Wire Size
Load in amps, distance in feet, and the gauge that satisfies both the ampacity table and the drop over the run.
Estimating only. Electrical work is permitted and inspected against the code edition adopted where you live — have a licensed electrician do or verify the work.
Buy this wire
12
AWG copper · 20 A breaker- Sized for
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- Table ampacity
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- Voltage drop
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- Volts at the load
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Sized by the ampacity table.
How wire sizing works
Two separate questions decide the gauge. The first is heat: NEC Table 310.16 lists how many amps a conductor may carry continuously without cooking its insulation, in three columns for 60, 75 and 90 °C terminations. The second is distance: every foot of copper has resistance, and a long run arrives at the load with less voltage than it left with. Short circuits are decided by the table; long ones are decided by the drop.
One rule overrides the table for small wire. NEC 240.4(D) will not let a 14 AWG copper conductor sit behind more than a 15 amp breaker, or 12 AWG behind more than 20 amps, however generous the 90 °C column looks. That is why a 20 amp kitchen circuit is 12 AWG and not 14 AWG — the table alone would have allowed it.
Common branch circuits
| Circuit | Copper | Breaker | Typical use |
|---|---|---|---|
| 15 A | 14 AWG | 15 A | Lighting, bedroom receptacles |
| 20 A | 12 AWG | 20 A | Kitchen, bathroom, garage receptacles |
| 30 A | 10 AWG | 30 A | Dryer (240 V), water heater |
| 40 A | 8 AWG | 40 A | Range, EV charger |
| 50 A | 6 AWG | 50 A | Range, welder, hot tub |
| 100 A | 3 AWG | 100 A | Subpanel feeder (75 °C terminations) |
Why the run length changes the answer
The drop is proportional to current times distance divided by the conductor's area, so doubling the run doubles the loss. A 20 amp circuit at 120 volts is fine on 12 AWG for the first 50 feet or so; run it 120 feet out to a workshop and the same wire arrives several volts short, dimming lights and making motors run hot. The calculator bumps the gauge until the drop fits your limit and says which of the two constraints decided it.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- Ampacity table
- NEC 310.16 — the 60/75/90 °C columns for copper and aluminum
- Small-conductor rule
- NEC 240.4(D): 14 AWG Cu capped at 15 A, 12 AWG at 20 A, 10 AWG at 30 A
- Continuous loads
- Sized at 125 % when the box is ticked — NEC 210.19(A)
- Voltage drop
- 3 % branch-circuit target — a recommendation in NEC 210.19 informational notes, not a rule
Questions from the aisle
What size wire do I need for a 20 amp circuit?
12 AWG copper. NEC 240.4(D) caps 14 AWG copper at a 15 amp breaker and 12 AWG at 20 amps regardless of what the ampacity table says, so a 20 amp circuit starts at 12 AWG — and gets bigger if the run is long enough for voltage drop to bite.
What size wire for a 100 amp subpanel 150 feet away?
Ampacity alone allows 3 AWG copper at 100 amps on the 75°C column, and at 100 feet that is the answer. Stretch it to 150 feet and the drop passes 3%, so the calculator bumps it to 2 AWG; at 200 feet it goes to 1 AWG. That progression is the whole reason to enter the distance rather than look the size up in a table.
Which temperature column should I use?
NEC 110.14(C) sizes conductors by the lowest-rated termination in the circuit. For equipment rated 100 amps or less that is the 60°C column unless the breaker and lugs are both marked for 75°C — which most modern panels are. This form defaults to 60°C, the conservative reading.
Is aluminum wire allowed?
Aluminum is still used for services and feeders, and it is cheaper per amp. It needs a size or two larger for the same load, listed connectors (AL/CU rated), and antioxidant paste at the terminations. It is rarely worth it for a small branch circuit.
The reasoning behind this one
Wire size, breakers and voltage drop
Where the assumptions above come from, what changes them, and the mistakes that cost a second trip. Free to read, no account needed.