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.

Расчет падения напряжения в трехфазной сети: баланс, перекос и нейтраль

Практическое руководство для систем 208, 400 и 480 В: трехфазная формула, несимметрия фаз, ток нейтрали и проверки NEC/IEC.

Hommer Zhao

Три фазы Падение напряжения Нейтраль NEC 215.2 IEC 60364

Не каждый отходящий кабель из трехфазного щита нужно считать как трехфазную нагрузку. Двигатель 480 В считают по линейному напряжению, а розеточную или осветительную ветвь - по фазному напряжению.

Расчет падения напряжения в трехфазной сети: баланс, перекос и нейтраль

Для симметричной нагрузки используйте линейное напряжение, ток и длину в одну сторону, затем сравните результат с 3% для ветви и 5% суммарно по NEC 210.19/215.2.

При токах 38 А, 24 А и 18 А среднее значение 26,7 А не подходит для выбора проводника самой нагруженной фазы. Нужно проверить фазу 38 А и ток нейтрали.

Load	Voltage	Method	Check
480 V motor	480 V line-line	Balanced 3-phase	Running drop + starting dip
208Y/120 V receptacle	120 V line-neutral	Single-phase leg	Worst phase and neutral
400 V pump	400 V line-line	Balanced 3-phase	IEC 60364-5-52 method
277 V lighting	277 V line-neutral	Single-phase branch	Driver input range
Mixed 208 V panel	208 V + 120 V checks	Hybrid review	Unbalance and farthest load

NEC / IEC checkpoints

NEC 210.19(A), NEC 215.2(A), NEC 310.15(C)(1), NEC 310.15(E), NEC 430, and IEC 60364-5-52 should be reviewed beside the numeric voltage-drop result. Voltage drop is a performance check; ampacity, temperature, grouping, terminal ratings, and equipment rules still control whether the installation is acceptable.

Сначала считайте фидер как трехфазный, затем отдельно проверьте самый дальний однофазный путь.

Voltage Drop Calculator Wire Size Calculator

Расчет падения напряжения в трехфазной сети: баланс, перекос и нейтраль

Расчет падения напряжения в трехфазной сети: баланс, перекос и нейтраль

NEC / IEC checkpoints

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