Running electric cable from ceiling to furnace - attach interupt switch to plenum?

Hi all,
I need an electrical wizard to advise me here. I am installing a new hot air wood furnace next to an existing electrical furnace. I have run the 20 amp cable along the basement ceiling ok and now I need to drop down to the furnace. I want to add a cut-off switch so that I can isolate the thing in an emergency.
Can I put this switch on the plenum? In a metal box? The other furnace has a small breaker box on its plenum with 3 breakers inside, so I can't se why I should not be able to do this. I have used standard orange 10-gauge romex cable.
Plenum temp is set typically to turn on fan at 170, and will open an emergency heat dump at 200 in case of a problem.
Thanks!
Dean
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no. "temperature rating of the insulation of the conductor due to the terminal limitations of 60 degrees centigrade of the terminals used in the circuits" 60C = 140F
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I doubt it.... Put the box on the cool lower part of furnace. Usuallythat ends up being the same height as a standard wall switch.
OR
Put thebox in the rafters above. Then run steel conduit from that ceiling box to the furnace.
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You should mount the box on the joists or wall, and you need to use shielded cable.

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basement ceiling, did it go along a joist, or across them? If it went across them, it had to go through them instead. You did that, didn't you? The alternative is to run it way in the corner. The idea is that it shouldn't be caught by something and pulled down.
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Thanks all for the replies. Actually I have not run the cable yet, its still sitting poking out through the garage to basement wall. I can run it along the piece of wood that is above the I-beam that goes the entire length of the basement. The othe rfurnace was done on the other side of this and passed all its inspections.
Good idea on the mounting it on the lower part of the furnace. If I do this would I need to put in a junction box on the joist to change it to encased metal cable, and then run this down?
Or should I put the switch on the joist itself? The ceiling is 9 feet tall.
Thanks!
Dean
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Most codes require that the disconnect be within easy reach of anyone working on the heater. Some even require one at the top of the basement stairs. If the ceiling is 9 ft., then mount a box, run BX down, and put the disconnect on the furnace.

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Ok great! Thanks.
I called the furnace manufacturer, and they said that the side of the furnace still gets very warm, even though it is insulated. I am wondering if there is any standard kind of insulator plate or something I can put in between the furnace and the disconnect? Any ideas?
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looks like you'll need nm cable on an extra 2x4 from the ceiling: http://www.faqs.org/faqs/electrical-wiring/part2/section-5.html
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Just make sure the 2 x 4 is outside the minimum clearances specified on the installation instructions.

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