Wire Sizing Calculator
NEC 310.15 compliant. Copper conductors at 75°C. Sizes for ampacity and voltage drop.
Wire Sizing
How wire sizing works
Wire must be sized to handle the load without overheating (ampacity) and without excessive voltage loss (voltage drop). This calculator checks both and selects the larger gauge if needed.
NEC 125% continuous load rule
Per NEC 210.20 and 215.3, conductors feeding continuous loads (on for 3+ hours) must be sized at 125% of the load. This calculator applies that automatically.
Ampacity table used (NEC 310.15)
- 14 AWG15A
- 12 AWG20A
- 10 AWG30A
- 8 AWG50A
- 6 AWG65A
- 4 AWG85A
- 2 AWG115A
- 1/0 AWG150A
Copper, 75°C, in conduit. Derate for temperature or conduit fill per NEC 310.15(B).
When to upsize for voltage drop
On long runs, the minimum ampacity gauge may cause excessive voltage drop. This calculator automatically upgrades the gauge if the drop exceeds 3%.
Save calculations with your jobs in CircuitDesk
CircuitDesk lets you run wire sizing, voltage drop, conduit fill, and more — directly inside each job. Quote and invoice from the same app.