Voltage Drop Calculator

Calculate voltage drop in an AC or DC circuit given wire gauge, voltage, current, and length. Determine the correct size for a circuit including the minimum wire gauge and maximum conductor length given an allowable voltage drop.

Calculate Voltage Drop



Calculate Minimum Conductor Size



Calculate Maximum Conductor Length




What is Voltage Drop

Voltage drop is the amount of voltage lost in a circuit due to resistance of the conductor. Voltage drop is an important consideration when planning a circuit to allow equipment using the circuit to run as designed. Excessive voltage drop could result in damage to equipment and devices or excess heat introducing in a fire hazard.

How to Calculate Voltage Drop

Voltage drop can be calculated using the following formula:
voltage drop VD = (M × K × I × L) ÷ CM

“M” = phase multiplier: Use 2 for a single phase or DC circuit and 3, or 1.732, for a three phase circuit.

“K” = direct current constant: Use 12.9 for a copper conductor and 21.2 for an aluminum conductor. This is equal to the resistance of a conductor that is one thousand circular mils and one thousand feet in length.

“I” = current: This is the current of the circuit in amps. Use our Ohm’s Law calculator to convert from watts to amps.

“L” = length in feet: This is the one-way length of the conductor in feet. Use our length conversion calculators to convert from metric measurements to feet.

“CM” = cross-sectional area: This is the cross-sectional area of the conductor in circular mils. Use our wire gauge calculator to find the area of a conductor in kcmil. To convert kcmil to circular mils multiply kcmil × 1000.

For example: Calculate the voltage drop of a 120V circuit drawing 15A using a 25′ long 14AWG copper conductor.

A 14AWG wire is 4.1067 kcmil which is 4106.7 circular mils.
VD = (M × K × I × L) ÷ CM
VD = (2 × 12.9 × 15 × 25) ÷ 4,106.7
VD = 9,675 ÷ 4,106.7
VD = 9,675 ÷ 4,106.7
VD = 2.35 volts

How to Estimate the Conductor Size Needed for a Circuit

Using the equation for voltage drop and a little algebra the minimum conductor size in circular mils for a circuit can be found using the following:
circular mils CM = (L × M × K × I) ÷ voltage drop
kcmil = CM ÷ 1,000

Substitute values in the formula to find the cross-sectional area in circular mils, then divide by 1,000 to find the size of the conductor in kcmil required. Use our wire gauge size chart to find the wire gauge with the correct cross-sectional area.

For example: Find the minimum wire gauge needed for a 120V circuit drawing 20A using a 40′ long copper conductor with a max voltage drop of 3%.

A 3% voltage drop would be 3.6 volts.
kcmil = ((L × M × K × I) ÷ voltage drop) ÷ 1000
kcmil = ((40 × 2 × 12.9 × 20) ÷ 3.6) ÷ 1,000
kcmil = (20,640 ÷ 3.6) ÷ 1,000
kcmil = 5,733 ÷ 1,000
kcmil = 5.733
12 AWG

How to Determine the Maximum Length of a Circuit

The maximum length of a conductor in a circuit can be determined by re-writing the formula for voltage drop like this:
L = (VD × CM) ÷ (M × K × I)

Just as before substitute known values in the formula to get the length in feet.

For example: Find the maximum conductor length for a 120V circuit drawing 15A using a 14 AWG copper conductor with a max voltage drop of 3%.

A 3% voltage drop would be 3.6 volts.
A 14 AWG conductor has a cross-sectional area of 4,107 circular mils.
L = (VD × CM) ÷ (M × K × I)
L = (3.6 × 4,107) ÷ (2 × 12.9 × 15)
L = 14785.2 ÷ 387
L = 38.2 ft

Also check out our electricity cost calculator to see how much it will cost to power a device.