On long wire runs, voltage drops along the way and can starve a circuit. The accepted limit is about 3% for branch circuits. This chart gives the approximate one-way distance before a 120V copper circuit hits 3%.
| Wire (AWG) | Load | Max one-way run |
|---|---|---|
| 14 | 15 A | about 50 ft |
| 12 | 20 A | about 60 ft |
| 10 | 30 A | about 64 ft |
| 8 | 40 A | about 76 ft |
| 6 | 55 A | about 88 ft |
Upsize for distance. Past these lengths, move up a wire gauge to keep the drop in check. The drop is calculated as 2 × K × I × length ÷ circular mils (K is about 12.9 for copper).
See the Wire Ampacity Chart and What Wire Size Do I Need.
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