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Wires shorting under the slab



 
 
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  #1  
Old August 11th 06, 10:17 AM posted to alt.home.repair
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Posts: 5
Default Wires shorting under the slab

Anybody ever run into this? I'm trying to figure out what's causing it.

I've got a 40-year old house in which much of the outlet wiring is in steel
conduit under the slab (or in it, for all I know). The wires are individual
#12, with alternate phases sometimes sharing a neutral.

In the first incident I suddenly lost one branch circuit completely. The
wire had no continuity from the panel to where it comes out of the slab. Two
other branch circuits in the same conduit started tripping their breakers
intermittently. I discovered that randomly the hot wires would have low and
variable resistance to neutral. (And yes, I measured with the breaker off
and absolutely nothing connected to the problem runs.)

I sort of shrugged off the first incident, but now it's happened again. This
time another run of conduit carrying just one branch circuit developed the
same intermittent low resistance to neutral problem, causing breaker trips.

The resistance measurements are particularly puzzling. Sometimes the meter
shows a few hundred ohms, gradually creeping up over minutes as if some
large capacitance is being charged. And then suddenly the resistance will
drop to 20 or 30 ohms or jump up to a few thousand ohms.

I've worked out fixes for both problems, but I'd really like to hear if
anyone has any idea what's going on. Could it be the slab settling on the
conduit? The floors are flat and level and I see no foundation cracks.


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  #2  
Old August 11th 06, 12:53 PM posted to alt.home.repair
RBM
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Posts: 1,690
Default Wires shorting under the slab

If the conduits are under the slab, water in the conduit can be causing the
problem, and if they're poured in the slab, concrete could have seeped in
and deteriorated the conductors insulation. Either way, if they're in
conduit, pull out the offending conductors and pull in new ones



"SteveR" wrote in message
...
Anybody ever run into this? I'm trying to figure out what's causing it.

I've got a 40-year old house in which much of the outlet wiring is in
steel conduit under the slab (or in it, for all I know). The wires are
individual #12, with alternate phases sometimes sharing a neutral.

In the first incident I suddenly lost one branch circuit completely. The
wire had no continuity from the panel to where it comes out of the slab.
Two other branch circuits in the same conduit started tripping their
breakers intermittently. I discovered that randomly the hot wires would
have low and variable resistance to neutral. (And yes, I measured with the
breaker off and absolutely nothing connected to the problem runs.)

I sort of shrugged off the first incident, but now it's happened again.
This time another run of conduit carrying just one branch circuit
developed the same intermittent low resistance to neutral problem, causing
breaker trips.

The resistance measurements are particularly puzzling. Sometimes the
meter shows a few hundred ohms, gradually creeping up over minutes as if
some large capacitance is being charged. And then suddenly the resistance
will drop to 20 or 30 ohms or jump up to a few thousand ohms.

I've worked out fixes for both problems, but I'd really like to hear if
anyone has any idea what's going on. Could it be the slab settling on the
conduit? The floors are flat and level and I see no foundation cracks.



  #3  
Old August 11th 06, 01:04 PM posted to alt.home.repair
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Posts: 312
Default Wires shorting under the slab

On Fri, 11 Aug 2006 01:17:38 -0700, "SteveR" wrote:

Anybody ever run into this? I'm trying to figure out what's causing it.

I've got a 40-year old house in which much of the outlet wiring is in steel
conduit under the slab (or in it, for all I know). The wires are individual
#12, with alternate phases sometimes sharing a neutral.

In the first incident I suddenly lost one branch circuit completely. The
wire had no continuity from the panel to where it comes out of the slab. Two
other branch circuits in the same conduit started tripping their breakers
intermittently. I discovered that randomly the hot wires would have low and
variable resistance to neutral. (And yes, I measured with the breaker off
and absolutely nothing connected to the problem runs.)

I sort of shrugged off the first incident, but now it's happened again. This
time another run of conduit carrying just one branch circuit developed the
same intermittent low resistance to neutral problem, causing breaker trips.

The resistance measurements are particularly puzzling. Sometimes the meter
shows a few hundred ohms, gradually creeping up over minutes as if some
large capacitance is being charged. And then suddenly the resistance will
drop to 20 or 30 ohms or jump up to a few thousand ohms.

