Wire Size Calculator
Enter the current, the distance and the voltage to get the wire gauge (AWG) that carries the load safely and keeps the voltage drop low, for house wiring or low-voltage DC.
| AWG | Ampacity | Voltage drop | OK? |
|---|
Common household circuits
| Circuit | Copper wire | Longest run at full load and 3% drop |
|---|---|---|
| 15 A circuit (lights, outlets) | 14 AWG | about 40 ft |
| 20 A circuit (kitchen, bath, garage) | 12 AWG | about 45 ft |
| 30 A (water heater, dryer) | 10 AWG | about 100 ft at 240 V |
| 40 A (range, EV charger) | 8 AWG | about 115 ft at 240 V |
| 50 A (range, hot tub, EV) | 6 AWG | about 145 ft at 240 V |
Longer than that, go up one size. The breaker protects the wire, so the wire must always be rated for the breaker: never put a 20 A breaker on 14 AWG.
How the calculator works
- Ampacity: the wire must carry the current without overheating. Values are from NEC Table 310.16, 60 °C column, the one used for NM-B (Romex) cable in homes.
- Voltage drop: 2 × distance × amps × resistance per 1,000 ft ÷ 1,000. The 2 is for the trip out and back. Resistances are from NEC Chapter 9, Table 8.
- The result is the thinnest wire that passes both tests. Continuous loads are sized at 125% of the current.
This is a planning tool. Wiring in a home must follow the NEC and your local code, which can require larger wire for conduit fill, high temperatures or bundled cables. When in doubt, ask a licensed electrician.
Wire diameters, areas and mm² equivalents are in the wire gauge chart; for extension cords, see the extension cord gauge chart.
Frequently asked questions
What size wire do I need for 20 amps?
12 AWG copper for a standard 20 amp, 120 V household circuit. If the run is long, about 45 ft or more at full load, the voltage drop passes 3% and 10 AWG is the better choice. The calculator works out the exact distance for your load.
What size wire for 100 feet?
It depends on the current. At 120 V and a 3% drop limit: 10 A needs 10 AWG, 15 A needs 8 AWG and 20 A needs 8 AWG over 100 ft (one way). At 240 V the same current can often use a thinner wire, because the same drop in volts is only half as much in percent.
What is an acceptable voltage drop?
The National Electrical Code recommends no more than 3% on a branch circuit and 5% in total from the service to the farthest outlet. For 120 V that's 3.6 V on the branch circuit. Motors and electronics run better with less.
Why do I enter the one-way distance?
Current flows out on one wire and back on the other, so the wire the electricity travels through is twice the distance. The calculator doubles it for you; enter the distance from the panel or source to the load.
Can I use aluminum wire?
Aluminum has about 1.6 times the resistance of copper, so you need a size about two gauges larger: 10 AWG aluminum carries roughly what 12 AWG copper does. It's common for large feeders and service cables, but use connectors and devices rated for aluminum (marked AL or CO/ALR).