How to use the Wire Size Calculator
- 1
Enter the load in amps. If it runs for three hours or more (a continuous load), choose yes so it's sized at 125%.
- 2
Enter the voltage, system type and one-way length, and your maximum voltage drop.
- 3
Choose copper or aluminum and the terminal temperature rating. The recommended size is the larger of the ampacity size and the voltage-drop size.
Two checks decide the wire size
Ampacity keeps the wire from overheating. NEC Table 310.16 lists how many amps each size can carry, and the terminal rating decides which column you can use: 60°C for most equipment rated 100 A or less unless it's marked 75°C.
Voltage drop keeps the equipment working well. On a long run the ampacity size is often too small, which is why long circuits regularly need a wire one or two sizes larger than the ampacity chart says.
Common wire sizes by amps (copper, 75°C)
These are the usual sizes for short runs. Long runs and continuous loads often need the next size up.
- 15 A: 14 AWG
- 20 A: 12 AWG
- 30 A: 10 AWG
- 40 A: 8 AWG
- 50 A: 8 AWG (6 AWG on 60°C terminals)
- 60 A: 6 AWG
- 100 A: 3 AWG
- 200 A: 3/0 AWG
Frequently asked questions
What size wire do I need for 50 amps?
8 AWG copper on 75°C terminals or 6 AWG on 60°C terminals for a short run. For long runs, check voltage drop: 100 feet at 240 V usually calls for 6 AWG.
What size wire for a 30 amp circuit?
10 AWG copper. NEC 240.4(D) limits 10 AWG copper to a 30 A breaker.
Can I use aluminum wire?
Aluminum is common for feeders and service entrances. It needs a larger size than copper and terminals rated for aluminum (marked AL or AL/CU).
What is a continuous load?
A load expected to run at its maximum current for three hours or more, like EV chargers, water heaters and commercial lighting. The circuit is sized at 125% of that load.
Does this include derating?
No. It assumes a 30°C ambient and no more than three current-carrying conductors together. Hot attics, rooftops and bundled cables need derating. Have a licensed electrician confirm the final size.