The calculator is preset to a 20 amp, 120 volt circuit. Type the one-way distance from the panel and it tells you whether 12 AWG still keeps the drop in check, or where you should go up a size.
How to use this calculator
- Leave the current at 20 amps for a general receptacle or lighting circuit: size the wire for the breaker, not for what you plan to plug in today.
- Enter the distance one way, measured along the path the cable will take, through the attic or along the joists, not in a straight line.
- Copper is the norm for circuits this small. Aluminum branch wiring is rare in new homes and needs devices marked for it.
- A dedicated 240 volt circuit of this size, for a small heater or a compressor, loses half the percentage: switch the voltage and compare.
Worked examples
Kitchen countertop receptacles
Load current: 20 A · Circuit voltage: 120 V · One-way distance: 35 ft
- Wire size
- 12 AWG copper
- Current the wire must carry
- 20.0 A
- Rated for
- 20 A
- Voltage drop
- 2.31%
- Longest run within 3%
- 45 ft
A detached garage receptacle
Load current: 20 A · Circuit voltage: 120 V · One-way distance: 110 ft
- Wire size
- 8 AWG copper
- Current the wire must carry
- 20.0 A
- Rated for
- 40 A
- Voltage drop
- 2.85%
- Longest run within 3%
- 116 ft
A baseboard heater on its own circuit
Load current: 16 A · Circuit voltage: 240 V · One-way distance: 60 ft · Runs for 3 hours or more at a time: Yes, a continuous load
- Wire size
- 12 AWG copper
- Current the wire must carry
- 20.0 A
- Rated for
- 20 A
- Voltage drop
- 1.58%
- Longest run within 3%
- 114 ft
Wire size for a 20 amp circuit by one-way distance
| One-way distance | 120 V, Copper | 240 V, Copper | 120 V, Aluminum |
|---|---|---|---|
| 25 ft | 12 AWG | 12 AWG | 10 AWG |
| 50 ft | 10 AWG | 12 AWG | 8 AWG |
| 75 ft | 8 AWG | 12 AWG | 6 AWG |
| 100 ft | 8 AWG | 10 AWG | 6 AWG |
| 150 ft | 6 AWG | 8 AWG | 4 AWG |
Each cell: the smallest size that carries the full current within the usual branch-circuit drop. Copper in NM-B cable; at this size the cable type does not change the answer.
Why 14 AWG cannot go on a 20 amp breaker
The ampacity table alone would let 14 AWG copper carry a little more than its everyday rating, but the code adds a separate cap for the three smallest sizes: 15 amps on 14 AWG, 20 amps on 12 AWG and 30 amps on 10 AWG. That cap is why a white-jacketed 14/2 cable on a 20 amp breaker is one of the most common inspection failures, and why a yellow 12/2 cable is the usual sight in kitchens, bathrooms, laundry rooms and garages, where the code calls for 20 amp receptacle circuits.
Long runs at 120 volts
Voltage drop grows with current and length and shrinks with voltage. A branch circuit is commonly kept within 3 percent, and at 120 volts and the full 20 amps, 12 AWG crosses that line well before the end of a long backyard run. Lights dim when a saw starts, and a compressor may struggle. Moving up to 10 AWG for the long stretch costs little and keeps the circuit honest; the breaker stays at 20 amps. The table shows the size each distance needs, and the voltage drop calculator gives the exact percentage for your own load.
Thicker wire and the receptacle terminals
Standard receptacles and switches accept 12 AWG easily. 10 AWG is stiff and fills the box, so many electricians run 10 AWG to the first box and finish with a short 12 AWG pigtail, which is allowed because the breaker protects both. Check the box fill and the device's label before you push heavy wire onto its screws.
Common questions
Can I use 14 gauge wire on a 20 amp breaker?
No. 14 AWG copper is limited to a breaker of 15 amps, so a 20 amp circuit needs 12 AWG or thicker, however short the run.
Is 12 gauge wire good for 20 amps?
Yes, 12 AWG copper is the standard size: the code allows it on a breaker of 20 amps. On a long run voltage drop can call for 10 AWG; enter your distance above.
How far can you run 12 gauge wire on a 20 amp circuit?
It depends on the load and the drop you accept. At the full 20 amps and 120 volts, staying within 3 percent limits 12 AWG to a fairly short run; the calculator shows the longest run for your numbers in its result.
Can I put 10 gauge wire on a 20 amp breaker?
Yes. A wire may always be bigger than the breaker needs; it only must not be smaller. Thicker wire on the same breaker is the usual fix for a long run.