Conductor Temp Rating

[joeyww12000]

What exactly does the conductor temp rating mean from table 310.16? Lets say I had a conduit that had only one 277v, 2 wire circuit, pulled in #12 THHN. I would have no reason to derate. Normally I would use an ocpd rated for 20a or less. If I had enough CCCs to have to derate I would use the 90c column which would tell me the ampacity of #12 THHN is 30a before derating. I guess Im confused on why Im using this column all of a sudden to size my ocpd, when without derating I dont even look at this table?

 

[chris kennedy]

Im confused on why Im using this column all of a sudden to size my ocpd, when without derating I dont even look at this table?

Because this says you can use the 90C column for derating.

110.14(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

 

[iwire]

You have to use the column from 310.16 that has a temperature rating that does not exceed the lowest rating of any equipment in the circuit.

Lets say your running a 60 amp feeder for HVAC unit, you are using pipe and THHN. The 60 amp breakers terminals are marked 75 C and the cheap 60 amp pull out disconnect has terminals rated at 60C.

In that case, even with 90 C wire you would be stuck using the 60 C rating because of the pull out disconnect rating.

You will not find 90 C rated equipment below 600 volts, so you will usually use the 60 or 75 C coulmn......Unless you are derating, then you could start derating from the 90 C column assuming you have 90 C wire.

 

[SEO]

I agree with iwire and chris. Another way to look at it is that a chain is only as strong as it's weakest link and wiring systems are the same.