Voltage Drop Calculator
Voltage drop in volts and percent for a given wire size, current, and run length.
- Voltage drop
- 7.902 V
- Voltage drop (percent)
- 6.58%
- Voltage at load
- 112.1 V
- Within 3% target
- No
What voltage drop actually does to a load
Every foot of wire has resistance. Push current through resistance and you lose voltage as heat. That heat is why an undersized wire warms up under load. The voltage lost to resistance is voltage drop, and the load at the end of the run runs on whatever voltage is left after the drop.
A resistive load like a heater or incandescent lamp puts out less heat or light at lower voltage. A motor pulls more current at lower voltage to try to maintain torque, which increases the drop further, which can eventually stall or burn out the motor. Electronic loads run through switching power supplies handle voltage drop better up to a point, then the supply drops out entirely.
The 3 percent and 5 percent guidelines
NEC does not enforce voltage drop, but industry practice targets no more than 3 percent drop on any branch circuit and no more than 5 percent total from the service entrance to the furthest load. These numbers come from motor manufacturers and lighting designers who found that beyond about 5 percent, most loads start to misbehave.
Sensitive equipment (medical, laboratory, some industrial) may specify tighter limits like 2 percent. Long agricultural pump feeders sometimes tolerate up to 5 percent alone because the alternative is a much larger wire. Know the load before you decide the target.
How to fix a voltage drop problem
Upsize the wire. Doubling the circular mils halves the voltage drop. Every step up in AWG (12 to 10, 10 to 8, 8 to 6) roughly cuts drop by 40 percent.
Raise the voltage. Converting a long run from 120 V to 240 V (with a 2-pole load) cuts current in half and voltage drop by a factor of 4. This is why detached shops are usually fed at 240 V even when most receptacles inside will be 120 V.
Shorten the run. Move the sub-panel closer to the load. Every foot removed cuts drop proportionally.
Reduce the load. If you cannot upsize wire, split the load across two runs.
Copper vs aluminum voltage drop
Aluminum has more resistance per circular mil than copper. The formula constant K is 12.9 for copper and 21.2 for aluminum. That means aluminum wire has 65 percent more voltage drop at the same size, or equivalently, aluminum needs about 65 percent more circular mils for the same drop.
In practice, aluminum feeders are always two AWG sizes larger than copper for equivalent performance. 4 AWG copper is equivalent to 2 AWG aluminum. 1/0 copper is equivalent to 2/0 aluminum. This is why aluminum feeders look oversized compared to copper: they have to be.
For electricians
Voltage drop calculations are recurring service work: sub-panel installs, shop feeders, well pumps, EV chargers. Peak Local Leads builds and ranks local electrical sites for one contractor per Texas market. Try the calls free for a week.
Frequently asked.
- How much voltage drop is acceptable?
- Industry target is 3 percent on any branch circuit and 5 percent total from service to load. NEC includes these as informational guidance, not enforceable limits.
- How do I calculate voltage drop?
- Vd = (2 x K x I x L) / CM for single phase. K = 12.9 copper, 21.2 aluminum. Multiply K by 1.732 (sqrt of 3) for three-phase.
- Does voltage drop cause fires?
- Not directly. Voltage drop causes wire heating and equipment underperformance. Overload above ampacity causes fires. But chronic voltage drop means chronic wire heating, which shortens insulation life.
- Does wire length affect amperage or voltage?
- Length affects voltage drop, not amperage. The load still draws the same amps; the wire just delivers those amps at a lower voltage.
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