Voltage Drop Calculator

Calculate voltage drop in a copper or aluminum wire from the AWG size, current, length and system voltage, and find the smallest wire that keeps the drop under 3%. Example: 20 amps over 100 feet of 12 AWG copper on 120 volts drops 7.90 volts, or 6.58%.

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System

Conductor material

Wire size (AWG)

Current

amps

One-way wire length

feet

System voltage

volts

How to use the Voltage Drop Calculator

Choose single phase or three phase and the wire material.
Select the wire size and enter the current and one-way length.
Enter the system voltage.
Press Calculate to see the drop, percent and the smallest wire that stays under 3%.

Formula and method

Single phase: VD = 2 x K x I x L / CM. Three phase: VD = 1.732 x K x I x L / CM. K is 12.9 ohm-circular-mils per foot for copper and 21.2 for aluminum, I is the current in amps, L is the one-way length in feet and CM is the wire area in circular mils. Percent drop = VD / system voltage x 100.

Worked example: 20 A, 100 ft, 12 AWG copper, 120 V

Step	Value
2 x 12.9 x 20 x 100	51,600
Divide by 6,530 circular mils	7.90 V
Percent of 120 V	6.58%
Smallest wire for 3% or less	8 AWG

Circular mils by wire size

AWG	Circular mils
14	4,110
12	6,530
10	10,380
8	16,510
6	26,240
4	41,740
2	66,360
1/0	105,600
4/0	211,600

Reducing voltage drop

Use a larger wire, shorten the run, raise the system voltage, or reduce the load current. Copper drops less than aluminum of the same size. Always check that the wire is also rated for the current, since voltage drop and ampacity are separate limits.

Frequently asked questions

What is voltage drop?

The loss of voltage along a wire because of its resistance. It grows with current and length.

What voltage drop is acceptable?

The NEC recommends 3% or less on a branch circuit and 5% or less for the whole feeder and branch.

How do I calculate voltage drop?

Multiply 2 x K x current x one-way length, then divide by the wire circular mils.

Why use one-way length?

The formula multiplies by 2 for the return path, so you enter the one-way distance.

Is aluminum worse than copper?

Aluminum has about 64% higher resistance, so it drops more voltage for the same size.

How do I fix too much voltage drop?

Increase the wire size or shorten the run.

Sources

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