Copper AWG comparison
How do you calculate voltage drop?
Every metallic conductor has internal electrical resistance. As current flows through a run of copper or aluminum wire, part of the source voltage is lost as heat along the cable path. Using Ohm's law (V = I × R) and circular-mil resistivity at 75°C, the voltage lost across a two-wire circuit is:
where:
- VD – Total voltage drop between the supply panel and the load terminals, measured in volts (need a target gauge instead? Use the wire size calculator);
- K – Conductor resistance constant at 75°C (K = 12.9 for copper and K = 21.2 for aluminum in Ω·CM/ft);
- I – Load current flowing through the circuit in amps (estimate from wattage with our watts to amps calculator);
- L – One-way wire distance in feet (the multiplier of 2 already accounts for the return conductor); and
- CM – Conductor cross-sectional area in circular mils from NEC Chapter 9, Table 8.
Three-phase AC voltage drop equation
In a balanced three-phase system, the return currents cancel in phase opposition, so the round-trip factor of 2 is replaced by √3:
Subtract VD from the source voltage (V_load = V_source − VD) to find the actual voltage delivered to the equipment terminals. Explore all related tools in our electrical calculators hub.
| Copper AWG | Estimated volts lost | Estimated percentage |
|---|---|---|
| 14 | 18.83 V | 7.85% |
| 12 | 11.85 V | 4.94% |
| 10 | 7.46 V | 3.11% |
| 8 | 4.69 V | 1.95% |
| 6 | 2.95 V | 1.23% |
| 4 | 1.85 V | 0.77% |
| 2 | 1.17 V | 0.49% |
Read across one row only after checking inputs
The percentages apply only to the stated example. At 120 V the same volts lost would be twice the percentage; at 60 A the estimated volts lost would double.
Conductor temperature and AC impedance can change actual operating voltage. Use the chart to understand direction, then calculate the real project inputs.
Copper is not a complete specification
- Identify insulation and conductor construction
- Verify both terminal temperature ratings
- Apply ambient correction and conductor-count adjustment
- Check overcurrent protection and continuous load
- Confirm raceway fill and physical product dimensions
Cross-check the selected conductor
Enter the chosen AWG in the voltage-drop calculator, then run the same circuit through the wire-size calculator. A mismatch usually indicates that the target, phase, material or distance inputs differ.