250.122 mike holt illustrated guide to 2014 NEC changes question

[mjmike]

Upon reviewing the 2nd question for 250.122 (page 97), I agree with the answer (EGC does not increase due to adjustments for 4CCC) but not the calculation method on selecting the conductor size.

It references a 40A circuit being increased in size from 8 to 6 for 4 CCC with 60 deg terminals. However, the answer calculation says "8 awg rated at 60C x 0.80 = 32A after adjustment factors is too small for the circuit example. 6 awg rated at 55A at 60C, is required (55A x 0.80 = 44A)."

However, my understanding is that for any adjustment factor for 4CCC would use the 90C column regardless of termination temperature rating per 110.14.C. #8 = 55A @ 90C, 55A x 0.80 = 44A. The number 8 would still be acceptable for the 40A circuit. What am I missing here?

I would also reference (as supporting data) page 52/53 210.19 question/answer on what size branch circuit for a 4-wire 45A nonlinear continuous load. Here the 4CCC adjustment was made using the 90C column.

 

[Dennis Alwon]

I would have to agree with you. The temp rating of the wire, if 90C, can be used as long as the final overcurrent protective device is not larger than the 60C rating that the entire circuit would allow-- #8 is good at 40 C and derating from 90C column allows the use of that conductor assuming it is 90C.

 

[Adamjamma]

I was taught you derate based upon the connectors... so since your connectors are 60 degree you derate on bundling using the same 60 degree column, even though you are using 90 degree wire... because it is the connectors that count... that is the same reason why another post today is considering a splice to use the higher degree rating and splice at other end to make the fitting in the box... all so you can use the higher ratings and thus smaller cable.

 

[david luchini]

I was taught you derate based upon the connectors... so since your connectors are 60 degree you derate on bundling using the same 60 degree column, even though you are using 90 degree wire... because it is the connectors that count... that is the same reason why another post today is considering a splice to use the higher degree rating and splice at other end to make the fitting in the box... all so you can use the higher ratings and thus smaller cable.

See the second paragraph of 310.15(B)

 

[infinity]

Upon reviewing the 2nd question for 250.122 (page 97), I agree with the answer (EGC does not increase due to adjustments for 4CCC) but not the calculation method on selecting the conductor size.

It references a 40A circuit being increased in size from 8 to 6 for 4 CCC with 60 deg terminals. However, the answer calculation says "8 awg rated at 60C x 0.80 = 32A after adjustment factors is too small for the circuit example. 6 awg rated at 55A at 60C, is required (55A x 0.80 = 44A)."

However, my understanding is that for any adjustment factor for 4CCC would use the 90C column regardless of termination temperature rating per 110.14.C. #8 = 55A @ 90C, 55A x 0.80 = 44A. The number 8 would still be acceptable for the 40A circuit. What am I missing here?

I would also reference (as supporting data) page 52/53 210.19 question/answer on what size branch circuit for a 4-wire 45A nonlinear continuous load. Here the 4CCC adjustment was made using the 90C column.

You didn't mention the insulation type it could be a 60° C conductor in Mike's example.

 

[Adamjamma]

See the second paragraph of 310.15(B)

So if this isreading right, if the multiple conductors or other correction factors drop the 90 degree rating to below the 60 degree non adjusted figure, you can use the 90 degree wire? Even though you are taught that you use the table based upon the connectors, not the wire?
Hmm...

 

[Dennis Alwon]

Adam, you base the ampacity of the circuit on the weakest link in the setup. 90C wire is used at a higher ampacity even though the terminals or the equipment may be rated for 75C. This is often the case. You would use the 90C wire in order to derate and not have to use larger wires.

Take NM wire-- it has # 12 wire rated at 90C by code--- It is rated 30 amps but the cable (NM) cannot be used at a final ampacity of 60C, #12 at 60C is 20 amps but using 90C wire I can still derate from 30 amps. I can have 4- 12/2 nm cables bundled even though it has 8 current carrying conductor's because the wire is rated 30 amps and derating to 70%, I still have 21 amps . As long as the load is not rated more than 20 amps you are good to go.

 

[mjmike]

You didn't mention the insulation type it could be a 60° C conductor in Mike's example.

Valid point, but in example page 52/53 for 210.19, the question didn't indicate the insulation rating either and they derated from the 90C column.

 

[mjmike]

I emailed this into the Mike Holt Corrections for the book for review. They will be making the correction.