Voltage Drop & Wire Size
Percent voltage drop from amps, one-way length, gauge, and material, plus the gauge that meets your 3% target, the maximum length at that target, and the watts you lose.
3% on the branch circuit and 5% total from the transformer is the design target most engineers use. The branch is half of it, which is what makes long runs expensive.
How the math works
- Formula: voltage drop = (2 × K × amps × one-way feet) ÷ circular mils for single phase, with 1.732 replacing the 2 for three phase. K is 12.9 for copper and 21.2 for aluminum at operating temperature.
- Circular mils: 14 = 4,170, 12 = 6,530, 10 = 10,380, 8 = 16,510, 6 = 26,240, 4 = 41,740, 2 = 66,360, 1/0 = 105,600, 4/0 = 211,600. Aluminum is 1.64x the resistance of copper at the same size, which is the 3-size-upsizing rule.
- Targets: 3% branch-circuit drop, 5% total from the service transformer, 10% cumulative on a feeder + branch chain. The NEC gives these as FINE notes (9005.5), not as a mandatory rule: a conductor that meets ampacity and protection is code-legal at 6% drop.
- Wasted power: drop × amps = watts lost as heat in the conductor. A 20 A branch at 5.4 V drop burns 108 W continuously, which is about 950 kWh a year on a 24/7 load.
- Max length: length = circular mils × target% ÷ 100 × volts ÷ (2K). At 3% and 20 A on 230 V, 12 AWG tops out at 75 ft one-way.
What is the code-allowed voltage drop?
The NEC recommends 3% for a branch circuit and 5% total for feeder plus branch, and calls them Fine (informative) notes, not mandatory. A circuit wired to ampacity and protection rules is legal at 6%, and most utilities and engineers push for 3% on branch, 10% cumulative.
How far can I run 10 gauge at 20 amps?
At 3% on a 230 V single-phase load: 10 AWG (10,380 cmil) tops out at 120 ft one-way. At 120 V, 10 AWG takes you 63 ft at 3% and 104 ft at 5%. Distance halves when voltage halves.
Do I size for voltage drop before ampacity?
Ampacity and overcurrent protection first: that is the code. Voltage drop is the performance check that tells you the wire is code-legal and the motor gets 208 V instead of 230. When the drop fails, you upsize the conductor, not the breaker.