Wire size calculator
Enter the load, the one-way run length and the voltage. This returns the gauge that satisfies both ampacity and a 3 percent voltage drop, and tells you which of the two set the answer.
It sizes copper conductors at the 75 degree C column, which is what terminations on most equipment are rated for even when the cable insulation is rated higher.
- Copper conductor
- 8 AWG
Governed by ampacity. Voltage drop over this run is 3.9 V, or 1.6 percent.
Copper at the 75°C column of NEC 310.16, three current-carrying conductors, 30°C ambient, 3 percent voltage-drop target. This is the general table, not the reduced NEC 310.12 dwelling-service allowance. Check local rules and derate for ambient temperature, conduit fill and continuous loads.
AWG chart and ampacity
The three columns are insulation temperature ratings. Use the 75 degree C column in practice: even with 90 degree C cable, the terminal blocks and breakers at each end are usually only listed to 75, and the lowest-rated part of the circuit governs.
The 90 degree C column is for derating calculations, not for picking a conductor outright.
| AWG | mm² | 60°C | 75°C | 90°C |
|---|---|---|---|---|
| 14 | 2.08 | 15 | 20 | 25 |
| 12 | 3.31 | 20 | 25 | 30 |
| 10 | 5.26 | 30 | 35 | 40 |
| 8 | 8.37 | 40 | 50 | 55 |
| 6 | 13.3 | 55 | 65 | 75 |
| 4 | 21.2 | 70 | 85 | 95 |
| 3 | 26.7 | 85 | 100 | 115 |
| 2 | 33.6 | 95 | 115 | 130 |
| 1 | 42.4 | 110 | 130 | 145 |
| 1/0 | 53.5 | 125 | 150 | 170 |
| 2/0 | 67.4 | 145 | 175 | 195 |
| 3/0 | 85.0 | 165 | 200 | 225 |
| 4/0 | 107.2 | 195 | 230 | 260 |
Wire size by breaker rating
This is the answer most people are actually after. Note the small-conductor rule in NEC 240.4(D): regardless of what the ampacity table says, 14 AWG is limited to a 15 amp breaker, 12 AWG to 20 amp and 10 AWG to 30 amp. That rule overrides the higher figures in the 75 and 90 degree columns.
These are the general NEC 310.16 sizes and they apply to feeders, subpanels and branch circuits. Do not use the reduced dwelling-service figures here by mistake.
| Breaker | Copper | Aluminium | Typical use |
|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | Lighting circuits |
| 20 A | 12 AWG | 10 AWG | General socket circuits |
| 30 A | 10 AWG | 8 AWG | Dryer, small welder, compressor |
| 40 A | 8 AWG | 6 AWG | Range, larger compressor |
| 50 A | 8 AWG | 6 AWG | Welder, EV charger |
| 60 A | 6 AWG | 4 AWG | Subpanel, workshop feed |
| 100 A | 3 AWG | 1 AWG | Subpanel |
| 125 A | 1 AWG | 2/0 AWG | Subpanel |
| 150 A | 1/0 AWG | 3/0 AWG | Large subpanel |
| 200 A | 3/0 AWG | 250 kcmil | Feeder |
The dwelling-service exception
A single-family dwelling service, or a feeder carrying the entire load of one dwelling, is allowed smaller conductors under NEC 310.12 than the general table requires. This is why a 200 amp house service is commonly run in 2/0 copper rather than the 3/0 the general table would demand.
The allowance is specific. It applies to the service or feeder carrying the whole dwelling load and nothing else. Using these sizes on a workshop subpanel, a commercial feeder or any circuit that is not the full dwelling load is a code violation and an undersized conductor.
| Service rating | Copper | Aluminium |
|---|---|---|
| 100 A | 4 AWG | 2 AWG |
| 125 A | 2 AWG | 1/0 AWG |
| 150 A | 1 AWG | 2/0 AWG |
| 200 A | 2/0 AWG | 4/0 AWG |
Voltage drop on long runs
Ampacity is about the cable not overheating. Voltage drop is about the equipment at the far end actually working, and on a long run it is voltage drop that decides the size.
For a single-phase copper run the formula is VD = 2 x K x I x L / CM, where K is 12.9 for copper, I is the current, L is the one-way length in feet and CM is the circular mil area of the conductor. Aim for 3 percent or less on a branch circuit and 5 percent total including the feeder.
A practical shortcut: on runs past about 30 metres, go up one gauge. Past 60 metres, go up two.
