Neutral Problem ?

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hardworker

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I am working in a commercial building with older wiring. I am relocating circuits from an old panel to a new panel. This particular small subpanel has 3 circuits and only 2 neutrals.

I surmise 2 of the circuits are sharing a neutral, so I take the following steps to determine which of the two circuits are sharing a neutral.

I remove one neutral conductor from the bar to see what goes out. Nothing happens. All three circuits are still hot.

I put that neutral conductor back on the bar and remove the other neutral. Nothing happens. All three circuits are still hot.

I remove both neutral conductors from the bar and everything goes out, except one circuit has a slight amount of current still flowing, because the lights were still on, but very dim.

The wiring is in EMT. This old wiring has no ground conductor.

I have not yet traced all the wiring.

What would you think is the problem?
 
hardworker said:
I am working in a commercial building with older wiring. I am relocating circuits from an old panel to a new panel. This particular small subpanel has 3 circuits and only 2 neutrals.

I surmise 2 of the circuits are sharing a neutral, so I take the following steps to determine which of the two circuits are sharing a neutral.

I remove one neutral conductor from the bar to see what goes out. Nothing happens. All three circuits are still hot.

I put that neutral conductor back on the bar and remove the other neutral. Nothing happens. All three circuits are still hot.

I remove both neutral conductors from the bar and everything goes out, except one circuit has a slight amount of current still flowing, because the lights were still on, but very dim.

The wiring is in EMT. This old wiring has no ground conductor.

I have not yet traced all the wiring.

What would you think is the problem?
Click to expand...

They are using the EMT as an EGC. When you remove the neutral you now are putting return current on the EMT. With a bad connection in the conduit you could very well place over 180v on one of those circuits when you lift the neutral. Invest in a circuit tracer before you have to pay for a bunch of burned up equipment.
 
Never lift a neutral to see which circuit goes out. See post above. Clamp a meter around the neutral and turn the breakers off one at a time. When the current drops, maybe you are not sharing a neutral. Maybe. Sounds like you may have both neutals tied together in a box some place and/or they are using the conduit as stated above. No meters? Call for someone that has them. Minor detail, but you should not be working that panel hot.
 
hardworker said:
I am working in a commercial building with older wiring. I am relocating circuits from an old panel to a new panel. This particular small subpanel has 3 circuits and only 2 neutrals.

I surmise 2 of the circuits are sharing a neutral, so I take the following steps to determine which of the two circuits are sharing a neutral.

I remove one neutral conductor from the bar to see what goes out. Nothing happens. All three circuits are still hot.

I put that neutral conductor back on the bar and remove the other neutral. Nothing happens. All three circuits are still hot.

I remove both neutral conductors from the bar and everything goes out, except one circuit has a slight amount of current still flowing, because the lights were still on, but very dim.

The wiring is in EMT. This old wiring has no ground conductor.

I have not yet traced all the wiring.

What would you think is the problem?
Click to expand...

The problem is you don't understand multiwire branch circuits and what happens when you open the neutral. Be very if you did not burn something up connected to these ciruicts. Never open a loaded neutral especially if it is known to be carrying current from more than one ungrounded conductor.

When you opened the one that is shared between more than one ungrounded conductor you placed each 120 volt circuit in series with the other and applied 240 across the whole thing. If each side had equal resistance or close to equal then nothing abnormal is even noticed. If resistance of each side varies greatly then most of voltage will drop across the side with higher resistance easily putting something like 185 volts across one load and the remaining 55 across the other. If the neutrals were tied together someplace although wrong it still balanced voltage, but when you opened both neutrals you placed loads from all three branch breakers in a state where things could get a bad dose of wrong voltage.
 
The two neutrals are connected together on the load side of where you disconnected them. You will have to trace the wiring and separate them.
 
When you opened the one that is shared between more than one ungrounded conductor you

When you opened the one that is shared between more than one ungrounded conductor you

Ok, don't pull neutrals, use a meter to measure drop. Got it. Will do.

How do you ever get 240vt on a single device, that is only supplied by a 120vt circuit. For example an outlet that is supplied by 1 hot and 1 neutral. If you pull the neutral how do you get 240vt.?

How is it any different, when 2 circuits are sharing a neutral and you pull or lose the shared neutral. For example you have two outlets on the two separate circuits sharing the neutral. How would either of the two outlets every get up to 240vt?
 
hardworker said:
Ok, don't pull neutrals, use a meter to measure drop. Got it. Will do.

How do you ever get 240vt on a single device, that is only supplied by a 120vt circuit. For example an outlet that is supplied by 1 hot and 1 neutral. If you pull the neutral how do you get 240vt.?

How is it any different, when 2 circuits are sharing a neutral and you pull or lose the shared neutral. For example you have two outlets on the two separate circuits sharing the neutral. How would either of the two outlets every get up to 240vt?
Click to expand...

