Is it acceptable to have two circuits with a shared hot but different neutrals?

I am working on cataloguing the hodge podge electrical system in my old house. There is an old fuse box that was converted to a junction box when they put the breaker box in. Inside this box there is one hot coming from the breaker panel that is used to power two different loads. The strange thing to me is that the loads have different neutrals.

Here is a simplified diagram of what I found:

The red and blue loads are connected to a common hot, but they lead to different neutrals. As you can imagine there are a bunch of different circuits coming from the breaker panel.

The house has old steel conduit running from light to light in the attic and all these lines are THHN or older cloth-and-rubber wire in conduit. Nothing is Romex. Here is the conduit grouping:

Conduit 1: 3/4" steel conduit from breaker panel to junction box. Contains 9 THHN conductors including the hot and both neutrals shown in diagram.

Conduit 2: 3/4" steel conduit from junction box with red load hot and neutral.

Conduit 3: 3/4" steel conduit from junction box with blue load hot and neutral.

• Several professional electricians read this stack and I am sure that as soon as one of them recovers from the emotional shock of seeing that diagram he will quickly cite the NEC rules being violated. Jul 31, 2017 at 16:06
• I expect that you will end up abandoning one of the neutrals between the breaker box and the old fuse box, and just using one of the neutrals for both loads. Jul 31, 2017 at 16:10
• Are the neutral wires routed with the hot wires? In other words do both the red wire and blue wire in your drawing each have a white wire routed or cables with it? The second neutral from the panel to the J-box is unnecessary and not Code. Jul 31, 2017 at 16:27
• I think the electrical experts will focus on the potential for magnetic induction heating where cables pass through knockout holes in steel panels. Jul 31, 2017 at 16:31
• "Old"? Metal conduit is AWESOME. Don't you dare rip any of it out. Romex has one solitary purpose: to make houses cheaper. And anyone would rather work in conduit if they had it available to them. Also you get to use stranded THWN which is wonderful stuff to work with compared to that stiff, miserable solid-core wire. Jul 31, 2017 at 17:29

Generally speaking, no.

In order to do it this way, they probably ran individual(unsheathed) wires, which is against NEC. From this related question, the relevant NEC is

NEC 300.3(B) requires that:

All conductors of the same circuit ... shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord [with certain exceptions that do not apply here].

The logic here is that you want to minimize the exposure of the wires. Sheathing helps prevent a small nick from turning into a big shock.

If you're going to upgrade this the proper way, you would run, say, a brand new 14-2 to your red fixture, and then from the red fixture to the blue fixture. Thus they would share the same hot and neutral circuits properly

EDIT: Since these are inside conduit, that's considered acceptable.

• @ Machavity, thanks for the answer! I updated the questions with descriptions of what conductors are in what conduits. Jul 31, 2017 at 16:43
• Correct, you are not allowed to unsheath multiconductor cable and use it in conduit -- because the internal conductors are not built, marked and rated for use as individual conductors. E.G. the nylon outer insulation on THWN-2 does not exist on Romex strands. Jul 31, 2017 at 17:33

It's fine. But it helps to mark it so the next guy knows what you did. Here's the litmus test:

Are current flows equal at every point inside every cable or conduit?

If you have 1 hot returning current on 2 neutrals, and they're all in the same conduit, that is OK. If you have 7A going out on the hot, 5A coming back on one neutral, and 2A coming back on the other neutral, you are OK. You can test this a couple ways:

• put a clamp meter around all wires at every entrance/exit to a cable or conduit. If currents are equal, it should read essentially zero on full load.
• hook all the wires of a circuit in question to a GFCI breaker, and also have good discipline about keeping hots and partner neutrals together.