Following on from my previous question about adding neutral wire.

I've read on here about neutral wires not being crossed between circuits, which makes sense after reading. But I'm trying to learn more and understand more about how neutrals work on a single circuit.

On a single circuit with multiple neutral wires, are those wires considered hub and spoke or can they be serial (or both)? For example, is it ok for a neutral wire to extend from one light to another on the same circuit or does each device need their own individual neutral tying back to the source neutral wire? Reason why?

  • 2
    A tree of any sort will do. No loops. The tree can branch, or not. It can branch many times.
    – Ecnerwal
    Sep 26, 2022 at 16:12

2 Answers 2


It really doesn’t matter. What is important is that the neutral follows the corresponding hot wire(s). This means either the same multi-wire cable or individual wires through the same conduit. The main reason for this is that this way, the magnetic fields generated by current through the wires cancel out; this prevents heating generated by the magnetic fields.

From there, you can have each set of wires running from outlet to outlet or each directly from the the breaker/fuse panel to a single outlet or any combination. Note that in all cases, the wires are connected in parallel: hot to hot to hot and neutral to neutral to neutral.

  • OK thanks. Would it be uncommon to find two lights sharing a neutral in serial where the neutral goes from source to light to light?
    – TheCleaner
    Sep 26, 2022 at 16:31
  • 2
    Assuming the hot follows exactly the same path, that is extremely common. Assuming the switch is near one switch, that is what I’d do. Whatever uses less copper.
    – DoxyLover
    Sep 26, 2022 at 17:29

OK, here's the thing. If you come from DC electronics, you're really accustomed to being able to route the + wire any way you want.... and the - wire any way you want... as long as it gets back to the battery or whatever.

That's not how it is with AC power. AC throws a considerable AC magnetic field because the current is considerable. Exploiting that is how transformers work. Those magnetic fields can cause lots of trouble if they are not managed. They can cause vibration in wires, and since copper and aluminum do not have a fatigue limit, that will crack them, causing hotspots or arcing. They can cause eddy current heating in metallic things nearby (including non-ferrous; inducing onto non-ferrous metal is how induction motors work).

So, in AC power installations it is required that current return the same way it came. This causes the equal and opposite magnetic fields to cancel each other out. All related conductors must be in the same cable or conduit. Documented in NEC 300.3.

A cable is several wires wrapped in a sheath.

So for instance if you have a bathroom fan+light, powered from the dual switch, you can use one /3 cable with red+black for the two switched hot wires, and the white for neutral. Whichever current goes up to the light/fan returns on the neutral, and so current is equal & opposite on the cable in all conditions.

Neutral MUST travel with the hot wires it is associated with. There is no option for running a network of neutral wires separately.

As a practical matter, this means cables in homes is in a tree topology. They can fork at any point, but they can never rejoin to form a complete "loop".

When you have power sources entering a box from different directions, e.g. violating the "no loops" principle, then you must keep each group of neutrals completely separate from each other. It's not required to use a physical box divider, but for novices it's not a bad idea.

  • Thank you for the detailed info! So they can fork at any point and continue on across multiple devices such as lights in serial as long as the wire can handle the load and the devices are meant to work simultaneously, right?
    – TheCleaner
    Sep 27, 2022 at 15:21
  • 1
    @TheCleaner right. Sep 27, 2022 at 19:07
  • Unless, of course, you are in the UK or anywhere else that blesses "ring" circuits.
    – Ecnerwal
    Sep 30, 2022 at 20:45

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