Old Wiring

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frank_n

frank_n

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Central NJ
I am trying to find an open wire in a house built in the late 40's. The house was wired with bx. The circuit I am working on has ceiling fixtures in the dining room, kitchen (2), and bathroom. First, I found an open neutral. I jumped a neutral wire from one circuit in the switch box, to the other circuit and all the lights came on. With that fixed, I changed lighting fixtures in the bathroom and kitchen. When I pulled the dining room fixture down, the hot shorted against the mounting bracket (short wires with worn insulation). I thought the breaker had tripped, so I taped up the insulation and installed the new fixture. When I went to the turn on the breaker, I discovered that the breaker didn't trip I had an open hot wire somewhere. I spent 5 hours looking for the open wire but no luck.

I could run a new RX to the circuit or I could use the hot from the switch box where I jumped the neutral. What I worry about is the circuit being fed from 2 different sources.

Any advice?

Frank
 
Re: Old Wiring

I'd be concerned about that jumpered neutral you did. Yes, it made the lights come back on, but now you have the current from one circuit returning to the panel via a neutral from a different circuit. Granted, one light fixture probably isn't going to seriously overload that neutral, but how far up/downstream is the jumper you made? Perhaps the entire circuit now has all of its current returning via the other circuit's neutral. You could potentially have created a good opportunity for overload on that neutral if the two circuits are on the same bus. Since neutrals are only as protected as the breakered ungrounded wires they are supposed to be paired up with, there's really nothing that would prevent that neutral from burning up if it does become overloaded. This is why you never want to mix neutrals between circuits (unless it's a properly wired multiwire circuit).

As for the open hot wire, are you sure you have one? If hot wire touching the mounting strap didn't trip the breaker, it could very well be because the mounting strap doesn't have a solid path for fault current. This would be the case if the wiring feeding the fixture box is K&T.
 
Re: Old Wiring

Yes, you should find that open wire. Make a note of things that are not working ( dead ). Then turn off the breaker for the circuit and see what else goes off. This should give a good picture of what is on the circuit. The first thing to check is the last item that is still hot.
I would guess that it's burned wire in a fixture junction box. In the old days they used 100W bulbs with no insulation between the bulb and the box. The fixture that you are looking for may either be in the attic or the crawl space. If you have some that are bad, check them all because they will only give you trouble later. Good Luck, it can take 4 or 5 hours to check out and repair a circuit in an old house because you don't always find just on bad connection but a lot of little problems. I have had to replace the wiring at the wiring at every fixture before I felt that the circuit was safe.
 
Re: Old Wiring

Thanks for your response. I agree with your point on "jumping the neutral" even though a few light fixtures shouldn't make much of a difference. If I also "jump the hot" on the same circuit, it should trip the circuit breaker if the neutral was overloaded.

I am positive that the hot wire is open and that the ceiling boxes are grounded.

Frank
 
Re: Old Wiring

Originally posted by frank_n:
First, I found an open neutral. I jumped a neutral wire from one circuit in the switch box, to the other circuit and all the lights came on. With that fixed...
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You didn't fix the open neutral, you dodged the problem. If all goes well, where the neutral opened, it is insulated from everything else. Or you might have set up more problems. I'd recommend throwing a hound on it and seeing where the open was, and either fixing it or removing it.

When I pulled the dining room fixture down, the hot shorted against the mounting bracket (short wires with worn insulation).
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Why are you performing work with the circuit energized? I realize, you probably turned the light off at the switch. Why didn't you just turn the light on, and shut off the power at the panel? You could have been killed by your assumption. :(

I thought the breaker had tripped, so I taped up the insulation and installed the new fixture. When I went to the turn on the breaker, I discovered that the breaker didn't trip I had an open hot wire somewhere.
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Are you sure there's only one panel?

I could run a new RX to the circuit or I could use the hot from the switch box where I jumped the neutral.... Any advice?
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If it's an option, rewire the whole mess. If some of that old wiring catches fire (as it sounds apt to do), you'll be held liable even if you didn't install it. And you'll rest easier, knowing all the circuit paths.

On a side note, how did you install the fixtures, if the supply conductors weren't rated for the new fixtures?
 
Re: Old Wiring

Frank, never say that a few light fixtures shouldn't make any difference because the last guy to work on the house may have had the same idea. I worked on a burn job( small house , old wire) and you wouldn't believe all the rigs I found in that house. Before it was over I had the place almost rewired. It was hard to find anything that was safe. I assumed that a handyman had been doing the electrical work and not an electrician. A handyman can rig the wiring to work but if you're an electrician you need to correct the problem. That's why we get paid more.
 
Re: Old Wiring

Frank, since you had an arc at this box, that's where I'd start. Often, when you have an arc that doesn't trip the breaker, the conductor ends up with a good melt, often severing the conductor. Check the wire above the burn.

If that isn't it, check for hot at the switch controlling this box. You want to work from the trouble back toward the source. Since you seem to be having multiple problems, don't check for hot only to this circuit's neutral.

I've found that carrying the end of an extension cord plugged into a known-good receptacle is a great troubleshooting tool. You can check for ground and neutral as well as hot by using the cord slots as references with your voltage tester.
 
Re: Old Wiring

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Frank, since you had an arc at this box, that's where I'd start. Often, when you have an arc that doesn't trip the breaker, the conductor ends up with a good melt, often severing the conductor. Check the wire above the burn.

Good advice especially with Bx from that time.It was 2 wire case ground and # 14 wire that had some type of rubber and cloth insulation that became very brittle over the years.It was common place to find a single fuse supply a 3 bedroom house or apartment and also that same fuse was on a 150 ft homerun :roll:
Grabbing a neutral from another circuit and feeding a circuit that might have power but it isn`t in contact but might be in contact if the door slams the right way is asking for trouble and dangerous to say the least.Most homes from that era were plaster and wood lath that didn`t hold up to well to glowing wires :eek:
 
Re: Old Wiring

Originally posted by georgestolz:
Why are you performing work with the circuit energized? I realize, you probably turned the light off at the switch. Why didn't you just turn the light on, and shut off the power at the panel? You could have been killed by your assumption. :(
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George, you must not do much service work. When searching for problems like this you pretty much need the circuit energized.
 
Re: Old Wiring

Scott, at this point he was not troubleshooting, according to what he wrote. He was replacing fixtures. There's no reason to do that hot.
 
Re: Old Wiring

If this is BX with metal boxes and the jumpered neutral is not in the same cable - then you will also have an inductive heating problem.

Mark
 
Re: Old Wiring

I find it odd that two seperate circuits were installed in a switch box,back in those days the circuitry was very basic.

jmo.
 
Re: Old Wiring

find it odd that two seperate circuits were installed in a switch box,back in those days the circuitry was very basic.


Agreed it doesn`t seem right.That old insulation over the years lost its color and many times it was hard to properly identify the hots from the neutrals.Could it be that the original open neutral was fed from the dining room fixture and there was actually an open hot coming in and the neutral was reading 120 volts to ground .when you fed what you thought was a neutral it was actually the open hot and when you gave it a return path all lights illuminated but reversed polarity :confused: That would put it in series though.Were the lights dim ?????
 
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