What is voltage drop?
Every wire has resistance, so some voltage is lost as current flows through it. The longer the run and the higher the current, the bigger the loss. Too much drop makes motors run hot, LEDs flicker and chargers slow down.
Voltage drop formula
L is the one-way length in feet, I is the current in amps, and R is the conductor resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8.
Example
10 AWG copper, 24 A, 100 ft, 240 V single-phase: 2 × 100 × 24 × 1.21 ÷ 1000 = 5.81 V, which is 2.42%. That is within the 3% recommendation.
Recommended limits
The NEC informational notes in 210.19 and 215.2 recommend no more than 3% on a branch circuit or feeder and 5% total from the service to the farthest outlet. These are recommendations, but many inspectors and equipment manufacturers expect them.
Resistance per 1,000 ft (NEC Ch. 9, Table 8)
| Size | Copper Ω | Aluminum Ω |
|---|---|---|
| 14 AWG | 3.07 | – |
| 12 AWG | 1.93 | 3.18 |
| 10 AWG | 1.21 | 2.00 |
| 8 AWG | 0.764 | 1.26 |
| 6 AWG | 0.491 | 0.808 |
| 4 AWG | 0.308 | 0.508 |
| 2 AWG | 0.194 | 0.319 |
| 1/0 AWG | 0.122 | 0.201 |
| 2/0 AWG | 0.0967 | 0.159 |
| 4/0 AWG | 0.0608 | 0.100 |
Frequently asked questions
What is an acceptable voltage drop?
3% or less on a branch circuit or feeder, and 5% or less in total, per the NEC informational notes.
Does voltage drop depend on voltage?
The volts lost depend only on current, length and wire. But the percentage is much smaller at higher voltage, which is why long runs use 240 V or 480 V.
How do I reduce voltage drop?
Use a larger wire, shorten the run, raise the system voltage, or split the load across more circuits.