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HeizerDesk Wire Size Calculator
Enter the load, the distance and the voltage. This returns the smallest conductor that satisfies both the ampacity requirement and the voltage-drop target — which are different limits, and on a long run the drop is the one that decides. A free wire size calculator you can use on the job: nothing to install, nothing to register.
Your numbers
Result
Wire size (AWG)
10
AWG
- Size for ampacity alone
- 12AWG
- Size for voltage drop alone
- 10AWG
- Actual drop at this size
- 2.07%
- Volts dropped
- 4.97V
- Ampacity of chosen wire
- 30A
- Circular mils
- 10,380cmil
- Circular mils needed for drop
- 7,167cmil
How this is calculated
- Circular mils needed = (phase factor × K × amps × one-way length) ÷ (allowed drop % × volts).
- The chosen size is the first AWG that satisfies BOTH the ampacity table and the circular-mil requirement.
- Ampacity figures here are the 75°C copper column of NEC Table 310.16, and aluminium carries less for the same size.
Before you order
- Two different limits are at work. On a short run ampacity decides; past about 100 feet voltage drop usually decides, and it is the one people forget.
- Ampacity has to be derated for ambient temperature, for more than three current-carrying conductors in a raceway, and for the terminal rating of the equipment — often the 60°C column, not 75°C.
- Aluminium needs roughly two sizes larger than copper for the same job, plus antioxidant compound and terminals listed for it.
- This is a planning tool. The NEC and your inspector are the authority.