I've worked out fixes for both problems, but I'd really like to hear if
anyone has any idea what's going on. Could it be the slab settling on the
conduit? The floors are flat and level and I see no foundation cracks.


I can only see one of two things happening.
1. The conduit separated at a fitting and the wires got pinched.
2. The conduit rusted out from moisture and the wires had bare spots
that shorted to the pipe or moist soil.

Can you pull any of them out? You will be able to see many things,
such as are the wires wet, so they look pinched, corroded, etc. ???

You might not be able to replace these wires if the conduit is
damaged, or for that matter you might not be able to remove them if
the pipes are bad.

If however you can remove a wire, maybe you should attach a stell
fishline (tool to fish wire thru conduit) and see if you can pull the
fish line out the other side. If you have a moisture problem, you can
maybe pull some UF cable thru to replace the old wire.

If you cant replace them, and you are on a slab with no basement, you
might have to run your wires up the wall, across thru the attic, and
back down. Otherside you may be opening walls.

Mark

  #4  
Old August 11th 06, 02:18 PM posted to alt.home.repair
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Posts: 6,205
Default Wires shorting under the slab

replace wires with underground type its the only way. wierdly this
sounds like a fun project.

probably metal conduit if its rusted out go up the walls, cant pull
cables thru sharp rusty edges

  #5  
Old August 11th 06, 03:38 PM posted to alt.home.repair
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Posts: 747
Default Wires shorting under the slab

SteveR wrote:
Anybody ever run into this? I'm trying to figure out what's causing it.

I've got a 40-year old house in which much of the outlet wiring is in steel
conduit under the slab (or in it, for all I know). The wires are individual
#12, with alternate phases sometimes sharing a neutral.

In the first incident I suddenly lost one branch circuit completely. The
wire had no continuity from the panel to where it comes out of the slab. Two
other branch circuits in the same conduit started tripping their breakers
intermittently. I discovered that randomly the hot wires would have low and
variable resistance to neutral. (And yes, I measured with the breaker off
and absolutely nothing connected to the problem runs.)

I sort of shrugged off the first incident, but now it's happened again. This
time another run of conduit carrying just one branch circuit developed the
same intermittent low resistance to neutral problem, causing breaker trips.

The resistance measurements are particularly puzzling. Sometimes the meter
shows a few hundred ohms, gradually creeping up over minutes as if some
large capacitance is being charged. And then suddenly the resistance will
drop to 20 or 30 ohms or jump up to a few thousand ohms.

I've worked out fixes for both problems, but I'd really like to hear if
anyone has any idea what's going on. Could it be the slab settling on the
conduit? The floors are flat and level and I see no foundation cracks.



Aww. If you have fixes for the problem, you've taken all the fun
out. g

40 years ago, I bet they were using type TW insulation.
It will deteriorate in about.......40 years when wet.

Would be fun if you could extract a length of the stuff to examine.

Jim
  #6  
Old August 12th 06, 03:22 AM posted to alt.home.repair
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Posts: 5
Default Wires shorting under the slab

Thanks to all for the suggestions. Jim, you're right, it is type TW. And
I've noticed that the insulation seems a little brittle even inside the
outlet boxes. As everybody suggested, I'm going to try pulling the wires
again. If I learn anything I'll post.

Steve


  #7  
Old August 12th 06, 05:23 AM posted to alt.home.repair
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Posts: 677
Default Wires shorting under the slab


"SteveR" wrote in message
...
Thanks to all for the suggestions. Jim, you're right, it is type TW. And
I've noticed that the insulation seems a little brittle even inside the
outlet boxes. As everybody suggested, I'm going to try pulling the wires
again. If I learn anything I'll post.

Steve

You will have a much better chance of success if you pull all of the wires
together as a unit, with a fish tape or new wire(s) attached to the tail
end. Make the splice as strong, small and flexible as you can, keep
everything as straight as possible, and lubricate with wire pulling
lubricant to give yourself all possible advantage. Push one end while
pulling the other, work carefully, and pull back and forth as seems
necessary. Cleaning out the conduit with compressed air or a vacuum cleaner
can be a help. Good luck.

