Wire size calculator
A 20 amp, 120 V circuit needs 12 AWG copper by ampacity at 75 °C terminations; at 50 ft one way it needs 10 AWG copper to keep the voltage drop within 3% (2.1%). Enter your own load, distance and voltage below.
From the panel to the load, not there and back
The run is long: ampacity alone allows 12 AWG, but a larger wire keeps the drop within 3%.
- Breaker
- 20 A
- Voltage drop
- 2.5 V (2.1%)
- Volts at the load
- 117.5 V
- Ground wire
- 10 AWG copper
How it is calculated
- Breaker: 20 A rounded up to a standard size = 20 A.
- Smallest copper conductor rated for 20 A at 75 °C: 12 AWG.
- Drop = 2 × K × 20 A × 50 ft ÷ 10,380 cmil = 2.49 V, target 3% of 120 V.
How to calculate wire size
Two checks size a circuit wire: the current it must carry (ampacity) and the voltage it loses along the run. The worked numbers use the calculator's defaults: a 20 A, 120 V circuit, 50 ft one way, copper wire and 75 °C terminations.
Step 1: breaker size from the load
Breaker = load current, rounded up to the next standard breaker size
Continuous load (3 hours or more): breaker = load current × 1.25, rounded up
Example A 20 A load that is not continuous: 20 A breaker.
Example The same 20 A as a continuous load: 20 A × 1.25 → 25 A breaker.
Step 2: smallest wire by ampacity at 60 °C or 75 °C
Wire = the smallest size whose ampacity in the termination column (60 °C or 75 °C) is at least the breaker rating
Small copper wires have a lower breaker limit than their ampacity: 14 AWG on 15 A, 12 AWG on 20 A and 10 AWG on 30 A at most
Example 20 A, copper, 75 °C: 12 AWG (25 A in the 75 °C column, protected by 20 A at most).
Step 3: voltage drop on that wire
VD = 2 × K × I × L ÷ cmil for single phase; three phase uses √3 in place of 2
K = 12.9 ohm·cmil/ft for copper and 21.2 for aluminum; I = amps; L = one-way length in feet; cmil = the wire's area in circular mils
Example 12 AWG: 2 × 12.9 × 20 A × 50 ft ÷ 6,530 cmil = 3.95 V.
Step 4: compare the drop with 3% and size up
VD % = VD ÷ circuit volts × 100
Above 3%: repeat step 3 with the next larger size until the drop is within 3%
Example 3.95 V ÷ 120 V × 100 = 3.29%, over 3%, so 12 AWG is too small for 50 ft.
Example 10 AWG: 2 × 12.9 × 20 A × 50 ft ÷ 10,380 cmil = 2.49 V = 2.07%, within 3%. Answer: 10 AWG copper.
Wire size by amps and distance
At 100 ft one way, a 20 A, 120 V circuit needs 8 AWG copper and a 30 A, 240 V circuit needs 8 AWG copper to keep the voltage drop within 3%. The table runs the same two checks as the calculator for common circuits: 15 and 20 A at 120 V, 30 to 60 A at 240 V.
| One-way distance | 15 A, 120 V | 20 A, 120 V | 30 A, 240 V | 40 A, 240 V | 50 A, 240 V | 60 A, 240 V |
|---|---|---|---|---|---|---|
| 25 ft | 14 AWG | 12 AWG | 10 AWG | 8 AWG | 8 AWG | 6 AWG |
| 50 ft | 12 AWG | 10 AWG | 10 AWG | 8 AWG | 8 AWG | 6 AWG |
| 100 ft | 8 AWG | 8 AWG | 8 AWG | 8 AWG | 6 AWG | 6 AWG |
| 150 ft | 8 AWG | 6 AWG | 8 AWG | 6 AWG | 4 AWG | 4 AWG |
| 200 ft | 6 AWG | 4 AWG | 6 AWG | 4 AWG | 4 AWG | 3 AWG |
| 300 ft | 4 AWG | 3 AWG | 4 AWG | 3 AWG | 2 AWG | 2 AWG |
- 15 A, 120 V
- 20 A, 120 V
- 30 A, 240 V
- 50 A, 240 V
How the wire size is chosen
Two limits apply to every circuit. The wire must carry the current without overheating, which is its ampacity. And the voltage lost along the run should stay small, usually within 3%, so that motors start and lights do not dim. On short runs ampacity decides the size. On long runs voltage drop does.
The termination rating matters as much as the wire itself. NM-B cable (Romex) is limited to the 60 °C column. Most modern breakers and panels are rated 75 °C. The 90 °C column is used for derating, not for choosing the final size.
When the hot wires are made larger for voltage drop, the ground wire grows in the same proportion. The calculator shows that size with the result.