NEC Cable Sizing Commercial
Three-phase NEC sizing follows the familiar two-check process, pulled from the NEC’s own AWG/kcmil-based reference data.
Three-phase commercial feeder sizing under the NEC follows the same structural logic as under PEC or AS/NZS — ampacity and voltage drop checked independently — pulled from Table 310.15(B)(16) and the NEC's own voltage drop guidance, in AWG/kcmil units.
Three-phase current (A) = Power (W) / (√3 × Voltage × Power Factor)
As covered in the dedicated NEC derating articles, the full ampacity chain requires Table 310.15(B)(16) for base ampacity, Table 310.15(B)(1) for temperature correction, and Table 310.15(C)(1) for bundling adjustment — all three applied together, not selectively.
Harmonic-heavy commercial loads — variable-frequency drives, LED and electronic-ballast lighting, computer power supplies — can require a neutral sized equal to or larger than the phase conductors, the same consideration covered under PEC and AS/NZS, applied here using NEC's own specific provisions.
Run your three-phase commercial load through the NEC Cable Sizing calculator.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks