Parallel Conductors: The 1/0 Club

Past a certain size, pulling one giant conductor stops making sense — 500 kcmil bends like a sapling and terminates like a wrestling match. The code's answer is paralleling: two or more conductors per phase sharing the current.

The membership rules — NEC 310.10(G)

Paralleled conductors must be 1/0 AWG or larger, and each parallel set must be identical: same length, same material, same size, same insulation, same termination style. The reason is current sharing — paralleled paths divide current in inverse proportion to impedance, so a shorter or fatter member quietly hogs more than its share and overheats while the code-math says everything is fine.

What paralleling does to drop

Two sets halve the effective resistance — drop and energy loss both halve. The calculator's parallel-sets field models up to eight sets. The interesting comparison: two 1/0 copper (2 × 105.6 kcmil = 211 kcmil of metal, 0.061 Ω/kft effective) versus one 4/0 (211.6 kcmil, 0.0608 Ω/kft) — nearly identical copper and resistance, but the parallel pair is far easier to pull, terminate and source. Above ~250 kcmil equivalent, parallel usually wins on installed cost.

The fine print that bites

• Ampacity derating: parallel sets in one raceway means more current-carrying conductors — the 310.15(C) adjustment factors apply. Separate raceways avoid it but must be arranged identically.
• Each conductor still needs ampacity for its share: two 1/0 copper at 75 °C = 2 × 150 A = 300 A ceiling.
• Equipment grounds are sized full-size in each raceway per 250.122(F).

Model your candidate set in the 3-phase calculator — sets and power factor included — before committing to the copper order.