Distance comparison at fixed inputs
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.
| One-way distance | Required circular mils | AWG candidate | Estimated drop with candidate |
|---|---|---|---|
| 50 ft | 5,375 | 12 AWG | 2.47% |
| 100 ft | 10,750 | 8 AWG | 1.95% |
| 150 ft | 16,125 | 8 AWG | 2.93% |
| 200 ft | 21,500 | 6 AWG | 2.46% |
Why the percentage drops after an AWG jump
Required area rises steadily with distance, but standard AWG sizes change in steps. When the result crosses into a larger conductor, the estimated percentage can fall even though the run became longer.
That step pattern is why extrapolating one known answer is unreliable. Recalculate the actual distance rather than assuming every additional 50 feet means exactly one AWG change.
Measure distance consistently
- 01
Trace the real route
Measure the planned conductor path, not straight-line map distance.
- 02
Record one-way length
Do not double the input before using a calculator that applies its own multiplier.
- 03
Separate multiple loads
A run with loads tapped along its length needs a method that reflects where current flows.
- 04
Rerun changed inputs
Voltage, current, material, phase or target changes invalidate the chart candidate.
When the chart does not apply
- Different system voltage or current
- Aluminum conductor
- Balanced three-phase circuit
- Loads distributed along the route
- Meaningful reactance, harmonics or motor-starting behavior
- Any use as final conductor approval