Voltage calculator: voltage drop and Ohm’s law
Check the voltage drop on a wire run and see the smallest wire size that keeps it under 3%. Below that, solve volts, amps, ohms and watts from any two. Everything is calculated in your browser.
Voltage drop
Ohm’s law results
Estimates only. Resistance values are NEC Chapter 9 Table 8 (DC, 75 °C, uncoated copper and aluminum). Real drop can be a little different with AC, conduit type and temperature. The 3% / 5% guideline is an NEC informational note, not a requirement, though some local codes and energy codes require it. Verify with your local code and a licensed electrician.
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Worked example
A 20 A load on 12 AWG copper, 100 feet away, at 120 V single-phase. Table 8 gives 1.93 ohms per 1,000 feet.
Voltage drop = 2 × 20 A × 1.93 × 100 ÷ 1,000 = 7.72 V. That is 6.4% of 120 V, so the load gets about 112.3 V.
That is over 5%. 8 AWG copper brings it under 3%.
How the math works
- Single-phase: volts dropped = 2 × amps × ohms per 1,000 ft × one-way feet ÷ 1,000.
- Three-phase: the same, with 1.732 (√3) in place of 2.
- % drop = volts dropped ÷ system voltage × 100. Voltage at the load = system voltage − volts dropped.
- Ohm’s law: volts = amps × ohms. Watts = volts × amps. For AC motors and other non-resistive loads, watts also depend on the power factor.
Questions
What is the maximum voltage drop allowed by the NEC?
The NEC suggests 3% for a branch circuit and 5% total for the feeder plus branch circuit, in informational notes (for example under 210.19 and 215.2). They are recommendations, not requirements, but many engineers, energy codes and inspectors use them.
Should I use one-way or round-trip length?
Enter the one-way length from the panel to the load. The formula already counts both wires for single-phase (the 2 in the formula).
Why is aluminum wire one or two sizes bigger?
Aluminum has about 60% more resistance than copper of the same size, so it drops more voltage. That’s why aluminum feeders are usually upsized.
Does this calculator pick the wire size for the amps?
No. It only checks voltage drop. Pick the wire for the amps from NEC 310.16 and the terminal ratings first, then use this to see if you need to go bigger for the distance.
Does the calculator save my numbers?
No. Everything is calculated in your browser. Nothing you type is sent to us or saved.