Wire size is set by two separate checks, and you use whichever gives the bigger wire:
- Ampacity — the current the conductor can carry without overheating. This sets the minimum size for the breaker/load.
- Voltage drop — on a long run, a wire that is thick enough for the current can still lose too much voltage. Long runs get sized up.
Wire size by amps (copper)
For standard copper building wire (60 °C / 75 °C, per NEC 310.16), the common pairings are:
| Breaker / load | Copper wire | Metric |
|---|---|---|
| 15 A | 14 AWG | 2.5 mm² |
| 20 A | 12 AWG | 4 mm² |
| 30 A | 10 AWG | 6 mm² |
| 40 A | 8 AWG | 10 mm² |
| 50 A | 6 AWG | 16 mm² |
| 60 A | 4 AWG | 25 mm² |
| 100 A | 1 AWG / 1/0 | 50 mm² |
Aluminum carries less current for the same size, so it steps up about two gauges — a circuit that takes 6 AWG copper needs roughly 4 AWG aluminum. It is common on larger feeders and service entrances.
When voltage drop makes you go bigger
A rule of thumb is to keep voltage drop under 3% for a branch circuit. Drop rises with distance and current, so a long run to a shop, well pump or EV charger often needs a size (or two) above the ampacity table. As a feel for it: a 20 A, 240 V circuit on 12 AWG is fine at 50 ft but drifts past 3% out around 100 ft — at which point you go to 10 AWG.
Cable Size & Wire Gauge Calculator
Enter the load, voltage, run length and allowed drop — it returns the exact AWG (or mm²) and the actual drop.
The two checks together
Size for ampacity from the table, then check voltage drop for the run length — and use the larger wire. A short 20 A kitchen circuit is ampacity-limited (12 AWG and done); a long low-voltage or solar-DC run is almost always voltage-drop-limited, where the wire ends up far bigger than the current alone would suggest.
This is a starting point, not the code
Real installations also account for the installation method, insulation temperature rating, how many conductors are bundled together, ambient temperature, and continuous-load derating (size to 125% of a load that runs 3+ hours). Always confirm against the NEC or your local wiring rules, and have an electrician sign off anything you are unsure of.