Another derating question...

[c2500]

I have done work both in the city I live and the county. When derating, the city has me derate based off of the wire type only. However, the county makes me derate based off of the lowest temp. in the system....normally 75 degree breakers in the panel.

So who is actually right in your opinions?

Just curious.

c2500

 

[wirebender]

For de-rating purposes you go by wire type, for instance 90 degree column for thhn, but you cannot use a value greater than the rating of the lowest temp in the system after you de-rate.

 

[infinity]

If they're following the NEC the city has it right, the county needs to buy a code book.

 

[cschmid]

we were always taught that if you use 90C rated wire and hooked to a 75c rated lug you use the lug as your baseline for the calcualtions..because it negates the 90c and makes them 75c for that circuit..

 

[infinity]

we were always taught that if you use 90C rated wire and hooked to a 75c rated lug you use the lug as your baseline for the calcualtions..because it negates the 90c and makes them 75c for that circuit..

That would be incorrect according to the NEC, but certainly wouldn't be unsafe or a violation. You may end up with using larger conductors than the NEC requires.

 

[iwire]

we were always taught that if you use 90C rated wire and hooked to a 75c rated lug you use the lug as your baseline for the calcualtions..because it negates the 90c and makes them 75c for that circuit..

That is true.....unitl you get to derating, then we can use the 90 C column if we have 90 C conductors

Take a look at 110.14(C), in particular the last sentence.

 

[cschmid]

yep normally do and installs last long time which is nice..

 

[cschmid]

okay I hope this is not to lame has this always been there or was it changed in last few code cycles..I guess I never picked up on that..

 

[cschmid]

wait a second here upon further review I have question..it says that conductor with high than specified temps are allowed to be used with termination of lesser values but does not say you use their rating for adjustments..does anyone have this on cd so we can have exact wording as I do not want to type it all out..

 

[iwire]

but does not say you use their rating for adjustments..

Actually it does say that.

Here is the sentence as typed by me.

Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

 

[infinity]

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.

This should be pretty clear that the temperature rating of the conductor may be used for derating.

 

[cschmid]

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.

Does that not say you need to use the lowest rating..the highlited part

Edited to add..
So if you use higher rated and derate if the anwser is greater than the 75 rated lug then you could not use them..would that be a correct statement..

 

[iwire]

Does that not say you need to use the lowest rating..the highlited part

Yes, and then the last sentence modifies it and says you can use the higher rating for the purposes of derating.

When it's all said and done after the derating you can still not exceed the terminals rating.

 

[cschmid]

thank You and hope this has clarified for the OP this issue..it has for me..better get my buns on the road gotta get some work done..later on gents

 

[Bob NH]

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.

Does that not say you need to use the lowest rating..the highlited part

Edited to add..
So if you use higher rated and derate if the anwser is greater than the 75 rated lug then you could not use them..would that be a correct statement..

The rationale for the code method (use the wire temperature rating for derating multiple conductors) is that the wire in the raceway must withstand the higher temperature from more current in more conductors in the raceway.

The fact that there are more conductors in the raceway doesn't affect the temperature at the terminals because the conductors are only permitted to carry Amps based on the temperature capability of the terminals. The enclosures that contain the terminals are not limited with respect to number of current carrying conductors.

If the calculation results in a current value greater than the 75 degree rated termination, the wire may be used but only at the 75 degree Amp rating.