Is it legal to put hots and neutrals in separate conduits?

No. NEC 300.3(B) requires all conductors of a circuit, including the neutral, to run in the same raceway or cable, and 300.20 requires grouping in metal raceways to prevent inductive heating.

What happens if AC conductors are separated in steel conduit?

The magnetic fields no longer cancel, inducing circulating currents that heat the conduit and raise circuit impedance. Parallel runs must contain a full set of phases, neutral, and ground in each raceway per 310.10(G).

Can You Run Hots in One Conduit and Neutrals in Another?

A user asked the app whether all the hots can go in one conduit and all the neutrals in a separate one. No. NEC 2023 300.3(B) requires all conductors of the same circuit, including the grounded conductor and equipment grounding conductors, to be in the same raceway, cable, or trench. Splitting hots and neutrals into different conduits is a straightforward violation, and with metal raceways it creates a genuine physical problem, not just a paperwork one.

The induction problem

Current flowing out on the hots and back on the neutrals produces magnetic fields that cancel when the conductors travel together. Separate them into two steel conduits and each raceway surrounds a net current. The alternating field induces circulating currents in the conduit walls, and 300.20(A) addresses exactly this by requiring conductors of the same circuit to be grouped in ferrous enclosures. In practice, separated AC conductors in steel conduit make the raceway heat up like a transformer core; on larger feeders the conduit can get too hot to touch and the added impedance chokes the circuit. This is also why a single conductor of an AC circuit may not enter a ferrous enclosure through its own individual knockout without slotting between holes per 300.20(B).

What inspectors will cite

Beyond 300.3(B) and 300.20, separating the neutral breaks the effective ground fault current path concept in 250.4(A)(5) for the grounded conductor's role at the service, and it wrecks the ability of breakers, especially GFCI and AFCI devices, to see balanced current. Multiwire branch circuits add 300.13(B), which prohibits depending on device terminals for neutral continuity, and 210.4(D) requires grouping hot and neutral of each multiwire circuit with ties where they enter the panel.

The narrow exceptions

Parallel installations under 310.10(G) allow a circuit to occupy multiple raceways, but each raceway must contain a complete set: every phase, the neutral, and the equipment ground, all matched in length and size. Isolated phase installations in nonmetallic raceways get a limited allowance in 300.3(B)(1) for single conductor cable arrangements, mostly seen in large underground services. For ordinary branch circuits and feeders, the rule is simple: every circuit travels as a complete family in one raceway.

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