Wire gauge, circuit amperage, and run distance all have to match, or you risk voltage drop problems and fire hazards. This calculator picks the right gauge for your circuit and tells you how many feet of wire to buy.
Circuit Details
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Scenario: You're running a 20-amp, 120-volt kitchen circuit with a 45-foot wire run from the breaker panel.
Result: A 20A circuit requires 12 AWG wire. Round trip is 90 ft plus 10 ft for connections, totaling 100 ft. With 10% extra, buy 110 ft of 12/2 NM-B wire (1 roll of 250 ft). Circuit capacity is 2,400 watts.
Use 12 AWG (gauge) wire minimum for 20-amp circuits. For runs over 75 feet, step up to 10 AWG to avoid excessive voltage drop. Always use NM-B (Romex) rated cable for residential interior wiring.
Use 10 AWG wire for 30-amp circuits (dryers, water heaters, small AC units). For runs over 50 feet, step up to 8 AWG. 30-amp circuits need a double-pole breaker and use 10/3 cable.
Measure the distance from the breaker panel to each outlet or fixture, following the path the wire will actually take through walls and ceilings. Add 10-20% for bends, stapling slack, and box connections. Don't forget the return path -- you need twice the one-way distance.
No. 14 AWG wire is only rated for 15-amp circuits. Using it on a 20-amp circuit is a fire hazard because the wire can overheat before the breaker trips. This violates the National Electrical Code.
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