Would like some input please

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bsmith8691

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Ellington, CT
I currently have an 8 floor apartment building. There are two three phase 120/208 owners panels that feed common areas with a common basement. The building was built and wired in 1963. I have a 1 inch rigid pipe run in the slab that goes directly from the panel to a 12 x 12 box in a utility closet. The conduit has 8 Branch circuits and three neutrals. I have a situation where phases A and C will trip when both of them are turned on. They work independently of each other. They feed LED stairwell lights and a Fob entry system. On a continuity check they do not ring phase to phase or phase to ground or phase to neutral. Because of the ampacity, at one point someone put three circuits on A phase and 3 circuits on C phase. I split each circuit and put it on its own breaker. I still have what appears to be a phase to phase short no matter how I split them up. Even if I take the circuit from A phase and put them on C phase and vice versa still trips. I have since put them on C phase in the tap box on a spare circuit​ coming from the panel. as they only draw 1.3 amps combined and haven't had any problems yet. Would anyone have any idea what in the world is going on?
Any and all help would be greatly appreciated.

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What happens if you disconnect all the associate loads and the j box and simply energize both branch circuits to the j box ?
 
I would turn off all breakers and disconnect all loads at the jb and see what you get. You will always get a short reading if you read thru a circuit that has a load connected. If you still have a short then you have a short in the wire
 
I did try that, put a meter on it and don't have continuity. Put it back together and still trips. I am completely baffled by this issue. In 27 years I have yet to come across this problem

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A phase and C phase. Even if I move to B phase, it still trips. It trips before the j box but doesn't come up any where else before that. I used different meters to check for continuity and still nothing.

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I didn't think of megging it...... what's your train of thought? A break down somewhere in the conductor in the conduit too slight to ring but enough to trip?

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bsmith8691 said:
A phase and C phase. Even if I move to B phase, it still trips. It trips before the j box but doesn't come up any where else before that. I used different meters to check for continuity and still nothing.

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What trips, all breakers, the main breaker? How many breakers on each phase, one for each circuit (8)? Is it only 1 breaker tripping? Which breaker?

Remember, we have not seen and do not know what you are looking at, so EVERY detail needs to be communicated by you to us if you are to receive good help.
 
Ok... Just breakers one and three trip with or with load connected. Phase orientation does not affect trip condition. Meaning if either b.c.c is moved to b phase then both breakers trip when turned on

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bsmith8691 said:
I didn't think of megging it...... what's your train of thought? A break down somewhere in the conductor in the conduit too slight to ring but enough to trip?

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Yes. Typical DVM only has maximum of 9V test signal being applied, your normal operation is 120 volts to ground and 208 phase to phase. If you put out a test signal of 250 volts (or more) you will find things you won't find with 9 volts test signal.

If you don't have or want to get a megger and the run to the junction box isn't all that long you may just want to pull conductors out and I bet you will see the problem once you do that. Sounding like you have some sort of insulation integrity issue between two conductors, but takes enough voltage to break down enough cause a fault. Eventually it may become more solid of a fault or burn at least one conductor completely open causing other noticeable issues if that would happen.
 
Good idea, I will try it in the a.m. The chances of getting the conductors out the conduit after 50 years are probably slim to none.

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bsmith8691 said:
Good idea, I will try it in the a.m. The chances of getting the conductors out the conduit after 50 years are probably slim to none.

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1 inch with what you seem to describe as 11 conductors - probably 12 AWG, maybe a problem if slab is on/below grade, slab between floors - might not be bad at all.
 
To help pull out the existing conductors, you need to get an air compressor and blow through the conduit soap and some water or pulling lube. Just be sure you have something to contain what is blown out the conduit. After you pull the wires out, run a mandrel through several times followed up with a rag several times. You don't want to pull new wires over debris left in the conduit.

How long is the conduit?

If you megger, disconnect both ends of every wire so you don't damage anything.
 
I've had real good luck with clearing conduits using white distilled vinegar. I used it a lot rewiring gas stations. Another contractor told me they used transmission fluid. I assume the vinegar ate at the corrosion and the transmission fluid was a good lubricant.

Our conduits went from inside to outside so we could blow out the conduits w/out having to worry. If you are inside to inside, different floors, it's gonna be a little more challenging.

Anyway, the point is, with some planning and creative thinking there is a good chance you can pull those conductors out.
 
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