3-Phase Voltage Drop Calculator

Three-phase voltage drop formula

Three-phase drop uses the same resistance basis as single-phase but a different multiplier, because current flows on three conductors 120° apart rather than out-and-back on two:

R is resistance in ohms per 1000 ft, I is line current in amps, and L is the one-way distance in feet. √3 (≈1.732) replaces the 2× round-trip factor used on single-phase, which works out to three-phase drop being about 86.6% (√3/2) of single-phase drop for the same conductor, current, and length. For conductors below #2 AWG, R comes from the K-factor (R = K × 1000 / circular mils); for #2 and larger it comes directly from NEC Chapter 9 Table 9.

Worked example

A 60A three-phase feeder on #6 copper, 208V, 150 feet one way:

At 3.68% this exceeds the NEC 210.19 branch-circuit recommendation of 3%. It's acceptable only as part of a combined feeder-plus-branch budget under NEC 215.2. Stepping up to #4 copper brings it back under the 3% branch recommendation.

Common mistakes

• Using the single-phase 2× factor by mistake. Three-phase uses √3 (≈1.732), not 2. Using the wrong factor overstates drop by about 15%. Enough to reject a conductor that actually passes.
• Entering line-to-neutral instead of line-to-line voltage. Three-phase feeder and motor circuits are sized against the line-to-line (phase-to-phase) voltage (480V, 208V, or 600V), not the 277V or 120V line-to-neutral voltage that single-phase loads on the same system use.
• One-way vs round-trip distance. Enter the one-way run length. The calculator applies the √3 multiplier internally. Entering the there-and-back length double-counts the conductor.
• Ignoring motor power factor. This calculator assumes unity power factor. Motors run at a lagging power factor, which increases real-world drop beyond what this tool reports. Treat the result as a floor for motor branch circuits, not a guarantee.

Frequently asked questions

Why does three-phase use √3 instead of 2 like single-phase?

Single-phase voltage drop uses a factor of 2 because current travels out on one conductor and back on another. A full round trip. Three-phase circuits have three current-carrying conductors 120° apart, and the line-to-line voltage relationship between any two of them works out to √3 (about 1.732) times the phase resistance drop rather than 2×. The result is that three-phase drop is about 86.6% (√3/2) of what the same conductor, current, and length would show on single-phase.

Should I enter line-to-line or line-to-neutral voltage?

Enter the line-to-line (phase-to-phase) system voltage. 480V, 208V, or 600V for typical commercial and industrial services. This calculator computes drop across the line-to-line voltage, which is the standard basis for three-phase feeder and motor circuit design. Line-to-neutral voltage (277V, 120V) is a different calculation and applies to single-phase loads fed from a three-phase system, not to the three-phase circuit itself.

Does the NEC require voltage drop calculations on three-phase feeders?

No. The 3% branch-circuit and 5% combined feeder-plus-branch figures are informational notes in NEC 210.19(A)(1) and 215.2(A)(1). Recommendations, not enforceable requirements, unless a local jurisdiction adopts them by amendment. That said, three-phase feeders are exactly where voltage drop tends to matter most in practice: they carry the highest currents over the longest runs in a building, and motors are sensitive to under-voltage at the terminals.

What's an acceptable voltage drop for a three-phase motor feeder?

NEC 430 does not set a voltage-drop figure, but motors are unusually sensitive to it: starting torque falls with the square of terminal voltage, and low voltage raises running current and heat. Many engineering specs hold motor feeders to 3% and total system drop to 5%, tighter than the NEC informational-note baseline. This calculator assumes unity power factor; motors run at a lagging power factor, which increases real-world drop, so treat the result as a floor, not a ceiling, for motor work.

Why is my three-phase result lower than a single-phase calculator gave me for the same load?

That's expected, not an error. It's the √3 vs. 2 factor described above. For identical voltage, current, conductor, and distance, three-phase drop is always about 86.6% of single-phase drop. If you're comparing a real single-phase branch to a real three-phase feeder, the two aren't equivalent circuits anyway. Check that you're using this tool for the actual three-phase load, not spot-checking against a different circuit type.

When does three-phase voltage drop matter most?

Long feeder runs, high current, and undersized conductors. The same drivers as single-phase, but three-phase feeders typically carry more current over longer distances (service entrances, panel feeders, large motor branches), so the absolute drop can be significant even though the √3 factor is smaller than single-phase's 2×. Watch it closely on 208V systems especially, since the lower system voltage means the same volt-drop is a larger percentage than on 480V.

Related calculators

• Single-Phase Voltage Drop Calculator: For standard 120/240V AC branch circuits and feeders.
• Wire Size Calculator: Size a conductor for a target ampacity and voltage drop.
• Wire Ampacity Calculator: Conductor ampacity with temperature and conduit-fill derating.