The short answer
12 AWG copper (6,530 circular mils) carrying 25 A on a 240 V single-phase circuit reaches 3% voltage drop at about 73 feet. At lighter loads it can go farther, as the tables below show.
Percent voltage drop at 240 V single-phase
Values over 3% are marked.
| Load | 25 ft | 50 ft | 75 ft | 100 ft | 150 ft | 200 ft | 250 ft | 300 ft |
|---|---|---|---|---|---|---|---|---|
| 5 A | 0.2% | 0.4% | 0.6% | 0.8% | 1.2% | 1.6% | 2.1% | 2.5% |
| 10 A | 0.4% | 0.8% | 1.2% | 1.6% | 2.5% | 3.3% | 4.1% | 4.9% |
| 15 A | 0.6% | 1.2% | 1.9% | 2.5% | 3.7% | 4.9% | 6.2% | 7.4% |
| 20 A | 0.8% | 1.6% | 2.5% | 3.3% | 4.9% | 6.6% | 8.2% | 9.9% |
| 25 A | 1% | 2.1% | 3.1% | 4.1% | 6.2% | 8.2% | 10.3% | 12.3% |
Longest run that stays under 3%
| Load | 120 V 1φ | 240 V 1φ | 208 V 3φ | 480 V 3φ |
|---|---|---|---|---|
| 5 A | 182 ft | 364 ft | 365 ft | 842 ft |
| 10 A | 91 ft | 182 ft | 182 ft | 421 ft |
| 15 A | 61 ft | 121 ft | 122 ft | 281 ft |
| 20 A | 46 ft | 91 ft | 91 ft | 210 ft |
| 25 A | 36 ft | 73 ft | 73 ft | 168 ft |
How it's calculated
Table 310.16 lists 12 AWG copper at 20 A (60 °C) and 25 A (75 °C), before any 240.4(D) limit on the breaker size. Voltage drop is a separate check. On long runs it usually decides the wire size.
Other copper sizes
10 AWG · 14 AWG · 8 AWG · 6 AWG · 4 AWG · 2 AWG · 3 AWG · 1 AWG · 1/0 AWG · 2/0 AWG · 3/0 AWG · 4/0 AWG
Open the full voltage drop 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.