how hot cables really get?

Old 06-14-15, 11:18 AM
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how hot cables really get?

So as I have been learning about wiring etc... I know there are strict rules about how many wires in conduit and I have seen many comments about cables generating heat as the current flows through it....which I was aware of BUT.... much heat is generating with 20A 12/2 running in the wall ?
What about today's highly insulated houses..... the insulation is packed between the studs and I bet you the cable has no space to "cool"

Just curious..... thanks!
Old 06-14-15, 11:55 AM
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Unless the cable wires are overloaded, there won't be much heat generated. Inordinate heat would cause the breaker to trip. This is the design.
Old 06-14-15, 01:33 PM
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Everything the NEC rules are for safety. They have done many test (or a listing agent has) on different components of a wring system form circuit breakers/fuses, to the final device on a branch circuit in all sorts of installations. I know that the rating of a wire/cable has a built in "fudge factor" for how it is rated. If you look in the code book, you will see that #12 THHN is actually rated for 30 amps. But then you have to take into consideration the rules of NM cable where you must follow the 60 degree for max rating, which is 20 amps. Also there is the small conductor rules for conductors #10 and smaller.

I know for a fact that #12 wire running 16 amps through it for a continuous load will get warm. But the point of the rules is so that conductor does not exceed the temperature rating of its insulation, not that the wire will melt.
Old 06-14-15, 04:26 PM
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with respect to the insulation and conduit I'm pretty sure that the conduit insulates much better than the fiberglass bat does. Try running your oven with the door open and piece of insulation taped over the opening.
Old 06-15-15, 07:56 AM
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To deal with the tightly sealed and insulated modern houses, the code was updated a few years ago to strictly limit "bundling" of NM cables inside insulated stud cavities exactly for the reason you brought up in your question. We also now have items like the plastic stacker cable clips to keep separation between cables in stud cavities.

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