Voltage Drop Calculator (Copper & Aluminum, Single and Three Phase)
Long runs lose voltage. Enter the circuit to see the voltage drop in volts and as a percentage of the supply voltage, and whether it stays within the 3% branch-circuit target in the NEC informational notes.
Voltage drop
6.32 V
Percent drop
5.27%
Voltage at load
113.7 V
vs 3% branch target
Above 3% — consider upsizing
How it is calculated
- Single phase: VD = 2 × K × I × L ÷ CM. Three phase: VD = 1.732 × K × I × L ÷ CM.
- K = resistivity constant: 12.9 Ω·cmil/ft for copper and 21.2 for aluminum (approximate values at 75 °C).
- I = load current in amps, L = one-way length in feet, CM = conductor area in circular mils (NEC Chapter 9, Table 8).
- Percent drop = VD ÷ supply voltage × 100. NEC informational notes suggest ≤3% on a branch circuit and ≤5% combined feeder + branch. It is a recommendation, not a mandatory rule; always confirm with the code adopted locally.
| Conductor size | Area (circular mils) |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 3 AWG | 52,620 |
| 2 AWG | 66,360 |
| 1 AWG | 83,690 |
| 1/0 | 105,600 |
| 2/0 | 133,100 |
| 3/0 | 167,800 |
| 4/0 | 211,600 |
FAQ
What is an acceptable voltage drop?
NEC informational notes recommend no more than 3% on a branch circuit and 5% total for feeder plus branch circuit. They are design recommendations, not enforceable requirements, unless the local jurisdiction adopts them.
Why is the voltage drop higher with aluminum?
Aluminum has higher resistivity than copper (K ≈ 21.2 vs 12.9 Ω·cmil/ft), so the same size conductor drops more voltage. Aluminum is usually upsized to compensate.
Should I use one-way or round-trip length?
Enter the one-way length from the source to the load. The formula already accounts for the return path (×2 for single phase, ×1.732 for three phase).
Is this calculator exact?
It uses the standard K-factor approximation, which is fine for estimating and sizing checks. It ignores reactance, power factor and conductor temperature, so final designs should be verified with the full NEC Chapter 9 Table 9 method.