Past roughly 4/0, one giant conductor stops making sense — and at 400 A it stops being possible (500 kcmil copper tops out at 380 A). Enter the load and the advisor below compares 2, 3 and 4 parallel sets against the single-conductor option by ampacity, drop and dollars.
Three reasons, in the order they bite: physics — above 380 A there is no single building-wire answer in copper, so paralleling is mandatory, not optional; installability — 500 kcmil bends like a young tree and pulls like a fight, while 2 × 3/0 handles like wire; and money — smaller conductors cost less per circular mil and per termination, which the advisor's cost column makes explicit using your own per-foot prices.
NEC 310.10(G): paralleled conductors must each be 1/0 AWG or larger (the advisor never offers 2 × 4 AWG, however tidy the arithmetic) and identical in length, material, size, insulation and termination style — mismatched sets share current unevenly and the short, fat member quietly overheats. Run each set in its own raceway where possible: stacking all sets in one conduit triggers the 310.15(C) bundling derate (80% at 4–6 current-carrying conductors), eroding the very ampacity you paralleled for. And budget terminations honestly — each set adds lugs, gutter space, and a full-size equipment grounding conductor per raceway (250.122(F)).
Two sets halve the effective resistance, so paralleling is also a voltage drop tool: a long 200 A feeder that needs 350 kcmil for drop alone often pencils out cheaper and easier as 2 × 2/0. Click any row's Use this and the whole calculator — chart, verdict, ROI line, PDF report — recalculates on that configuration.
NEC 310.10(G) sets a hard floor: conductors smaller than 1/0 AWG may not be paralleled (narrow exceptions exist for control circuits). The rule exists because current-sharing tolerance gets worse as conductors shrink.
Not required, but standard practice — one raceway per set avoids the bundling derate and keeps the runs naturally matched. If sets do share a raceway, apply 310.15(C) and re-check the ampacity.
Industry practice keeps set lengths within about 10% of each other, tighter on big services. Current divides inversely with impedance, so a meaningfully shorter conductor carries more than its share — permanently.