Running power to a detached garage, a shed, a well pump or a backyard outlet? On long runs, the wire size that’s fine for ampacity may still be too small, because of voltage drop.
What voltage drop is
Every wire has some resistance. As current flows, a little voltage is used up pushing it through the wire, so the voltage that reaches the far end is lower than at the panel. The longer and thinner the wire, and the more current it carries, the bigger the drop.
Why it matters
- Motors in pumps, compressors and power tools run hotter and can fail early.
- Lights may dim when a load starts.
- Energy is wasted as heat in the wire.
The common target: about 3%
The National Electrical Code includes informational notes suggesting about 3% maximum drop on a branch circuit and about 5% total from the service to the farthest outlet. These are recommendations for good performance rather than strict requirements in most cases, but they’re widely used as design targets.
How to calculate it
voltage drop = 2 × K × I × L ÷ CM
K = 12.9 for copper, I = amps, L = one-way length (ft), CM = circular mils of the wire
Example: 20 A at 120 V, 100 ft one-way
12 AWG (6,530 CM): about 7.9 V = 6.6% → too much
10 AWG (10,380 CM): about 5.0 V = 4.1%
8 AWG (16,510 CM): about 3.1 V = 2.6% → within 3%In this example, a 20-amp circuit that would normally use 12 AWG needs 8 AWG to stay within 3% over 100 feet. Our Wire Size Calculator runs this check automatically.
Ways to reduce voltage drop
- Use a larger wire. The most common fix.
- Use 240 V where possible. The same power at double the voltage needs half the current, so the percentage drop falls sharply.
- Shorten the route. Measure the real path the wire will take, not the straight-line distance.
- Install a subpanel at the outbuilding for several circuits; a licensed electrician should design and install it.
Upsizing wire is about performance; the breaker still has to match the circuit’s design. For the basics of matching wire to breakers, see What Size Wire Do I Need?
