Voltage Drop Calculator
Calculate voltage drop across conductors for DC, single-phase, and three-phase circuits per NEC standards. Includes live chart visualization.
Voltage Drop vs Wire Length
How to Use the Voltage Drop Calculator
Calculate voltage drop in three simple steps.
Choose Circuit Type
Select DC, single-phase AC, or three-phase AC. Enter the system voltage and load current for your circuit.
Configure Conductor
Select the AWG wire size, conductor material (copper or aluminum), and enter the one-way wire length in feet or meters.
Review Results & Chart
See the voltage drop in volts, voltage at the load, percentage drop, and a color-coded pass/warn/fail status. The live chart shows how drop increases with length.
NEC Voltage Drop Guidelines
Recommended voltage drop limits per National Electrical Code.
| Circuit Type | Recommended Max Drop | Notes |
|---|---|---|
| Branch Circuits | 3% | Lighting, receptacles, and general loads |
| Feeder Circuits | 3% | Conductors feeding subpanels |
| Combined (Feeder + Branch) | 5% | Total from service to farthest outlet |
| Motor Circuits | 5% | Starting and running conditions |
| Sensitive Electronics | 2% | Computers, medical equipment |
Frequently Asked Questions
Common questions about voltage drop calculations.
Voltage drop is the reduction in voltage as electricity travels through a conductor. Excessive voltage drop causes equipment to operate inefficiently, reduces motor torque, causes lights to dim, and wastes energy. NEC recommends keeping drop below 3% on branch circuits.
For three-phase circuits, use Vd = √3 × I × (2 × L × R / 1000). The √3 factor (approximately 1.732) accounts for the phase-to-phase voltage relationship in a balanced three-phase system.
The K constant represents conductor resistivity. K = 12.9 for copper and K = 21.2 for aluminum at 75°C. Vd = (2 × K × I × L) / (CM / 1000) where CM is circular mil area. This calculator uses direct resistance values for greater accuracy.
Yes, conductor resistance increases with temperature. This calculator uses values at 75°C (167°F), which is the standard NEC temperature rating for typical building wire. Higher temperatures increase resistance and voltage drop.
Yes! Select the "DC" phase type. DC voltage drop uses the same formula as single-phase: Vd = 2 × I × (L × R / 1000). There is no power factor consideration in DC circuits.