Voltage Drop Calculator

This page estimates voltage drop from the direct-current resistance of the conductor. It is the approximation electricians punch into a calculator when they want a fast check, not a power-system study.

For single-phase, and for a two-wire look at one leg of a 120/240 V circuit, the drop in volts is 2 × K × I × L ÷ CM. The 2 covers the go and return conductors. For three-phase, the constant is the square root of 3 instead of 2, because the phase conductors are 120 degrees apart: √3 × K × I × L ÷ CM.

• K is 12.9 for copper and 21.2 for aluminum, in ohm-circular mils per foot, near 75°C.
• I is the load current in amperes.
• L is the one-way length in feet. Meters are converted before the formula runs.
• CM is the conductor area in circular mils.

Percent drop is that voltage divided by the voltage you typed, times 100. People often try to keep a branch circuit near 3% and the feeder plus the branch near 5%. Those are common guidelines for reasonable operation. They are not a mandatory maximum this site can certify, and a result inside them is not a code compliance certificate.

Changing the inputs needs JavaScript. The worked example is also written out below, so you can check the arithmetic with the page open and the network off.

The form opens on this run: single-phase, 240 V, 30 A, 100 ft one way, copper 10 AWG (10,380 circular mils), K = 12.9.

VD = 2 × 12.9 × 30 × 100 ÷ 10,380 = 77,400 ÷ 10,380 = 7.456647 V, reported as 7.46 V. That is 3.106936% of 240 V, reported as 3.11%. The guideline flag is above 3%, within 5%: over the usual 3% branch figure, still inside the usual 5% combined figure.

The same function on a three-phase check:

480 V, 100 A, 200 ft, copper 3/0 AWG (167,800 circular mils). VD = √3 × 12.9 × 100 × 200 ÷ 167,800 = 2.663106 V, reported as 2.66 V, which is 0.55% of 480 V. That one sits within 3%.

If your phone shows different rounded digits, compare the substituted line in "Show the work" with 2 × 12.9 × 30 × 100 ÷ 10,380. The headline volts and percent are rounded to two decimal places.

• Direct-current resistance only. Power factor, inductance, and the extra AC resistance of large conductors are left out. On a long run of big wire at low power factor, the real drop can be higher than this estimate.
• K is one approximate 75°C number. A hotter conductor drops more voltage. A cold one drops less. The tool does not adjust K for temperature.
• Circular mils are nominal areas for the size you picked, not a measurement of a specific cable.
• Enter one-way length. The 2 or √3 already accounts for the path through the conductors. Doubling the length yourself double-counts the return.
• Percent uses the voltage you typed as the base. It does not solve a loaded panel voltage or a multiwire unbalance.
• It does not size the wire for ampacity, terminations, continuous load, ambient temperature, or fault current. Use the wire size calculator as a separate screen, then the code book.
• It is not a substitute for the NEC, the manufacturer's data, or the authority having jurisdiction.