Watch 480Sparks' powerpoint.

http://forums.mikeholt.com/showthread.php?t=89280&p=711889#post711889

To read:

http://www.mikeholt.com/mojonewsarc...ngDangersMultiwireBranchCircuits~20020218.htm
 
hardworker said:
For example you have two outlets on the two separate circuits sharing the neutral. How would either of the two outlets every get up to 240vt?
Click to expand...
By virtue of the fact that there is 240 volts between the phase A bus bar and the phase B bus bar. To build a MWBC, you connect (let us say) the black wire to a breaker on phase A, run the wire to an outlet, and run a white wire back to the neutral bar. Then you connect (let us say) a red wire to a breaker on phase B, run the wire to another outlet, and run a short ?jumper? wire to the first outlet, so that the two outlets will share the one white wire that runs back to the neutral bar.

If you lift the white wire at the neutral bar, then current will flow from the phase A breaker, along the black wire to the first outlet, along the ?jumper wire? to the second outlet, along the red wire back to the second breaker, and thus returns to the panel at the phase B bus bar. Therefore, there will be a total voltage drop across the two outlets of 240 volts. Now the question is, how much of that 240 volts will be dropped across the first outlet, and how much across the second outlet. The answer will depend on the resistances of the two loads that are connected to the two outlets. If the two loads are identical, then the voltage across each will be the same (i.e., 120 volts). If the two loads have different resistance values, then the voltage will be higher across one than across the other. If the two loads have very, very different resistance values, then the voltage could be close to 240 volts across the one, and close to zero across the other.
 
240vt example

240vt example

Ok, very good explanation. I followed it with a diagram. If the outlets did not have anything plugged in, then I take the 240vt could not happen, because the jumper would not be able to transfer the two buses into one outlet. Am I correct.
 
If you removed the neutral at the panel, and if one of the two outlets does not have a load connected, then you do not have a complete circuit. The outlet with the load plugged in would not be able to run that load.
 
hardworker said:
I am working in a commercial building with older wiring. I am relocating circuits from an old panel to a new panel. This particular small subpanel has 3 circuits and only 2 neutrals.

I surmise 2 of the circuits are sharing a neutral, so I take the following steps to determine which of the two circuits are sharing a neutral.

I remove one neutral conductor from the bar to see what goes out. Nothing happens. All three circuits are still hot.

I put that neutral conductor back on the bar and remove the other neutral. Nothing happens. All three circuits are still hot.

I remove both neutral conductors from the bar and everything goes out, except one circuit has a slight amount of current still flowing, because the lights were still on, but very dim.

The wiring is in EMT. This old wiring has no ground conductor.

I have not yet traced all the wiring.

What would you think is the problem?
Click to expand...

Hopefully you have learned about shared neutrals here from the advice of the others. This is a really important electrical concept and it is imperative that it is well understood, as the damages can be very expensive. If it's just a 100 watt light bulb on each circuit, no big deal. If you have say, a 1500 watt heater on one side and an expensive TV on the other....well, you should get the idea. You are probably just lucky that the circuit you are working on was probably miswired with the 2 neutrals tied together elsewhere (as others pointed out) when you lifted the neutrals seperately at the panel. Might have saved you some damage.
 
charlie b said:
If you removed the neutral at the panel, and if one of the two outlets does not have a load connected, then you do not have a complete circuit. The outlet with the load plugged in would not be able to run that load.
Click to expand...

That works okay as long as there are only outlets on the circuits and no lights. If you are pulling neutrals to find out what feeds what then most likely you don't know whether both circuits have lights on them, right?
 
texie said:
This is a really important electrical concept and it is imperative that it is well understood, as the damages can be very expensive.
Click to expand...

I saw one house where the garage door machines, the furnace control board, and the AC unit control board all got fried.
 
jumper said:
I am sorry, I am a tired: Can you rephrase this question? I am not sure of what you are asking.
Click to expand...

It was a rhetorical question aimed at the OP. He was saying that if nothing is plugged into the outlets on the circuits then an open neutral wouldn't result in an overvoltage. I was pointing out that if the reason he is disconnecting neutrals is to discover what is plugged into them, then he can't be too sure beforehand that nothing is plugged into them.
 
jaggedben said:
It was a rhetorical question aimed at the OP. He was saying that if nothing is plugged into the outlets on the circuits then an open neutral wouldn't result in an overvoltage. I was pointing out that if the reason he is disconnecting neutrals is to discover what is plugged into them, then he can't be too sure beforehand that nothing is plugged into them.
Click to expand...

I think I get what you are saying. The simpified version of what I am thinking is if you are lifting neutrals to find what they supply and there is no connected load .... you are not going to find anything. If there is load it will stop operation, if there is MWBC load - the load(s) may be damaged.
 
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