Derating conductors

[mrvaliant360]

The question of derating came up in calculating conductors for a circuit.
If you have for example 7 current carrying conductors in a raceway or cable, the conductors will need to be derated per 310.15(B)(2)(a) at 70%, which we all know. The question was brought up that, if we use 7- #12 THHN 90 degree conductors in a raceway or cable that we can use Table 310.16 allowable ampacity listed in the 90 degree column to start our derating.

I was always taught that per 110.14(C) we need to first start with our temperature terminations due to this being our weakest link in the circuit or system and if we are terminating to 75 degree terminations we need to use the allowable ampacities listed in the 75 degree column to start our derating calculation.

Who's right here???

 

[iwire]

If you have 90 C conductors you can start your derating from the 90 C column but the finished result can not exceed the terminal rating.

 

[JohnJ0906]

Start with the conductor rating - for THHN, 90 deg.

The final result is still restricted to the lower rating.

 

[Dennis Alwon]

Who's right here???

Bob is right. 7 ccc in the conduit with 90 C wire will not affect your ampacity at 75C

 

[infinity]

7 ccc in the conduit with 90 C wire will not affect your ampacity at 75C

I would preface this by saying that "Most installations with". There are times when derating for 7 CCC's may change the ampacity of conductors to a point so low that a larger set of conductors may be necessary. Since no specific installation type was given in the OP a blanket statement may not always be correct.

 

[Dennis Alwon]

I would preface this by saying that "Most installations with". There are times when derating for 7 CCC's may change the ampacity of conductors to a point so low that a larger set of conductors may be necessary. Since no specific installation type was given in the OP a blanket statement may not always be correct.

I was assuming #12 at 20 amps since the OP mentioned #12's but you are correct depending on the install.

 

[iwire]

Good point Rob, as soon as we get above 30 amps we run into derating issues with more then 3 current carrying conductors.

Sneaking in a few small conductors with a feeder could really bite you in the rear.

 

[mrvaliant360]

If you have 90 C conductors you can start your derating from the 90 C column but the finished result can not exceed the terminal rating.

So, what your saying is, as long as the ampacity of the derated 90 degree conductor does not exceed the allowable ampacity of the termination temperature rating your ok.

 

[iwire]

So, what your saying is, as long as the ampacity of the derated 90 degree conductor does not exceed the allowable ampacity of the termination temperature rating your ok.

Yes, that is it. :smile:

 

[Smart $]

So, what your saying is, as long as the ampacity of the derated 90 degree conductor does not exceed the allowable ampacity of the termination temperature rating your ok.

IMO, it is a double comparison and easier to look at it this way...

derated conductor ampacity ≥ circuit ampacity ≤ 60? or 75?C ampacity*​

*whichever is pertinent, of the same conductor type (Cu or Al) and size as that being derated.

Note the comparison is to the circuit ampacity, which is not necessarily the ocpd rating.