Wire Gauge Calculator
Find the correct AWG wire size for a given current load. Based on NEC ampacity tables for copper conductors.
How it works
This calculator uses NEC Table 310.15(B)(16) ampacity values for copper conductors in conduit. The insulation temperature rating determines the ampacity column used: 60°C (140°F), 75°C (167°F), or 90°C (194°F). Most receptacle and switch terminations are rated 75°C, so the 75°C column is used even if the wire insulation is rated 90°C.
Example
A 40A electric range circuit needs a wire size. Using the 75°C column, AWG 8 copper is rated for 50A — which covers the 40A load with adequate margin. For a continuous load (running more than 3 hours), multiply by 1.25: 40 × 1.25 = 50A minimum, still satisfied by AWG 8.
Frequently asked questions
Can I use aluminum wire instead of copper?
Yes, but aluminum requires a larger gauge for the same ampacity. For example, where AWG 12 copper handles 20A, you need AWG 10 aluminum. Aluminum is common for service entrances and large feeders but requires anti-oxidant compound and appropriate terminals.
What does AWG mean?
AWG stands for American Wire Gauge. The higher the number, the thinner the wire — AWG 14 is thinner than AWG 10. Above AWG 4/0, wire size is measured in kcmil (thousand circular mils).
What is a continuous load?
A continuous load is one that runs for 3 hours or more without interruption, such as commercial lighting or HVAC. The NEC requires the overcurrent device to be rated at 125% of the continuous load current.
Should I always use the next larger wire size?
Going up a size adds safety margin and reduces voltage drop on long runs. It also gives room to add loads to the circuit in the future without rewiring.
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