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.

حساب هبوط الجهد ثلاثي الطور: الاتزان وعدم الاتزان وتيار المحايد

ليس كل حمل خارج من لوحة ثلاثية الطور يحسب كحمل ثلاثي. محرك 400 او 480 فولت يحسب بين الخطوط، اما حمل 120 او 230 او 277 فولت فيحسب بين الخط والمحايد.