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Voltage drop

Voltage drop on a run, and the smallest wire that keeps it within your target.

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Phase
More optionsAWG / kcmil · Feet · Copper · Target drop 3 %
Wire sizes in
Length in
Conductor metal

12 AWG copper, 100 ft, 16 A at 120 V single-phase

5.15% against 3% target and 5%5.15%3% target5%
How it's calculated
  • Single-phase: drop = 2 × amps × resistance per metre × one-way length.
  • Resistance: 1.9316 Ω per 1,000 ft for this conductor, DC at 75 °C.
  • From resistivity: copper 1/58 Ω·mm²/m, aluminum 0.028264 Ω·mm²/m at 20 °C, rising 0.393% (Cu) or 0.403% (Al) per °C.
  • Checked against NEC Chapter 9 Table 8: 12 AWG solid copper = 1.93 Ω per 1,000 ft.

An estimate from DC resistance that ignores reactance, so it reads a little low for large conductors. NEC informational notes suggest at most 3% for a branch circuit or feeder and 5% overall.

Worked examples

12 AWG copper, 100 ft, 16 A at 120 V single-phase

  • Voltage drop: 5.15%
  • Drop in volts: 6.18 V
  • Voltage at the load: 113.8 V
  • Smallest size for your target: 8 AWG

4/0 AWG aluminum, 250 ft, 150 A at 208 V three-phase

  • Voltage drop: 3.07%
  • Drop in volts: 6.38 V
  • Voltage at the load: 201.6 V
  • Smallest size for your target: 250 kcmil

10 AWG copper, 150 ft, 20 A at 277 V single-phase

  • Voltage drop: 2.63%
  • Drop in volts: 7.29 V
  • Voltage at the load: 269.7 V
  • Smallest size for your target: 10 AWG

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