For 20 amps at 300 feet (one-way), you need 6 AWG copper at 240 V or 3 AWG copper at 120 V to stay within the NEC-recommended 3% voltage drop. In aluminum at 240 V: 4 AWG.
| Drop budget | 120 V copper | 240 V copper | 240 V aluminum |
|---|---|---|---|
| 3% (NEC branch) | 3 AWG | 6 AWG | 4 AWG |
| 5% (NEC total) | 6 AWG | 8 AWG | 6 AWG |
Distance is one-way; the round trip is built into the formula. The 3% figure is the NEC branch-circuit recommendation — use it unless the load is tolerant and the path already meets the 5% total budget. Verify the final pick against breaker size and termination ratings.
The absolute volts lost are identical, but they are a larger percentage of 120 V — so the budget runs out twice as fast. Running the same power at 240 V also halves the current, compounding the advantage.
No — 300 ft is the one-way route distance. You will buy roughly twice that in conductor for a single-phase circuit, which the calculation already accounts for.