Circuit Wire Gauge to Amp Calculator
Convert electrical wire gauge (AWG) to amperage capacity based on wire length, material, and installation method.
Includes voltage drop calculations.
Select a wire gauge and enter the circuit details to see amperage capacity and voltage drop.
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, 75 degrees C rated):
| AWG Gauge | Diameter (in) | Diameter (mm) | Max Amps (NM cable) | Common Use |
|---|---|---|---|---|
| 14 AWG | 0.064 | 1.63 | 15 A | Lighting circuits, receptacles |
| 12 AWG | 0.081 | 2.05 | 20 A | Kitchen, bathroom, garage outlets |
| 10 AWG | 0.102 | 2.59 | 30 A | Dryers, water heaters, AC units |
| 8 AWG | 0.128 | 3.26 | 40 A | Range/oven circuits, large AC |
| 6 AWG | 0.162 | 4.11 | 55 A | Subpanels, large appliances |
| 4 AWG | 0.204 | 5.19 | 70 A | Feeder circuits, RV hookups |
| 2 AWG | 0.258 | 6.54 | 95 A | Large subpanels, service entrance |
| 1/0 AWG | 0.325 | 8.25 | 125 A | Service entrance, main feeds |
| 2/0 AWG | 0.365 | 9.27 | 145 A | 150A service panels |
| 4/0 AWG | 0.460 | 11.68 | 195 A | 200A residential service entrance |
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, but the circuit amperage is limited by the lowest-rated component in the system, which is usually the outlet or switch rated at 75 degrees C.
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 would be about 6.4 volts (5.3 percent), which exceeds the recommended 3 percent. Upgrading to 10 AWG copper reduces the drop to about 4 volts (3.3 percent), which is acceptable. Always size up when voltage drop is a concern.