Voltage Drop Calculator
Choose single-phase or DC, or three-phase. Enter the system voltage, line to line for three-phase, then the current in amps and the one-way length of the run in feet. Pick a wire size from 14 AWG to 2/0 and copper or aluminum, then press Calculate Voltage Drop. You get the drop in volts and as a percentage of the supply, the voltage left at the end of the run, and how the percentage compares with the 3% and 5% in the NEC informational note on voltage drop. The result is an estimate and does not replace an electrician.
How to use the Voltage Drop Calculator
- Enter your numbers in the Voltage Drop Calculator above.
- Press the Calculate Voltage Drop button to see the result.
- To try other numbers, change any value and press the button again.
The voltage drop formula
V_drop = 2 × L × I × R (two-wire circuit) · · · V_drop = √3 × L × I × R (balanced three-phase) · · · R from the AWG table, e.g. 12 AWG copper = 1.588 Ω per 1,000 ft
The 2 counts the wire out to the load and back. L is the one-way length in feet, I the current in amps and R the resistance per foot. The calculator uses the resistance of solid copper wire at 20 °C (68 °F) from the American wire gauge table on Wikipedia, which notes that these are not the NEC Chapter 9 Table 8 values. For aluminum it multiplies the copper figure by 1.61. For a three-phase circuit, √3 (about 1.732) takes the place of the 2, and the voltage is line to line.
Worked example
100 ft of 12 AWG copper carrying 15 A: V_drop = 2 × 100 × 0.001588 × 15 = 4.764 V. On a 120 V circuit that is 4.764 / 120 = 3.97%, leaving 115.24 V at the end of the run. The calculator marks this as within 5%, the figure for feeder and branch circuit combined, but above the 3% for a branch circuit. Three-phase: 200 ft of 8 AWG copper carrying 40 A at 480 V line to line gives √3 × 200 × 0.0006282 × 40 = 8.705 V, or 1.81%, leaving 471.3 V.
Frequently asked questions
Why does voltage drop matter?
Informational Note No. 4 to NEC section 210.19(A) says branch circuit conductors sized to keep the drop at the farthest outlet within 3%, with no more than 5% on feeders and branch circuits together, "provide reasonable efficiency of operation." The calculator compares your result with those two figures. IAEI Magazine points out that informational notes are explanatory and not mandatory Code rules, and that other sections, such as 647.4(D) and 695.7, do require a calculation. Section numbers can change between NEC editions. The lost voltage turns into heat in the wire.
How do I fix excessive voltage drop?
Use a thicker wire (a lower AWG number), shorten the run, reduce the load or use a higher voltage. For the same power, 240 V draws half the current of 120 V, so the drop in volts halves and the drop as a percentage falls to a quarter. You can try each change in the calculator.
Aluminum vs. copper?
The calculator treats aluminum as 1.61 times the resistance of copper of the same gauge, so the same run drops 61% more voltage. It shows the drop for the wire you pick and does not choose a wire size for you.
How do I calculate three-phase voltage drop?
Choose Three-Phase and enter the line-to-line voltage. The calculator uses √3 × L × I × R, with L as the one-way length, so the same wire and current drop less than on a two-wire circuit: 8.705 V against 10.051 V in the example run. The formula holds for a balanced load only, so the figure does not apply to an unbalanced one.
Why can the real voltage drop be higher than this result?
The resistances here are for wire at 20 °C, and wire in service runs warmer. EC&M works a three-phase example at 75 °C: 80 ft of No. 1 aluminum at 100 A on 208 V, which drops 3.5 V. This calculator gives 2.764 V for the same run. On AC, 2/0 wire also needs an adjustment for skin effect, which the calculator leaves out, and it ignores power factor and reactance.
Can I size a circuit with this calculator?
No. It estimates voltage drop and nothing else. It does not check ampacity, breaker size, terminal ratings or your local code, and a low drop does not make a wire safe for the load. It does not replace an electrician. Have a licensed electrician check the circuit before you install or change wiring. Your local inspector decides what the code requires.