12 AWG copper at 20 amps?
NEC 240.4(D) limits 12 AWG copper to a 20 amp overcurrent device even though the 90 degree column shows 30 amps. The calculator applies that rule automatically.
Should I use the 60, 75, or 90 degree column?
Use the column that matches the lowest-rated termination or device in the circuit. Most breakers and lugs are rated 75 degrees, so the 75 degree column is the safe default.
Does this account for voltage drop?
No. This sizes for ampacity only. On long runs, check voltage drop separately and increase the wire size if the drop exceeds 3 percent on a branch circuit.
Copper or aluminum?
Copper is smaller for the same load and common on branch circuits. Aluminum is cheaper for large feeders and services but needs a larger size and listed terminations.
Estimates only. Sizes for ampacity under standard conditions. Apply ambient and bundling derating (NEC 310.15), check voltage drop on long runs, and confirm termination temperature ratings.