Wires shorting under the slab

[SteveR]

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.

[RBM]

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

[maradcliff]

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

[hallerb]

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:(

[Speedy Jim]

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

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

[SteveR]

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

[Don Young]

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

[SteveR]

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

[wiseoldsage]

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:

[Don Young]

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

[maradcliff]

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.

[Thomas D. Horne, FF EMT]

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.

[SteveR]

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 short.

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.

Steve

[Speedy Jim]

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.

[SteveR]

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 walls.

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.

Steve