Don Young


  #8  
Old August 12th 06, 08:07 AM posted to alt.home.repair
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Posts: 5
Default Wires shorting under the slab

Thanks, Don. Would THHW stranded be a good choice for the new wire?

"Don Young" wrote in message
...

"SteveR" wrote in message
...
Thanks to all for the suggestions. Jim, you're right, it is type TW. And
I've noticed that the insulation seems a little brittle even inside the
outlet boxes. As everybody suggested, I'm going to try pulling the wires
again. If I learn anything I'll post.

Steve

You will have a much better chance of success if you pull all of the wires
together as a unit, with a fish tape or new wire(s) attached to the tail
end. Make the splice as strong, small and flexible as you can, keep
everything as straight as possible, and lubricate with wire pulling
lubricant to give yourself all possible advantage. Push one end while
pulling the other, work carefully, and pull back and forth as seems
necessary. Cleaning out the conduit with compressed air or a vacuum
cleaner can be a help. Good luck.

Don Young



  #9  
Old August 12th 06, 02:08 PM posted to alt.home.repair
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Posts: 2
Default Wires shorting under the slab

Similar readings to those you have are often the result of insects or
other small creatures across connections. Spiders will cause this but
more likley is worms or slugs. Corrosion or degredation of the
insulation on its own will not cause this, but a hot spot as caused by
a poor connection is a magnet to little creatures who try to get as
close to the heat as possilbe and then .... ZAP! Often there is
enough current flow to burn out light gauge cables, particularly where
conventional fuses are used (mcbs will almost certainly trip). The
trouble is that when you pull the wire out you might not see any
evidence of the intruder as it may well get left behind in the tube,
although is now harmless!


SteveR wrote:
Anybody ever run into this? I'm trying to figure out what's causing it.

I've got a 40-year old house in which much of the outlet wiring is in steel
conduit under the slab (or in it, for all I know). The wires are individual
#12, with alternate phases sometimes sharing a neutral.

In the first incident I suddenly lost one branch circuit completely. The
wire had no continuity from the panel to where it comes out of the slab. Two
other branch circuits in the same conduit started tripping their breakers
intermittently. I discovered that randomly the hot wires would have low and
variable resistance to neutral. (And yes, I measured with the breaker off
and absolutely nothing connected to the problem runs.)

I sort of shrugged off the first incident, but now it's happened again. This
time another run of conduit carrying just one branch circuit developed the
same intermittent low resistance to neutral problem, causing breaker trips.

The resistance measurements are particularly puzzling. Sometimes the meter
shows a few hundred ohms, gradually creeping up over minutes as if some
large capacitance is being charged. And then suddenly the resistance will
drop to 20 or 30 ohms or jump up to a few thousand ohms.

I've worked out fixes for both problems, but I'd really like to hear if
anyone has any idea what's going on. Could it be the slab settling on the
conduit? The floors are flat and level and I see no foundation cracks.


  #10  
Old August 13th 06, 04:57 AM posted to alt.home.repair
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Posts: 677
Default Wires shorting under the slab


"SteveR" wrote in message
...
Thanks, Don. Would THHW stranded be a good choice for the new wire?

I do not feel qualified to comment on that. Generally, solid wire is used in
conduit as it is cheaper and slightly smaller. The flexibility of stranded
wire might help in this case. I would definitely want water resistant
insulation.
Don Young

"Don Young" wrote in message
...

"SteveR" wrote in message
...
Thanks to all for the suggestions. Jim, you're right, it is type TW. And
I've noticed that the insulation seems a little brittle even inside the
outlet boxes. As everybody suggested, I'm going to try pulling the wires
again. If I learn anything I'll post.

Steve

You will have a much better chance of success if you pull all of the
wires together as a unit, with a fish tape or new wire(s) attached to the
tail end. Make the splice as strong, small and flexible as you can, keep
everything as straight as possible, and lubricate with wire pulling
lubricant to give yourself all possible advantage. Push one end while
pulling the other, work carefully, and pull back and forth as seems
necessary. Cleaning out the conduit with compressed air or a vacuum
cleaner can be a help. Good luck.

Don Young





 




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