A wire has to pass two separate tests. It must carry the current without overheating, which is its ampacity in the National Electrical Code table, and it must not lose too much voltage on a long run. On short runs the current sets the size; on long runs voltage drop often calls for a thicker wire.
How to use this calculator
- Enter the load current in amps. If you only know the watts, divide them by the voltage.
- Tap the circuit voltage and enter the one-way distance from the panel to the load.
- Pick copper or aluminum. For copper, choose NM-B cable, the usual cable inside walls, or single THHN wires in conduit: the code rates them from different columns of its table.
- If the load runs at full current for 3 hours or more at a time, such as a car charger or a space heater, mark it as continuous. The wire must then carry 125 percent of the current.
Worked examples
A workshop circuit
Load current: 16 A · Circuit voltage: 120 V · One-way distance: 50 ft
- Wire size
- 12 AWG copper
- Current the wire must carry
- 16.0 A
- Rated for
- 20 A
- Voltage drop
- 2.64%
- Longest run within 3%
- 57 ft
A subpanel in a detached garage
Load current: 40 A · Circuit voltage: 240 V · One-way distance: 120 ft · Wire type: THHN/THWN wires in conduit (75 °C)
- Wire size
- 6 AWG copper
- Current the wire must carry
- 40.0 A
- Rated for
- 65 A
- Voltage drop
- 1.96%
- Longest run within 3%
- 183 ft
A car charger that runs for hours
Load current: 32 A · Circuit voltage: 240 V · One-way distance: 60 ft · Wire type: THHN/THWN wires in conduit (75 °C) · Runs for 3 hours or more at a time: Yes, a continuous load
- Wire size
- 8 AWG copper
- Current the wire must carry
- 40.0 A
- Rated for
- 50 A
- Voltage drop
- 1.24%
- Longest run within 3%
- 145 ft
A long aluminum feeder
Load current: 80 A · Circuit voltage: 240 V · One-way distance: 150 ft · Conductor: Aluminum
- Wire size
- 1 AWG aluminum
- Current the wire must carry
- 80.0 A
- Rated for
- 100 A
- Voltage drop
- 2.53%
- Longest run within 3%
- 178 ft
How the wire size is chosen
First comes ampacity: the current a wire may carry without overheating. NM-B cable is rated by the lowest temperature column of the code table, and single wires in conduit by the next one up, as long as the breaker and the device they connect to are rated for it. The thinnest sizes have one more limit: the code caps the breaker on 14 AWG copper at 15 amps, on 12 AWG at 20 amps and on 10 AWG at 30 amps, whatever the table says.
Then comes voltage drop. A branch circuit is commonly kept within 3 percent drop, so motors start and lights stay bright at the far end. The calculator works out the drop for each size over your run and keeps the larger of the two answers.
When to go up a size
- The run is long: the table below the result shows how much each size drops, so you can see where it crosses the limit.
- Cables run through a hot attic, or more than three current-carrying wires share a conduit or bundle. The code then lowers the ampacity, which this calculator does not.
- The breaker or the device is rated only for the lowest temperature column. Many are rated for the higher one, but check the label.
- You plan to add load later. A size up costs little next to opening the walls again.
Common questions
What size wire do I need for a twenty-amp circuit?
In copper NM-B cable, 12 AWG: the code allows a breaker of 20 amps on it. On a long run, voltage drop can call for 10 AWG instead, so enter your distance in the calculator.
What size wire do I need for a fifty-amp circuit?
It depends on the cable and the run. Single copper wires in conduit at the higher temperature column usually come out at 8 AWG, and NM-B cable at 6 AWG. Enter your current and distance to check voltage drop too.
Can I put 14 AWG wire on a twenty-amp breaker?
No. The code limits 14 AWG copper to a breaker of 15 amps, so a twenty-amp circuit needs 12 AWG or larger, even on a short run.
Why is NM cable rated for less current than THHN wire?
NM-B cable bundles its wires in a jacket, often inside insulation, so the code rates it by its lowest temperature column. Single THHN wires in conduit may use the next column when the breaker and devices are rated for it, which allows more current for the same size.
What is a continuous load?
One that runs at its full current for 3 hours or more, such as a car charger, a water heater or a space heater. The wire and breaker for it must be sized for 125 percent of that current.