Voltage Drop Calculator
Excessive voltage drop causes equipment malfunction, motor overheating, and dimming lights. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder plus branch. This calculator determines actual voltage drop for a given wire run.
Circuit Parameters
Based on NEC Chapter 9, Table 8 conductor resistance values at 75°C. NEC Article 210.19(A) recommends max 3% drop for branch circuits, 5% total (feeder + branch). This is a recommendation, not a requirement, except where specified by local code.
Voltage Drop Formula
Single Phase: VD = 2 × I × R × L / 1000
Three Phase: VD = √3 × I × R × L / 1000
Where I = current (amps), R = resistance (ohms per 1000 ft), L = one-way distance (ft).
NEC Voltage Drop Recommendations
| Circuit Type | Max Recommended Drop |
|---|---|
| Branch circuit (outlets, lights) | 3% |
| Feeder (panel to subpanel) | 3% |
| Total (feeder + branch combined) | 5% |
| Motor circuits | 3% at full load |
| Sensitive electronics | 2% or less |
Frequently Asked Questions
How do I fix excessive voltage drop?
Three options: (1) Increase wire size — going up one gauge roughly halves resistance. (2) Reduce circuit length — relocate the panel or subpanel closer. (3) Increase voltage — a 240V circuit has half the current of 120V for the same power, halving voltage drop.
Does voltage drop waste electricity?
Yes. Voltage drop means power is being dissipated as heat in the wire itself (I²R loss). A 5% voltage drop means roughly 5% of the power is wasted in the conductors.