VOLTAGE ON NEUTRAL 277 MULTIWIRE BRANCH CIRCUIT

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Houston714

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Location
Nashville Tn
Hi all, how much voltage should be on a neutral (neutral to ground) that is on a multiwire branch circuit, whether at the panel or on a break at the neutral down stream. 45 lights. 277, 3 hots per neutral.
One light is A leg, one light is B leg, one light C is leg and runs like that all the way down to the end. I'm reading 200 plus volts on the neutral (neutral to ground) midway down. Thanks for some great input. I read on some post from years back that if there is load on the circuit I should see voltage.
 
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bwat

EE
Location
NC
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EE
Houston714 said:
Hi all, how much voltage should be on a neutral (neutral to ground) that is on a multiwire branch circuit, whether at the panel or on a break at the neutral down stream. 45 lights. 277, 3 hots per neutral.
One light is A leg, one light is B leg, one light C is leg and runs like that all the way down to the end. I'm reading 200 plus volts on the neutral (neutral to ground) midway down. Thanks for some great input. I read on some post from years back that if there is load on the circuit I should see voltage.
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You shouldn't see anywhere close to that high.

At the panel where the N-G bond is, it should be zero by definition. Out in the field, if there is a phase imbalance because of the MWBC, you'll see a small amount just because of voltage drop and the impedance of the neutral conductor back to the panel. However, depending on length, current, and size of wire this should be just a few volts between neutral and ground. Can you give more details?

And when you say 'break at the neutral' you don't mean the neutral is broken, do you? That changes things, and you'd read something closer to what you're seeing.
 
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Houston714

Member
Location
Nashville Tn
Yep! There are 60 lights. the first 15 are fine. Then at the break (stop working).... the next 45 are all out except for one about 20 down. Voltage from any leg to neutral is 277-281. But for some reason it would spike to 447 between any leg and the neutral. Then go back to normal voltage. So I checked neutral to ground (conduit/structural beam) and would get 200 plus up to 261. But it would only be for a brief few seconds and then drop to 4-6 volts.
 
charlie b

charlie b

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You have a serious problem on your hands. The voltage from neutral to ground should be very small, nowhere near 200 volts. I suspect there is a disconnected neutral wire somewhere along the circuit.

Think of a single circuit consisting of one light with an ungrounded conductor leading to the light and a neutral wire leading back to the panel. This can be viewed as a set of three resistors in series. The resistors are the ungrounded conductor, the light itself, and the neutral conductor. You might expect about 3-4 volts to be dropped along the ungrounded conductor, so there would be 273 volts available to the light. Then about 270 volts would be dropped while current passes through the light. That would leave about 3-4 volts to ground at the downstream side of the light. Those 3-4 volts would be dropped along the neutral conductor. So at no point along the neutral wire should you be able to measure a voltage higher than about 3-4 volts.
 
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Houston714

Member
Location
Nashville Tn
I was thinking it was maybe a bad connection, but at the light prior that does work, connections are fine.... They used connectors for 3 hots and neutral through each fixture all the way down the line. Any thoughts on how to resolve. We already are going to replace 45 blown drivers. Not sure where to start on finding the bad connection if the light that does work is fine before getting to the next one which is the one with the weird voltage readings.
 
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Houston714

Member
Location
Nashville Tn
THIS IS A POST FROM 2007

Break the circuit at that point and both sides of the break could potentially be 277V to ground.

Break the circuit and at the point of the open you should have 0-5 VAC on the end coming from the distribution panel (if there is any load still on this circuit). The other side of the circuit (towards the load) should read 277 VAC, if there is still load connected. If this is lighting and the light switches are off the reading should be 0 (zero).

Wouldn't the voltgage reading depend on WHERE this this point of open is in the circuit?
At the panel "0" and 277V
In the middle of the circuit 277V both ends?
Considering the switch is on.

POST FROM 2007

If you break the neutral on a circuit, the side of the break that is still electrically connected to the neutral bar in the panel will be near 0V, as determined by any voltage drop and current in the remaining portion of the circuit. The side connected to ungrounded conductors via any loads will be anywhere up to the full system voltage, depending upon the combination of loads on that side of the break.

ANY THOUGHTS.....
 
H

Houston714

Member
Location
Nashville Tn
Yeah I'll do that. Seems as though I did that already LOL. But will do it again... I was called out to look at the lights and troubleshoot them. I've never came across a situation like this one before. Always something new to learn.
 
R

Russs57

Senior Member
Location
Miami, Florida, USA
Occupation
Maintenance Engineer
I find it ironic that my most challenging neutral troubleshooting problem was in Nashville (at Hubbard hospital).

You are going to have fun and you are going to learn.

There comes a time for the brute force/shotgun approach. How long does it take to redo 180 wire nut connections vs the price of all those driver boards?

In my case, it seems they had left the wire ends stripped and ready to be made up, then spray painted the conduit to mark it as emergency power, and the bare copper ends got coated with paint. Then everything was made up and everything worked.....until one day it "browned out" and I got involved. Didn't help that it worked most of the time, like every time I was around to try and troubleshoot it.

At least your problem is remaining a constant so it shouldn't be hard once you verify how the wiring is laid out.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
ActionDave said:
You have an open neutral somewhere. If it is not in the last fixture that is working try the first one that is not working. Turn the power off on two of the breakers so you stop blowing up drivers.
Click to expand...
This is where I'd start, and what I would have said if not mentioned.
 
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