Conduit heats up

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If you have 3 conductors in a conduit, using the heat dissipation assumptions of table 310.16 and 'back calculating' from 310.16, then #12 conductors carrying 17A should result in an insulation temperature of about 20C above ambient; a bit more than 100F in a 70F room.

The above is a very 'sketchy' calculation, since there is probably a considerable amount of rounding error in the table and the assumption about heat dissipation could be totally out to lunch. Additionally, I believe that the OP has more than 3 conductors in the conduit. In the original poster's installation, I would not be surprised by a temperature rise of anywhere from 10C to 40C depending upon conditions.

100F is quite warm (body temperature), but _not_ hot. Thus I am not content to simply say 'the wire is overloaded and it will get hot', even though I suspect that it is the problem. I believe that there is a real possibility of some other cause for this conduit heating.

If I understand the theory behind ampacity ratings, 17A on 12ga conductors is _not_ 'too much', even for a continuous load. It is probably too much for the breaker (most breakers are only listed for continuous operation at 80% of the trip level). It is also probably causing rapid wear on the switches, even if they are 20A switches.

-Jon
 
OK here we go. This is getting strange. since many of you were in the realm of the heat coming from the switches or the wire here is what I did.

now remember the switch box has two conduits going to a j-box 5' above it at that point it T's the conduit is warm 10' in either direction of the j-box. #12 is run throughout and after 10' there is another j-box at each side which feed receptacles from diferent circuits. So I didn't really think the wire size or the switches were the problem remember #12 THHN is rated for 25Amps and the 20 A breaker is not tripping off any of the 4 circuits or are they hot. But I put #10 THHN in the area that was warm to replace the #12 THHN. The conduit is not painted there is no insulation near it it has plenty of air movement around it. Still getting warm! I left and thought about it for awhile on another job. I figured it out by what some of you had said. I'll tell you later what it was HAHAHAHAHAHAHAHAHA.
 
Does a class 1-3 cable, or external EGC, follow outside the heated run, perhaps providing enough imbalance to inductively heat, NEC 300-20, the ferrous conduit between?
 
Here we go. I was lost as to what to try next, but here is what I did, since there were 2 phases in each conduit and they were running parrallel to each other I tried this- I put all 3 phases A B C in one conduit and the 4 circuit which is A phase in the other conduit and I ran a nuetral down one conduit and up the other creating a loop. If anyone can tell me why this worked I would be happy to understand it but the heat is gone. Customer is happy and I am PAID! In the Air force we called this FM (F------ Magic)
 
So, the "mystery" was in the unbalanced currents in the conduit. Can you show us in a diagrammatic format before and after?
 
Code Violation

Code Violation

Has anyone commented that a 17A lighting load on a 20A CB would be a Code violation, as it is a continuous load? Max allowed is 16A.
 
Okay, if I understand things correctly, He basically had 2 of 3 phases in each conduit. In a normal 2wire circuit with the neutral, the neutral is out of phase with the hot and cancels magnetic field. In this case the wires (travelers) are in phase with the feed and will not cancel out anything. Have any of you ever seen what happens when someone goofs on parallel feeds going to a MCC or Large panelboard. I have seen 3" RMC get literally red hot from the improper phasing. Charlie and someone else gave the right answer much earlier in these posts. He had 2 conduits in parelell and 2 out of 3 phases in each one. Conduits were heated by nothing more than induction. :)
 
Larry,
In this case the wires (travelers) are in phase with the feed and will not cancel out anything.
Click to expand...
The magnetic field of hots with travelers or hots with switch legs cancel because the current flow is in the opposite direction on the conductors. There should be no excessive heating of the conduit from those conditions.
Don
 
stopped back today to see if everything was the same. curious of a few things I checked the amp reading on the lighting circuits. 3 circuits they read 14.6, 15.2, and 13.6. each of these had read in the range of 17A before with the highest being 17.5. I wasn't concerned with 1.5 amps before since the breaker was cold to the touch. and #12 THHN is rated for 25A. the fan circuit which was in one of the conduits that was hot only reads 6.7A and it was reading 8.5 before. go figure. any way the customer is happy and please remember I was not the original installer of this just the trouble shooter. Thanks for the help.
 
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