Wire insulation question

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paul

paul

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Snohomish, WA
I do service work for a large grocery store chain. I got a call today where they said an outlet wasn't working. I get there and when I see it in the bakery, I realize exactly what the problem is, as this is the 4th time I've seen it. When they did the slab work, the installing electricians are running the conduit underneath the ovens. The pipes still get water in them and then you add the head from these large ovens, it cooks the THHN wire. In that, I mean it comes out bubbled and white.

Since this is a problem and replacing the wire with THHN again will result in the same problem down the road, I'm wondering what else out there may work. I'm leaning towards XHHW-2. Does anybody have any thoughts on this, or maybe have dealt with something similar. I need something to withstand the higher temp, and be water resistant.

There is always the option of re-piping this, but that's a last resort because of the distance and expense.

Thoughts?
 
Re: Wire insulation question

XHHW-2 is still only a 90 degree wire, as is THHN. I see in table 310-18 that there are higher temperature conductors such as type Z, but I don't know if they are rated for wet locations. If the ambient temperature is high enough that 90 degree conductors are being damaged then the ampacity of the conductors will need to be drastically reduced. It seems that the best permanent solution is to reroute the conductors to aviod the overheated areas.
 
Re: Wire insulation question

Have you tried contacting the oven manufacturer and asking what they use in the internal wiring of the ovens?

I looked at 310.13, and all of the conductors require a dry location to utilize their above 90c ratings. Now, since the part of the wiring in the oven is dry, you may be able to use Type ZW-2, which is good up to 150c when dry.

mike
 
Re: Wire insulation question

Paul,

It would take an extreme amount of heat to penetrate thru 8" of concrete and still melt thhn insulation.

Maybe during installation they knicked the wire?
 
Re: Wire insulation question

If I remember to take my camera when I pull these wires out, I'll take a picture for you. I've never seen anything like it before. It's definately not a nicked wire installation. All the conductors in the conduit had their insulation swelled, for lack of a better word. The #12's were the size of #10's and bubbley looking, I want to use the term marshmallow like. Both hots, the neut and ground all looked the same. In three different buildings, but the same situation. Figure they are subject to the heat 8 hrs a day, 6 days a week. These ovens don't have a bottom. They roll racks into the ovens and the concrete is the bottom. Also in all three situations the conduit had water in it. So I'm assuming it was caused by a combination of the heat and the water.

I was told by an electrician who's been doing work for the chain that the specs specifically say not to run any conduit in the slab in the area of the ovens.
 
Re: Wire insulation question

Use type TGGT.

That's what we use when wiring lights in oven rooms and such. I think it's good to 250*C or something thereabouts.
 
Re: Wire insulation question

Originally posted by benaround:
It would take an extreme amount of heat to penetrate thru 8" of concrete and still melt thhn insulation.
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As I recall from a way back time when I did a lot of heat loss calculations, it takes eight feet of concrete to equal one inch of fiber glass insulation.

The thermal mass of the concrete will take a lot of heat before the temperature rises close to the oven temperature, and will take just as much time to release the heat after the oven is off.
 
Re: Wire insulation question

Figure they are subject to the heat 8 hrs a day, 6 days a week.
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OK, then what about the ambient temp. correction factors below table 310-16? If the concrete is 350 degrees, the #12 is good for what - 2 amps? the table only goes as high 176 degree F ambient, and the #12 is good for 10 amps. What's the load at the receptacle 180 VA, or an appliance?

Fix it right. Re-pipe it.
 
Re: Wire insulation question

that amount of heat would have to spread to the front of the ovens. The walking surface, even if the concrete is only 180 degrees under the oven, and even if it is two feet away from where it is 180 degress, would have to hit 150 degrees. that melts most shoes. I think something else is goung on here. Lye from the concrete or something.

paul
 
Re: Wire insulation question

If the conduit is getting hot enough to destroy the conductors than I don't see how it would contain water. The water would boil and evaporate. And if the conduit contained water I would think that the water would help to cool the conductors.
 
Re: Wire insulation question

Just a thought. I remember way back where the chemicals in a plant was eating away wire insulation. Maybe chemical being used for cleaning the ovens is the cause of the trouble and not heat. I have seen fumes do funny thing over the years.
 
Re: Wire insulation question

iggy,
If the concrete is 350 degrees, the #12 is good for what - 2 amps?
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That would be an ambient temperature greater than the rating of the insulation, so the conductor is good for zero amps.
Don
 
Re: Wire insulation question

There are three fundamental forms of heat transmission; convection is only one of them. Radiance and conduction are the other two and they will both transmit heat to the UG just fine. And, as Al noted, the UG could very likely reach a quiescent temperature very near that of the oven which it will then hold for a long time even after the oven is off.

With regard to the OP, as a thermoset rather than a thermoplastic insulation, XHHW -2 may last a bit longer but it would still likely be a misapplication. The best solution is just what the specs called for ? ?the specs specifically say not to run any conduit in the slab in the area of the ovens.?
 
Re: Wire insulation question

Paul,

I was just thinking about the type of conduit used in the underground, either pvc or ridged,and you don't see alot of ridged these days so the pvc must be totally disformed from the heat.

The point being,can you even get new conductors thru the existing raceway?

Even if you run overhead as soon as you get close to this inferno it would have to be at least very,very,very hot as far as the insulation on the conductors go!!
 
Re: Wire insulation question

I do service work for a large grocery store chain. I got a call today where they said an outlet wasn't working. I get there and when I see it in the bakery, I realize exactly what the problem is, as this is the 4th time I've seen it. When they did the slab work, the installing electricians are running the conduit underneath the ovens. The pipes still get water in them and then you add the head from these large ovens, it cooks the THHN wire. In that, I mean it comes out bubbled and white.
Click to expand...
I want to know, since you have seen this on 4 other stores what you did to fix them if you're asking for suggestions now.
 
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