TradesHow this works
A big feeder can run as one large conductor or as several smaller ones in parallel, each set carrying its share of the load. This sizes both ways: it splits the load across the sets, finds the smallest conductor that carries each share, and checks the NEC 310.10(G) rule that anything paralleled has to be at least 1/0.
It also handles the raceway question. Bundle every set into one conduit and the extra current-carrying conductors trigger the fill adjustment, so the conductors size up. Give each set its own raceway and that penalty goes away. The recommendation leans toward the fewest sets that stay easy to pull, since a single conductor past about 4/0 is stiff and hard to land.
Common questions
What is the minimum size for parallel conductors?
NEC 310.10(G) sets the minimum at 1/0 AWG. Below that you cannot run conductors in parallel and have to use a single conductor per phase. Above 1/0, you can split the load across matched sets, which is how large feeders are usually run because a single conductor gets unwieldy.
Do the parallel conductors have to match?
Yes. Every set that makes up a phase has to be the same length, the same material, the same size in circular mils, and the same insulation type, and be terminated the same way. That is what makes the sets share the current evenly. Mismatched sets overload the shorter or smaller one, so this is not a place to mix leftovers.
Does putting all the sets in one conduit change the size?
It can. When every set shares one raceway, the current-carrying conductors add up fast, and once you pass three the NEC 310.15(C) adjustment derates them. That means the conductors have to be bigger to carry the same load. Running a separate raceway per set avoids the adjustment and can drop the conductor a size, at the cost of more raceways.
Sources: NEC (NFPA 70) 310.10(G) conductors in parallel (1/0 minimum) (2023) · NEC Table 310.16 ampacity + 310.15(C)(1) adjustment (via necData/deratingData) (2023). Engine version 1.0.0. Risk tier 2.