Voltage Drop Calculator
Voltage drop is the loss along a conductor between the source and the load. It is not a code violation in itself — the NEC states its 3% branch-circuit and 5% total figures as recommendations in informational notes, not requirements — but equipment is rated to operate within a voltage band, and a long run that drops too far will leave a motor starting hard, a contactor chattering, or a drive faulting on undervoltage. On a long feeder it is usually voltage drop, not ampacity, that forces the conductor up a size.
Run the numbers
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The formula
VD = (1.732 × K × I × D) / CM [three-phase]
| K | Resistivity constant: 12.9 for copper, 21.2 for aluminium, in ohm-cmil/ft |
| I | Load current in amperes |
| D | One-way circuit length in feet |
| CM | Conductor cross-section in circular mils |
| 2 or 1.732 | Accounts for the return path: two conductors single-phase, the square root of three for three-phase |
Worked example
A 100 A three-phase load at 480 V, 150 ft away, on 2 AWG copper (66,360 cmil):
VD = (1.732 × 12.9 × 100 × 150) / 66,360 = 5.05 V
As a percentage: 5.05 / 480 = 1.05%. Comfortably inside 3%.
Move the same load to 400 ft and it becomes 13.5 V, or 2.8% — still inside the limit, but with nothing left for the branch circuit beyond it.
Which standard governs this
The K-factor method is the standard hand calculation. NEC 210.19(A) and 215.2(A) informational notes give the 3% branch and 5% total figures. For exact work, IEEE Std 141 (the Red Book) covers the full method with reactance included.
What this calculation does not account for
This is a resistance-only calculation. It ignores conductor reactance, which matters on larger conductors and longer runs — above roughly 250 ft, or above about 4/0, the reactive component starts to dominate and this method under-reads. It also assumes a 75°C conductor temperature and a balanced load.
Common mistakes
Using one-way distance where the formula wants it and then doubling it again — the 2 and the 1.732 already account for the return path. Sizing to ampacity first and never checking drop on a long run. Applying the single-phase formula to a three-phase circuit, which over-states drop by about 15%.