How to Choose the Right Wire Size

1. Work out the load

Find the current the circuit will carry. If you know the power, divide by the voltage: a 1,800 W heater at 120 V draws 15 A. For continuous loads that run for 3 hours or more, the NEC sizes the circuit at 125% of the load.

2. Check the current rating

Choose a wire whose ampacity is at least the load, and match the breaker to the wire. In the US, 14 AWG copper goes with a 15 A breaker, 12 AWG with 20 A and 10 AWG with 30 A.

3. Check the voltage drop

Measure the one-way length of the run and check the voltage drop with the calculator. If it is more than 3%, go up one size at a time until it is within the limit. Long runs to garages, pumps and outbuildings are where this matters most.

4. Correct for heat and bundling

Ampacity tables assume an ambient temperature of 30 °C (86 °F) and no more than three current-carrying conductors together. Attics, rooftops in the sun and full conduits reduce how much current a wire can safely carry, so you may need a larger size.

5. Low-voltage DC systems

At 12 V, voltage drop is the main problem. A 10 A load 5 m away needs 6 mm² copper (about 10 AWG) to keep the drop under 3%. Keep battery cables short, and fuse every positive wire close to the battery.

Checklist

Before you buy wire, check these points:

• The load in amps, including any continuous loads at 125%.
• The one-way length, measured along the actual route.
• The voltage drop at the chosen size is within 3%.
• The breaker or fuse matches the wire size.
• The conductor type and insulation suit the location (wet, buried, outdoors, high temperature).
• The design follows your local code and is checked by a qualified electrician.