Voltage Drop Calculator
0% voltage drop
| Wire Size | Resistance (Ω/1000 ft) | Voltage Drop | Drop % |
|---|
How Voltage Drop Is Calculated
This calculator uses the standard conductor-resistance method referenced in NEC Chapter 9, Table 8. Each wire gauge has a known DC resistance per 1000 feet based on its cross-sectional area and material. For a single-phase or DC circuit, voltage drop equals VD = 2 × L × R × I / 1000, where L is the one-way distance in feet, R is the conductor's resistance in ohms per 1000 feet, and I is the current in amps; the factor of 2 accounts for the round trip through both the outgoing and returning conductor. Three-phase circuits use VD = √3 × L × R × I / 1000 instead of doubling the length, since the phase geometry cancels part of the return-path resistance. The resistance values used here are standard uncoated-copper and aluminum figures at approximately 75°C; actual resistance varies slightly with temperature, conductor coating, and manufacturer, so treat results as a close engineering estimate rather than an exact lab measurement.
Why the 3% Rule Matters
The National Electrical Code doesn't mandate a maximum voltage drop, but its informational notes recommend keeping branch circuits under 3% drop and the total from service to farthest outlet under 5%, since excessive drop wastes energy as heat and can cause motors, lights, and electronics to underperform or run hot. If your result exceeds 3%, the usual fix is stepping up to a larger wire size (lower resistance) rather than raising the source voltage. Pair this with the resistor calculator or Ohm's law calculator if you need to work through the underlying circuit math in more detail.
Long Runs Need Bigger Wire, Not Just a Bigger Breaker
A common misconception is that as long as a wire's ampacity rating covers the load, any length is fine. Ampacity only concerns heat and insulation limits at a given length; it says nothing about how much voltage is lost over distance. A 12 AWG circuit that's perfectly safe at 20 feet can drop well over 3% at 150 feet, which is why long outbuilding, well-pump, or shop-panel feeders routinely need to be upsized beyond what the load current alone would suggest.
Frequently Asked Questions
How much voltage drop is acceptable?
The NEC doesn't set a hard legal limit, but its informational notes recommend keeping branch-circuit voltage drop at or below 3%, and the combined drop from the service entrance to the farthest outlet at or below 5%. Beyond that, equipment can run hot, dim, or underperform, and energy is wasted as heat in the wire.
Why does three-phase use a different formula than single-phase?
Single-phase and DC circuits use a multiplier of 2 because current travels out on one conductor and back on another, doubling the effective resistance path. Three-phase circuits use a multiplier of the square root of 3 (about 1.732) instead, since the phase-to-phase geometry means the return current is split across the other two conductors rather than doubling back through a single path of equal resistance.