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.
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
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.
I finally got a chance to try pulling the wires out of one of the bad runs,
but no dice - I couldn't budge them.
I made sure they were free to move at the other end, wrapped them tight
around a dowel and put my back and legs into it. Tried pulling both together
and each singly. Tried steady pulls and jerks. Nothing doing.
This should have been the easy run - two #12 solid in what looks like
half-inch RMC, 15 feet box-to-box and two 90s. So either I just don't have
the strength (could it be that hard?) or the wires are pinched somewhere in
the conduit. If they are pinched that's probably the cause of the resistive
We're coming up on a remodel soon and I'll have the electrical guy see what
he can do. Meanwhile I've bypassed the run with WireMold.
Anyway, thanks to everyone for all the helpful advice.
HaHa WireMold to the rescue! Love it.
The conduit may have filled with concrete slurry when poured,
or the steel rusted around the conductors. A short run like
that should have been an easy pull for you.
Heh. Yeah, I know the WireMold is amateurish. But hey, I *am* an amateur. It
was a quick fix to get the power back up until we remodel and a real
electrician looks at it.
Even so, due to the structure of the house I don't think there are any ways
to bypass the run that won't be difficult and expensive. Come to think of
it, that's probably why the builder put this short run between boxes in a
conduit below grade to begin with. Most of the other short runs are in the
I would love to know what happened in those two conduit runs, and whether I
should expect more such incidents. But short of busting up the slab it's
beginning to look like I never will.
Aww. If you have fixes for the problem, you've taken all the fun
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.
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.
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.
I still think you should use UF cable if it will fit. You can get a
12-3 or 14-3 if you got 2 circuits, but be sure the breakers are on
opposite legs of the main for those 2 circuits. If your old wires are
wet, thats even more reason to use UF.
UF is not more water resistant than THWN. It is more resistant to
physical damage hence its larger cross section. It's use inside raceway
is a waste of resources and an invitation to a difficult time pulling it.
Well we aren\'t no thin blue heroes and yet we aren\'t no blackguards to.
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!
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