Circuit Wire Gauge to Amp Calculator

Understanding Wire Gauge and Amperage

Electrical wire size is measured using the American Wire Gauge (AWG) system. A smaller AWG number indicates a thicker wire that can carry more current. Selecting the proper wire gauge is critical for safety because undersized wire overheats, which can cause fires. The wire gauge you need depends on the amperage of the circuit, the length of the wire run, and whether the wire is copper or aluminum.

Wire Gauge to Amperage Capacity (Copper):

The amp column below is the 60 degrees C figure from NEC Table 310.16, which is the one that applies to NM cable (the flat white or yellow “Romex” used throughout most houses) and to any circuit of 100 amps or less landing on ordinary breakers and devices. Higher-temperature insulation does not raise it, because the terminations are the limit.

Voltage Drop Calculation:

For long wire runs, voltage drop becomes a significant concern. The National Electrical Code (NEC) recommends no more than 3 percent voltage drop for branch circuits and 5 percent total from the service panel to the furthest outlet. The formula for single-phase voltage drop is:

Voltage Drop = (2 x Length x Current x Resistance per foot) / 1000

Where length is the one-way distance in feet and resistance is the wire resistance in ohms per 1000 feet. For a 120-volt circuit, 3 percent drop equals 3.6 volts. For a 240-volt circuit, 3 percent equals 7.2 volts.

Copper vs. Aluminum Wire:

Aluminum wire is about 61 percent as conductive as copper, so it requires a larger gauge for the same amperage. For example, where 12 AWG copper handles 20 amps, you need 10 AWG aluminum for the same capacity. Aluminum wire is lighter and cheaper per foot, making it common for main service entrance cables and large feeder circuits. Copper is preferred for branch circuits due to its better conductivity and easier termination.

Temperature Rating:

Wire insulation is rated for different temperatures, which affects the maximum amperage. Common ratings are 60 degrees C (TW insulation), 75 degrees C (THW, THWN), and 90 degrees C (THWN-2, XHHW-2). Higher temperature ratings allow more current on paper, but the circuit is limited by the lowest-rated component in it, and that is almost always the termination rather than the wire. NEC 110.14(C) is explicit: for equipment rated 100 amps or less, use the 60 degrees C column unless the device is specifically listed for 75 degrees C. So a 90 degrees C conductor landed on a standard breaker and a standard receptacle is still a 60 degrees C circuit. The 90 degrees C rating earns its keep only as headroom for derating, when several conductors share a conduit or the run passes through a hot attic.

Practical Example:

Running a 20-amp circuit to a workshop 100 feet (30.5 meters) from the main panel at 120 volts: using 12 AWG copper wire, the voltage drop is 2 x 100 x 20 x 1.98 / 1000 = 7.9 volts, or 6.6 percent, which is more than double the recommended 3 percent. Going up to 10 AWG brings it to 5.0 volts (4.1 percent) - better, but still past the branch-circuit target. It takes 8 AWG to get under 3 percent, at 3.1 volts (2.6 percent).

That surprises people, and it is worth saying why the answer comes out worse than most rule-of-thumb charts suggest. This calculator uses the NEC Chapter 9 Table 8 resistance values at 75 degrees C, which is the conductor temperature assumed when a circuit is actually loaded. Charts based on 20 degrees C resistance give a figure roughly 25 percent lower and make marginal runs look fine. Long runs to detached garages and workshops are exactly where that difference decides the wire size.