Reverse formula
Every value must match the planned circuit. Increasing allowed drop increases the calculated distance, but it does not relax ampacity, protection or equipment requirements.
How do you calculate wire size?
The size of an electrical conductor refers to its cross-sectional area (measured in circular mils (CM) or AWG in the US). The larger the cross-sectional area, the more current the wire can transmit with lower resistance—very similar to water flowing through a wider pipe with less pressure loss. We derive the required conductor area from Ohm's law and conductor resistivity:
where:
- V_drop – Voltage drop calculated between the supply source and the farthest end of the wire, measured in volts (V_drop = V_source × (% drop ÷ 100));
- I – Maximum current running through the conductor, measured in amps (convert power using the watts to amps calculator);
- K – Specific resistance constant of the conductor material at 75°C (12.9 Ω·CM/ft for copper and 21.2 Ω·CM/ft for aluminum);
- L – One-way length of the wire run from panel to load, measured in feet; and
- CM – Cross-sectional area of the conductor, measured in circular mils (where 1,000 CM = 1 kcmil).
Calculating wire sizes for three-phase AC
For a balanced three-phase AC circuit, three current-carrying phase conductors are used instead of a two-wire loop. The calculator accepts line-to-line voltage and line current, modifying the required area of a single conductor to:
The factor of √3 ≈ 1.732 converts between phase and line quantities in a balanced three-phase system, replacing the factor of 2. The calculator automatically selects the next AWG size up from its listed 14 AWG – 4/0 table so the voltage-drop limit is never exceeded. To check apparent power feeding that circuit, use the kVA to amps calculator.
Example maximum lengths
| AWG | Circular mils | Maximum one-way distance |
|---|---|---|
| 14 | 4,110 | 38.23 ft |
| 12 | 6,530 | 60.74 ft |
| 10 | 10,380 | 96.56 ft |
| 8 | 16,510 | 153.58 ft |
| 6 | 26,240 | 244.09 ft |
Use the number as a boundary check
If a measured route is close to the calculated boundary, use the actual unrounded values in the calculator and preserve margin for changed load, route or operating conditions. Do not treat a rounded chart distance as a construction tolerance.
Reasons a real limit can be shorter
- Higher actual current
- Lower operating voltage
- Aluminum instead of copper
- Connections and conductor temperature
- Motor starting or equipment minimum voltage
- A stricter project voltage-drop target