The short answer
For a 20 amp load 100 feet from the panel on a 240 V single-phase circuit, use 10 AWG copper or 8 AWG aluminum. That keeps voltage drop at or under 3% and meets NEC Table 310.16 ampacity at the 60 °C terminal rating.
Voltage drop decides the size here. The smallest copper wire rated for 20 A is 12 AWG, which would drop 3.29% over 100 ft. That's over the 3% guideline, so the next size up is needed.
If the load is continuous (on for three hours or more, like heaters, EV chargers and lighting), the conductor must be rated for 125% of the load, or 25 A. That calls for 10 AWG copper.
20 amps at 100 feet on other systems
| System | Copper | Aluminum | Drop (copper) |
|---|---|---|---|
| 120 V single-phase | 8 AWG | 6 AWG | 2.6% |
| 240 V single-phase | 10 AWG | 8 AWG | 2.07% |
| 208 V three-phase | 10 AWG | 8 AWG | 2.07% |
| 480 V three-phase | 12 AWG | 10 AWG | 1.43% |
Three-phase systems carry the same current with less voltage drop, so they often allow a smaller conductor over the same distance.
20 amps at other distances (240 V copper)
| Distance | Copper size | Governed by |
|---|---|---|
| 25 ft | 12 AWG | ampacity |
| 50 ft | 12 AWG | ampacity |
| 75 ft | 12 AWG | ampacity |
| 100 ft | 10 AWG | voltage drop |
| 150 ft | 8 AWG | voltage drop |
| 200 ft | 8 AWG | voltage drop |
| 250 ft | 6 AWG | voltage drop |
| 300 ft | 6 AWG | voltage drop |
How this was calculated
K is 12.9 for copper and 21.2 for aluminum. CM is the conductor's area in circular mils. Distance is one-way. Change any input in the calculator above to check your own circuit.
These sizes assume no more than three current-carrying conductors in the raceway and 30 °C ambient. Derate under NEC 310.15 for more conductors or hotter locations, and size motor circuits under Article 430.
Open the full wire size calculator with these numbers
Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.