FFieldDeskElectrical and HVAC

Voltage drop · screening

Check voltage drop on a one-way run.

Drop uses 2 × K × I × D / cm for single-phase and 1.732 for three-phase. K is 12.9 copper and 21.2 aluminum. D is one-way feet.

Input values to calculate

Calculated results

Amperes the wire must carry
25.0 A
Wire for those amperes
12 AWG
Wire that also checks drop
12 AWG
Estimated drop
6.32 V · 2.63%
Within the limit
Yes
Report calculation

What this sheet does

  1. You enter the load in amps, the voltage, and how far the wire runs. Distance is one way.
  2. If the load runs for three hours or more, leave the continuous box on. The sheet adds 25% before it picks a wire.
  3. It finds the smallest wire that can carry those amps.
  4. It then checks how much voltage that wire loses on the run.
  5. If the loss is over the limit you set, it steps up to a larger wire until the loss is inside the limit, or until the table runs out.

Worked example: 20 A, 240 V, 80 ft

The distance is one way. At 20 A, not continuous, 14 AWG copper can carry the amperes, but the drop is about 10 V, or 4.2%. That is over a 3% limit.

12 AWG copper drops about 6.3 V, or 2.6%, which is inside 3%. Three percent on a branch and five percent total is the usual recommendation, not a code maximum.