temperature derating

[Ngarcia92]

we have a 8amp fuse in a control panel for a boiler that was blowing. the cause was determined as a short in the wires that lead to e-stops that were added to the system.

a continuity test and megger were used to determine there was no short in the wires. Now, the wires (2- 14awg thhn) run into a 3/4 conduit that passes directly under the boiler for about 8'. After removing these e-stops from the boiler, the fuse held.
-the temperature under the boiler is 300 degrees f.
-the length of conduit between controller and bottom of boiler is about 6'

should this wire have been de-rated due to this? and could this cause the fuse to blow.

thanks

 

[augie47]

As far as derating, IF the conductor was rated for a temperature higher than it's 90°C, the exception to 310.15(A)(2) would likely cover you plus the load is only 8 amps HOWEVER, THHN is a 90°C wire and should not be subjected to a higher temperature {310.15(A)(3)}.
Did you check the conductor for signs of damage ??

 

[infinity]

If a 194° F conductor is subject to 300° F for an extended period of time it will eventually fail. The raceway needs to be moved or you could use a higher temp conductor. Derating won't solve the problem if you've exceeded the conductor temperature.