How to calculate voltage drop

Voltage drop is V = 2 × K × I × L ÷ CM for DC and single-phase circuits. Each term is set out below with a worked example you can check against the calculator.

The formula

V = 2 × K × I × L ÷ CM

V: voltage drop in volts.
K: the material constant, in ohm·circular-mils per foot. It is 12.9 for copper and 21.2 for aluminum, on the 75 °C basis used in the NEC voltage drop note.
I: load current in amps.
L: one-way length in feet, from the source to the load.
CM: conductor area in circular mils. A 12 AWG wire is about 6,530 CM.
2: the return conductor. The current goes out and back, so the loop is twice the one-way length.

For a three-phase circuit the 2 becomes √3, which is out of scope for this guide and the calculator.

Worked example

A 120 V single-phase circuit carries 15 A over 50 ft one way in 12 AWG copper. The target is 3%.

Example120 V · 15 A · 50 ft · copper

Step 1: loop length	2 × 50 ft = 100 ft
Step 2: resistance of the loop	K ÷ CM × 100 ft = 12.9 ÷ 6,530 × 100 = 0.1976 Ω
Step 3: drop	I × R = 15 A × 0.1976 Ω = 2.96 V
Step 4: percent	2.96 V ÷ 120 V = 2.47 %
Step 5: at the load	120 V − 2.96 V = 117.04 V
Result	Within the 3% target for 12 AWG.

The calculator computes the same numbers with the same formula.

Open this example in the calculator

Mistakes that cause wrong answers

Using the total loop length as the one-way length. Enter the distance from source to load only. The formula counts the return.
Mixing units. The constants are in feet and circular mils. Convert metres and mm² before you start.
Using K for the wrong material. Aluminum has a larger K, so it needs a larger size for the same run.
Forgetting the target. A drop can be correct for the formula and still over your chosen target.

Next steps

Run your own numbers in the calculator. Then check whether ampacity also needs a look on the ampacity guide, and the constants on the method page.