PEC Cable Sizing Commercial
Three-phase sizing follows the same two-check logic as single-phase, with a different current formula and neutral consideration.
Three-phase commercial feeders follow the exact same ampacity-plus-voltage-drop logic as single-phase circuits — but the current calculation itself and the neutral conductor question are genuinely different.
Three-phase current (A) = Power (W) / (√3 × Voltage × Power Factor)
Forgetting the √3 factor — or using a single-phase formula out of habit — is a common transcription error that produces a badly wrong design current before any sizing logic even runs.
A balanced three-phase load theoretically carries no neutral current at all. In practice, commercial buildings often carry meaningful harmonic content (from electronic ballasts, variable-frequency drives, computer power supplies) that shows up specifically on the neutral, sometimes requiring the neutral to be sized equal to or even larger than the phase conductors rather than assumed negligible.
The voltage drop formula changes shape too — it's not a simple doubling of the one-way resistance the way single-phase circuits calculate it, since three-phase geometry cancels some of that effect. Use the code's actual three-phase voltage drop method rather than adapting the single-phase formula by intuition.
Run your three-phase load through the PEC Cable Sizing calculator.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks