Voltage Drop Calculator Formula

Understand the math behind the voltage drop calculator. Each variable explained with a worked example.

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Formulas Used

Voltage Drop

vdrop = cmils > 0 ? 2 * 10.8 * amps * distance / cmils : 0

Drop Percentage

vdrop_pct = cmils > 0 && voltage > 0 ? (2 * 10.8 * amps * distance / cmils) / voltage * 100 : 0

Voltage at Load

voltage_at_load = cmils > 0 ? voltage - 2 * 10.8 * amps * distance / cmils : voltage

Variables

VariableDescriptionDefault
ampsCurrent(A)20
distanceOne-Way Distance(ft)75
cmilsWire Area (Circular Mils)(CM)6530
voltageSource Voltage(V)120

How It Works

Voltage Drop Formula

Vdrop = 2 x K x I x D / CM

K is 10.8 for copper (12.9 for aluminum), I is current in amps, D is one-way distance, and CM is the wire cross-section in circular mils. The factor of 2 accounts for the round-trip path.

Worked Example

20A on 12 AWG (6,530 CM) copper over 75 ft at 120V.

amps = 20distance = 75cmils = 6530voltage = 120
  1. 01Vdrop = 2 x 10.8 x 20 x 75 / 6530 = 4.96V
  2. 02Percentage = 4.96 / 120 x 100 = 4.14%
  3. 03Voltage at load = 120 - 4.96 = 115.0V
  4. 04This exceeds 3%; consider upsizing to 10 AWG.

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