Shared Neutral problem


Old 09-24-19, 01:55 PM
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Shared Neutral problem

A couple weeks ago I replaced 2 old florescent fixtures with new LED ones. These fixtures are on two separate switches and run parallel to each other about 4' apart. The old fixtures were very old, had ballast problems, and the molded plastic ends were brittle & breaking apart. The building is a commercial construction fraternity house with the wiring in that area probably 50 years old. All wiring is contained in conduit.

Within the old fixtures someone had made several wire splices like they cut the wrong wires (fixture wires) at some point & had to reconnect. There were several splices contained in those old fixtures and even sections of unused wires. Those old fixtures spanned across 2 separate junction boxes. Supply power and neutral came from one junction box but a random white (neutral) also came / went from the other box at the other end.

So I on my new LEC install, I wired completely from the 1st junction box, and then wirenutted off the random neutral in the other box & put a cover over it. Now I find that lights in another adjacent room do not work. Thus I believe the random neutral in the other junction box is a shared neutral with the other room. Some of those splices in the old fixture were probably to keep the shared neutral to the other box.

The problem is that access to the back side of any of these junction boxes is very difficult, with the only way being cutting access holes in the ceiling panels along side the fixtures & boxes, which I really don't want to do. At this point I see 2 options:

1. Cut the access holes in the ceiling & running new flexible conduit between the two junction boxes thus extending the neutral between the two boxes.

2. Adding another new LED fixture to the ends of the fixtures I just put up. The new fixtures are connectable (via pin connectors at each end) up to 8 units, thus my total length will span across both junction boxes. That leaves an unused neutral wire from that last LED fixture to connect the stray neutral in the end junction box.

The location is a couple hours drive away, so I am trying to figure out what I need before going to fix this.

Is option 2 acceptable? Codes it meet code? Will it work? My worry is that the neutral i'm going to connect to at the end might not work. Might there be some type of stepped down voltage due to the LED transformer or circuit board from that 1st connected fixture thus not providing a clean neutral connection for the circuit in the other room. Here is the link to the new LED fixtures.

Your help is greatly appreciated. Thanks Mike
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Old 09-24-19, 06:37 PM
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This is a fraternity house ? On a school campus or property ?
Be careful..... this can be a shaky area for a DIY'er. Many places like this require a licensed electrician due to insurance liabilities.

It is very common to see wires spliced within a fluorescent fixture. If the fixture is indeed 50 years old.... the ballast has more than likely been replaced at least once.

It's very hard to follow what is going on there. If the light is connected to a circuit in conduit...... there needs to be at least one white and one black (hot color) coming out of the conduit to the light. If there were two white wires.... possibly they used the fixture as a splice point but that is not typically done in a conduit system.

Why would you need access to the back of the boxes ?
If they are interconnected by conduit..... run the correct wiring or trace what is there.
Old 09-25-19, 06:14 AM
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Except within a switch loop you may not have hot from the panel coming from one direction (conduit) and neutral back to the panel coming from another direction (conduit).

Specifically, if a neutral to a room goes bad then you may not run a neutral by itself from an outlet box or fixture in the next room as a remedy.

If a neutral goes off to serve another light fixture then the hot for that light fixture must go with it (in the same cable or conduit).

The switch loop exception: When power comes to a light fixture, raw hot, switched hot, and neutral for dimmers or continuation go together down to the switch. A second white wire does not come back from the switch box but rather neutral from the panel is connected directly to neutral for the light (and to the single white wire going to the switch box but not to the switch itself) at the light fixture's junction box.

Older switch loops may have just two wires, white and black, both connected to the switch but the white wire is connected to hot, not to white, up at the light fixture.

In most areas, commercial, multi-family, and non-owner occupied buildings and residences need a licensed electrician.

Before disconnecting any wires anywhere, label them. The information you write down should at least be enough that you could put everything back the way it was. Many simple tasks turn into major projects just because of the lack of such labeling.


Last edited by AllanJ; 09-25-19 at 06:42 AM.
Old 09-25-19, 08:07 AM
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PJ & Allen, thanks for the reply. Also I understand & appreciate the comments about working in a building / structure such as this.

Before I did any work, I checked with local City codes (actually spoke with the codes person) to see if a licensed electrician is required. City code allows a non-electrician to make simple replacements such as replace existing broken fixtures & switches with new, but no new wiring.

Pete, in regards to the conduit lines being connected. I am not sure as I cannot see into that section of the structure, however that would be nice. In other sections of the building, the conduit was attached to the underside of the decking above or to the steel trusses, then junction boxes were placed along those main lines to allow for flexible conduit directly down to light fixtures or ceiling boxes. Again, while I cannot see in this section, I think the two junction boxes are not directly connected.

Allen, I will try to explain further. Within the room I am in with the new fixtures, there are 2 ceiling boxes. Box A has the hot & neutral coming from the switch & it is the hot & neutral for the new fixtures. Box B is 8' away and has some hot wires running thru it, then a white neutral, which at this time is not connected to anything. That white neutral in Box B used to be connected to the white neutral from Box A, but not directly via conduit run, but rather going within the old fixture. It was also not spliced directly with the building neutral from A, but rather at the far end of the old fixture to one of the white wires of that fixture. I do believe (I have not checked / tested those hots) that one of the sets of hot wires running thru Box B then goes directly to the adjacent room. Thus the Hot & neutral are together once the begin their run to that room. I don't know but believe those hots in box B do not run within or thru the conduit from box A. But rather come via some other run.

So Allen if I read you correctly, what is the fix to this situation? The prior neutral was not ran previously within the conduit to Box B and I do not believe Boxes A & B are connected via conduit. One of my suggested fixes was to run new conduit run between Boxes A & B, with only a neutral within it.
Can you guys suggest a fix? Thanks Mike
Old 09-25-19, 10:12 AM
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Some quick notes:

All junction boxes must be accessible. While they may be above suspended ceilings, the ceiling panels need to be removable.

Specifically you may not run the conduit with just a neutral inside. The matching hot wire(s) have to accompany it.

You will need to analyze box B to find out which hot wire is hot from the panel, which hot wires are continuations to other boxes or lights or devices, and which hot wires, if any, are switch controlled hot from a switch box.

You need to find out how the hot wire from the panel to box b got to box B. The neutral for box B has to have come the same way.

I am not sure whether the interior of a light fixture can be a junction box, namely if the hot supply for box B was connected to the hot in Box A but was routed through the light fixture. If so then you can temporarily run the neutral
Old 09-25-19, 10:26 AM
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Allen, thanks for the quick reply. Your right about the access issue, I had not thought about the differences. In the other parts of the building, the ceiling was a drop ceiling, this is not. Those other boxes were accessible thru the dropped ceiling. This is not, so the conduit runs must be different. I have a lot more sleuthing / investigating to do before I attempt this fix. Maybe a small access hole from the adjacent room will give me some visual help from which to begin.

I do not have the tools to trace a particular hot from one box to the next. I might just end up hiring an electrician to get this done. I will do some more investigating, then make a decision. Thanks for your help here.

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