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I have a bathroom that was re-wired to add a bathroom fan some years ago. I am wondering if it was wired correctly. Originally, I believe the overhead light was wired in a switch loop to a single wall switch. When the fan was added, two new switches (for the fan, and the fan's light) were added on the wall.

To make this work, the electrician pulled a new wire pair from the switch to the overhead light. The old wire pair is now in parallel with the single circuit from the breaker panel, it provides power to the three wall switches. All of the neutral wires at the light are tied together, and all of the neutral wires in the wall are tied together. Because there are now two wire pairs between the overhead light gang box and the wall switch gang box, this creates a circular neutral path.

In the image, the neutrals are tied together at A and B, and the circle / parallel path for neutral is created by cables 1 and 2. Grounding wires are not shown.

wiring diagram

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    Could you provide a pic of the wiring ? Are there 2 or 3 switches if just 2 switches 3 cables the electrician swapped the switch loop to be the power to the fan and the other cable is for the light with the last one being the supply. This would be correct. Just because it was a switch loop doesn’t mean the wires weren’t changed , without a wiring diagram I would believe that the electrician did it right. But you state 3 switches and 2 devices so a better description of the wiring is needed not how you believe it is wired. – Ed Beal Aug 30 at 13:44
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    Break the neutrals at the fan / light and everything will be fine. I know parallel grounding is not allowed but grounded conductors might be ok, , the grounding issue parallel becomes a path for objectionable current on the grounding conductors. It’s one of those things that is difficult to prove if you don’t have the specific code reference handy. Understanding the grounded conductor vs the grounding is where we can start splitting hairs. – Ed Beal Aug 30 at 19:54
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    @Ed Beal that sounds like an answer. I understand that you mean that the four neutrals in the (one) wire nut should be separated into two wire nuts--one neutral for the fan and one for the light. Right? Looks like a simple solution and one wonders why the electrician didn't do that in the first place. Also there are two different places where there are four neutrals in one wire nut. Is it possible that this will fix this wrong wiring? – Jim Stewart Aug 30 at 21:18
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    They are on the same branch circuit you are right Jim. Split neutrals at light fan and I missed the extra at the other light. That needs to be pulled out of the bundle and wired direct to the lamp. And it should be good. I doesn’t have time to search code so I can’t provide a reference. Cable 1 is supply and cable 2 is hot neutral to the lamp. – Ed Beal Aug 30 at 21:29
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    At the fan / light and the light they are ok in the switch box. The neutral in the switch box and the hot is the source from that point. – Ed Beal Aug 30 at 22:10
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I think you've figured it out, but here it is in diagram form anyway, for future readers who may be more visually oriented:

made this an answer because cannot attach pic to comment

As you have indicated, you will separate the neutrals at two locations.

  • What about the four neutrals at Point A? Do those stay as they are in the OP's diagram – Jim Stewart Aug 31 at 1:47
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    @Jim Stewart: Yes. If you separate them in any way, at least some of the loads will stop working. – A. I. Breveleri Aug 31 at 1:49
  • Thanks for saving me 1/2 hour wiring an answer! +1 – Harper - Reinstate Monica Aug 31 at 5:31
  • In carrying out these changes how would one know which wires are to be separated? In the fan/light combo it is obvious what to do. In the region of Point B one separates the white in the same cable as the switched hot to the light. then lands that on the neutral side of the light. So in both places no electrical testing is required. Is this right? – Jim Stewart Aug 31 at 9:58
  • @Jim Stewart: You certainly wouldn't expect to do any more electrical testing. But at point B you would need to identify cable 2 out of the four cables coming in to the box -- as you say, it's the one whose hot wire is connected to the lamp. – A. I. Breveleri Aug 31 at 12:18
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This is not allowed. If I understand your description correctly, the neutrals are in parallel. So for example if only the fan is powered there will be current only in the hot conductor for the fan, but the current in the neutral side will be divided between two separate cables.

Parallel current paths for the neutral are not allowed because in any cable the current in the hot conductor(s) and the neutral conductor are supposed to be exactly the same (but they will be in opposite directions) so the magnetic fields cancel out.

EDIT Light and fan combinations are normally wired with a single cable which has a common neutral but separate conductors for each hot. The total of the currents in the two (or three) hots will equal the current in the common neutral.

This has come up numerous times on this site and there are code experts here who could give the reference.

EDIT2

This may be the reference (https://www.ecmag.com/section/codes-standards/web-code-requirements):

The basic requirement is found in 300.3(B): “(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment-grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).”

  • I added a diagram, but I think your understanding is correct. Could you provide a code reference or a link where I could read more to solve this on my own next time? Referencing the diagram, to correct the problem I think I should disconnect the neutral from cable 2 at point B, and make it connect only to the light's neutral in that gang box. I should also separate the neutrals at the fan itself. – John Aug 30 at 17:58
  • May be NEC 300.3(B). See my edit. – Jim Stewart Aug 30 at 19:54
  • Looks like @Ed Beal may have the solution to fix this circuit. See his comment above. I would have to draw my own diagram and ponder it for hours. – Jim Stewart Aug 30 at 21:36
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Since we have these very nice drawings, here's a great way of telling if you've complied with 300.3(B).

Open the drawing in a Paint program, Photoshop is overpowered but it will do. Select the "paint can" (autofill) tool. Click in an open area. If your drawing is 300.3(B) compliant, the paint will fill the entire diagram, except inside boxes and between wires in cables.

Note that on A.I.Breveleri's diagram it will do just that.

This mistake arose because the last guy didn't understand: Neutral is not ground. The guy saw all the grounds glommed together just like that (which is fine), and sees neutrals clustered onto a bar just like grounds (often the same bar), and figured "Neutral is [like] ground". And you can get away with that safety defect because nothing tells you not to. At least until you install a GFCI device.

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