OT: Oven melts wires under concrete.

Kind of interesting, we had half a dozen 120V circuits in our factory short out and start tripping their breakers.

First one went, and we assumed that someone drilled through the conduits under the concrete while mounting a machine to the ground. So we replaced the circuit and away it goes. Then 2-3 weeks later the rest shorted out, and we pulled out the circuits, the wires were completely black, and the insulation gone. And it was hot. The first one was hot too, but not very, we assumed the short circuiting had heated up the wires.

Scratching our heads, we tried to find out what caused it. Turns out there was an oven (no floor, just walls on top of the concrete with a natural gas burner) sitting right on top of where the conduits for these circuits travelled. The oven runs at 395F almost 24/7. THHN is rated for 190F.

So, the heat from the oven melted the insulation of the wires inside the underground conduit. I check out the ratings of various wires, but it seems that everything rated for 390+F is also for dry locations only, and I'm pretty sure buried under concrete counts as a wet location in the code book.

Anyone know of a wire rated for high temp and wet locations? Or am I just going to have to run the circuits through a new conduit running up to the ceiling instead of the ground?

Tharon said:

...run the circuits through a new conduit running up to the ceiling instead of the ground?

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Best option.

This may be something that you could look at.
http://www.awcwire.com/ProductSpec.aspx?id=MGT-1000

If the conduit is indoors under the slab, you should be fine with dry location wiring. Were the wires you pulled out wet? Is the conduit PVC, I assume it would be melted too as it's 90 degree (celsius) rated, same as your wires.

mhulbert said:

If the conduit is indoors under the slab, you should be fine with dry location wiring. Were the wires you pulled out wet? Is the conduit PVC, I assume it would be melted too as it's 90 degree (celsius) rated, same as your wires.

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Yes, it was PVC. I didn't think about that melting as well. And just going by code definitions, I don't think it matters if the area is physically wet to be classified as a wet location. I was pretty sure buried under concrete was all that it required, and didn't think there was any indoor/outdoor exceptions. (Though, I could be wrong, I'm not an expert).

I'm pretty sure we will be moving the circuits and abandoning the existing conduit. I don't think it's a fire hazard since everything that comes out through the concrete is rigid metallic conduit. The PVC is all contained under concrete.

I wish I could find the code section dealing with this...any code wizards out there?
Good luck with your setup, sounds like moving the wires is the right thing to do

Are you sure the wires weren't hot because one or more of them was damaged and no longer capable of carrying the load demanded from it? Were they hot the whole length, or mainly around the melted one?

How deep in the concrete below the gas fired oven were they?

Can you stick a thermocouple down there and measure ambient?

Perhaps the conduit is not a total loss. I would test before and verify spending...but that's just me.

Wire sized for actual temp might be possible and avoid downtime and the expense of relocating pipe.

We paced out the length of wire and it just so happens that the area the wire was damaged was roughly where the oven had been placed.

I believe the concrete is around 6 inches thick.

We assumed that the hot wires at first were due to the damage and overload situation. But the heated portion was isolated to the general area around the burn, and not the whole length of wire.

yeah, 6 inches...I don't know the volume of the oven or how well insulated it is, but I am sure that depth the heat transfer was significant. I would abandon that pipe at that depth based on country boy guesstimation. Now if you had said 12" or more...

I'd be really curious as to how hot it really does get under 6" of concrete with an oven of unknown construction radiating heat downwards.

Thermocouple wire costs what? $0.75/foot? give or take?

I'd spend $20 of the company's money and make a thermocouple from T/C wire (twist the ends together) and pull it through the conduit until the hot end was under the oven. I'd connect something that could read that thermocouple (calibrator, indicator, controller, whatever) while the 24-7 oven is running and see what temperature it gets to under 6" of concrete and an oven of unknown construction.

The oven is a pretty simple thing. No floor, just four walls, cieling with the burners in it. About a 10 foot cube. I can't tell you the BTUs, but it's a natural gas burner in the ceiling and a blower to circulate the air. The concrete is either 6 or 8 inches, not very thick.

So there is no insulation at all between the concrete and the hot air.

I do have some spare temperature controllers on the shelve and I couple spools of T/C wire. I never really thought of making a thermocouple just using the wire twisted together at the ends. Now that I realize I can do that I may push some down the conduit and take some measurements just for curiosities sake.

The oven had been sitting there running since about late 2008. Just now started having issues. Maybe the heat wasn't very significant but over the couple years degraded the wire insulation enough to cause problems.

Here are some hints that may or may not be any help for this particular problem.
1. We did this under some glass furnaces many years ago. Set up enclosures or junction boxes in the conduit runs on both sides of the "Hot" zone to make termination points. Use wire rated to 250 C. between them. Silicone Rubber insulated is one type.
Link:
http://www.radix-wire.com/250.htm

2. Another option, if physically possible, is to pour some Vermiculite Insulating Concrete as an overlay on the floor concrete. Here is a link to help evaluate this.
Link:
http://www.schundler.com/vermcon.htm