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Voltage Drop Calculator

Enter the supply voltage, load current and one-way cable length, and choose copper or aluminium and the wire size. The calculator gives the voltage drop, percentage and voltage at the load. For US circuits it compares the result with the NEC’s 3% branch and 5% total recommendations. 120 V, 20 A over 100 ft of 12 AWG copper drops 7.90 V (6.6%).

Estimates only. Electrical work must be designed and carried out by a licensed electrician in line with your local wiring rules.

Measurements

Units

Distance from the panel to the load, not the round trip.

Results

Voltage drop

7.9 V

Voltage drop (%)

6.6 %

Voltage at the load
112.1 V
  • Check: about 3% or less for a branch circuit (NEC recommendation)
  • Check: about 5% or less for feeder plus branch (NEC recommendation)

The NEC's 3% and 5% figures are informational notes, not enforceable rules. Conductor size must also meet ampacity, protection and your local code. Electrical work must be done by a licensed electrician.

Show the working
  1. VD = 2 · K · I · L ÷ CM2 × 12.9 × 20 A × 100 ft ÷ 6530 cmil = 7.9 V
  2. Percent7.9 ÷ 120 × 100 = 6.59%
Paper and orientation
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How to use the voltage drop calculator

  1. Choose Imperial (AWG sizes, feet) or Metric (mm², metres).
  2. Choose single-phase or three-phase.
  3. Enter the supply voltage and load current.
  4. Enter the one-way cable length.
  5. Choose copper or aluminium and the wire size.

How the formula works

In imperial mode, VD = 2 × K × I × L ÷ CM for single-phase and √3 × K × I × L ÷ CM for three-phase. K is 12.9 Ω·cmil/ft for copper and 21.2 for aluminium. Circular mils come from the AWG definition (ASTM B258). In metric mode, ΔU = 2 × L × I × ρ ÷ A (single-phase) or √3 × L × I × ρ ÷ A (three-phase), with ρ = 0.0178 Ω·mm²/m for copper and 0.0287 for aluminium. The percentage is VD ÷ supply voltage × 100.

Worked examples

120 V, 20 A, 100 ft, copper. 12 AWG (6,530 cmil): 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V, which is 6.6%. 10 AWG: 4.97 V (4.1%). 8 AWG: 3.13 V (2.6%), within the 3% recommendation.

230 V, 16 A, 30 m, 2.5 mm² copper. 2 × 30 × 16 × 0.0178 ÷ 2.5 = 6.84 V, which is 2.97%.

How to interpret the results

A large voltage drop wastes energy and can make motors and electronics misbehave. Compare the percentage with the limit that applies where you live; the NEC checks only appear in imperial mode because they are US recommendations.

Tips and common mistakes

  • Use the one-way length; doubling it yourself counts the return path twice.
  • Size for ampacity first, then check voltage drop; both must be satisfied.
  • Long runs to outbuildings often need a larger conductor than the breaker alone suggests.
  • Aluminium needs a larger size than copper for the same drop.

Assumptions and limitations

The calculation uses DC resistance at typical temperatures and ignores reactance, which matters for large conductors and long runs. Electrical design and installation must be carried out by a licensed electrician in line with your local wiring rules.

Frequently asked questions

What is an acceptable voltage drop?

The US National Electrical Code recommends, in informational notes, no more than about 3% on a branch circuit and 5% for feeder plus branch. These notes are guidance rather than enforceable rules. Other countries set their own limits.

How do I calculate voltage drop?

For single-phase in US units: VD = 2 × K × I × L ÷ CM, where K is about 12.9 for copper, I is current in amps, L is one-way length in feet and CM is the wire’s circular mils. Three-phase replaces 2 with √3.

How is the metric formula different?

ΔU = 2 × L × I × ρ ÷ A for single-phase, with L in metres, ρ the resistivity in Ω·mm²/m (0.0178 for copper) and A the cross-section in mm².

What size wire for 20 A at 100 ft?

At 120 V, 12 AWG copper drops about 6.6%, 10 AWG about 4.1% and 8 AWG about 2.6%. To stay under 3% you would need 8 AWG. Wire size must also meet ampacity and protection rules.

Should I use one-way or round-trip length?

Enter the one-way distance from the panel to the load. The formula already doubles it for the return conductor in single-phase circuits.

Does temperature affect voltage drop?

Yes. Resistance rises with conductor temperature. The constants used here are typical values; a professional calculation may adjust for operating temperature.

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