NEC Cable Sizing Commercial
NEC Article 310 has specific requirements for running conductors in parallel, mirroring the same core principle covered under PEC.
As covered in the PEC parallel conductors article, very high current feeders eventually make single-conductor-per-phase sizing impractical. The NEC addresses this in Article 310, with its own specific requirements for how parallel conductors must be configured.
Parallel conductor sets must be the same length, same conductor material, same cross-sectional area, same insulation type, and terminated together at both ends — ensuring current divides evenly rather than concentrating disproportionately on whichever path happens to have lower impedance.
The NEC specifies a minimum conductor size below which paralleling isn't permitted at all — paralleling exists specifically to solve the practical problem of oversized single conductors, not as a general-purpose technique applicable to any circuit regardless of size.
Routing parallel sets through raceways of different lengths — even a seemingly minor routing difference changes relative impedance enough to skew current sharing away from the intended even split, undermining the entire justification for paralleling in the first place.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks