Wire size is chosen so the wire can safely carry the circuit’s current without overheating. In the US it’s measured in AWG (American Wire Gauge), where a smaller number means a thicker wire — and the thicker the wire, the more amps it can handle.
Standard residential copper sizes
| Wire (copper) | Max breaker | Common use |
|---|---|---|
| 14 AWG | 15 A | Lighting, general outlets |
| 12 AWG | 20 A | Kitchen, bath, garage outlets |
| 10 AWG | 30 A | Dryer, water heater, A/C |
| 8 AWG | 40 A | Range, large appliances |
| 6 AWG | 50 A | Sub-panels, EV chargers |
These pairings come from the National Electrical Code, which caps 14 AWG at a 15 A breaker, 12 AWG at 20 A, and 10 AWG at 30 A.
Don’t forget distance
Over long runs, voltage drops along the wire, so you may need to go up a size to keep voltage adequate at the load — a common reason a 20 A circuit feeding a far shed uses 10 AWG instead of 12. Consider upsizing once a run exceeds ~50 feet.
Frequently asked questions
What wire for a 20-amp circuit? 12 AWG copper.
Why upsize wire for long runs? To limit voltage drop, which dims lights and strains motors.
Is aluminum sized the same? No — aluminum needs to be one to two sizes larger for the same amps.