What voltage should I use for a three-phase voltage drop calculation?

Use the line-to-line voltage for a three-phase load: 208 V, 400 V, 415 V, or 480 V are common examples. For a line-to-neutral load on one phase, calculate that leg at the line-to-neutral voltage, such as 120 V on a 208Y/120 V system or 277 V on a 480Y/277 V system.

Why does the three-phase formula use 1.732?

The 1.732 factor is the square root of 3. It represents the phase relationship in a balanced three-phase circuit, where the line currents are 120 electrical degrees apart instead of sharing the same outbound-and-return path used in a single-phase two-wire calculation.

Is the NEC 3 percent voltage drop rule mandatory?

For general branch circuits and feeders, the common 3 percent branch-circuit and 5 percent combined recommendation appears in NEC informational notes, including NEC 210.19(A) and NEC 215.2(A). It is not usually a hard prescriptive rule, but many specifications and AHJs treat it as the expected design target.

How do I handle unbalanced three-phase voltage drop?

Do not rely only on the average three-phase current. Check the heaviest phase or calculate each line-to-neutral load path separately. On a 208Y/120 V panel with 38 A, 24 A, and 18 A single-phase loads, the 38 A leg usually controls conductor selection even though the average is only 26.7 A.

Does neutral current affect voltage drop?

Yes, when line-to-neutral loads are unbalanced or nonlinear. A shared neutral in a 3-phase, 4-wire wye system can carry imbalance current, and triplen harmonic current from electronic loads can add in the neutral. NEC 310.15(E) and IEC 60364-5-52 both matter when evaluating neutral loading.

When should I upsize a three-phase feeder for voltage drop?

Upsize when the calculated drop exceeds the project target, commonly 2 percent for a feeder on sensitive equipment or 3 percent for a general feeder. For example, a 120 A, 480 V, 3-phase load at 250 ft may push 1 AWG copper near the 3 percent range while 1/0 copper gives more operating margin.

三相壓降計算：平衡負載、不平衡支路與中性線檢查

三相壓降不能只看盤體是三相。真正決定計算方式的是負載：480 V馬達可用線電壓三相公式，120 V插座支路要按線對中性線檢查。