310.16

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A/A Fuel GTX

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Is there ever a time when the conductors in the 90C column after derating for temperature and 310 (B) (2) (a) factors can be used at their rated ampacity without going to the 75C column? I know if we had 90C terminals we could but I've never seen that.
 

jim dungar

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Yes.

The only time conductors must be used at the 75C rating is when they terminate at a device. So you could use the 90C column when you are going from a splice to second splice.

UL has no listings for overcurrent protective devices at the the 90C column ratings
 

charlie b

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jim dungar said:
So you could use the 90C column when you are going from a splice to second splice.
That would, of course, be of limited value. Eventually, the conductors are going to have to land somewhere. That termination point is probably going to impose the most limiting constraint on the ampacity of the entire run.
 

ramdiesel3500

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Please correct me if I am wrong, but if a circuit originates and terminates in locations with low ambient temps, and the circuit passes through a location with a higher ambient, would it not be allowable to figure the derating of the conductors that pass through the higher ambient based on the 90 degree column values? I see this condition often when a branch circuit is routed through a blistering hot attic.
 

charlie b

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If you are using a conductor with a 90C rating, then you can always start the derating process with the values in the 90C column. 110.14(C).
 

ramdiesel3500

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Thanks Charlie
Now I can actually show folks why this is permissible. I have been told in the past that the temperature derate has to be base on the same column as is used for the rating of the terminal lugs. The explanation had to do with coppers tendancy to transmit heat along the conductor to the termination point! BOGUS!
Thanks again!
 

charlie b

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Do keep in mind that once you derate from the 90C column, you still can't use a final number that is higher than the 75C column.
 

A/A Fuel GTX

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So generally speaking, the values in the 90C column could never be used to carry the ampacities from that column. Correct?
 

jim dungar

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No.

For example:
If you start at a circuit breaker with 75C rated conductors and go (a distance, like 4ft) to a junction box, then you can switch to 90C conductors for any distance you want, then use another junction box and go back to 75C conductors, finally run a final distance (again say 4ft) to a panelboard.
 

A/A Fuel GTX

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But the ampacity of the circuit would be based on 75C so in reality the use of the 90C conductor gains you nothing. Which brings me to my next question....why even bother producing 90C insulation?
 

infinity

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Jim's example is making an assumption that the 75 degree portion of the circuit has a larger conductor than the 90 degree portion of the circuit. The splice would typically use wirenuts listed at 105 degrees.
 

George Stolz

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m73214 said:
But the ampacity of the circuit would be based on 75C so in reality the use of the 90C conductor gains you nothing. Which brings me to my next question....why even bother producing 90C insulation?
It sure makes derating less brutal. You derate from a higher ampacity than allowed, and work your way back to the 75? rating or below. A higher starting point results in a better chance that derating won't result in us having to compensate by installing a larger conductor, or a smaller OCPD. :)
 

jim dungar

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Infinity is correct, I was misleading in my example.

Circuit breaker with terminations rated at 75C.
4 ft of conductor sized using 75C column.
Junction point with 90C minimum rated splices
???ft of conductor sized using 90C column.
Junction point with 90C minimum rated splices.
4 ft of conductor sized using 75C column.
Panel with terminations rated at 75C.

This is only one possibility. Eventually there is an point where it is not economical.
 

George Stolz

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macmikeman said:
why even bother producing 90C insulation?

Now I am the one getting confused. Isn't it so you can use the wire in an enviroment that exceeds 75 deg such as attics?
Mike, as soon as the ambient exceeds 30?C (86?F), we would need to start adjusting the ampacity of any conductor/cable. IMO, NM is towards the bottom of the attic insulation, so it's not exposed to the extreme temperatures of the attic itself. The temperature around the NM would be closer to the temperature of the room below.

If the NM is in the attic space, I think you take an average of the ambient temperature year-round. In my area, I would guess that an attic gets as low as 25?F in the winter, and as high as 140?F in the summer. So, a wild wild guess...
140 - 25 = 115
115 / 2 = 57.5
57.5 + 25 = 82.5

...would put the average attic temperature at 82?F.

So, if you're into wildly speculative math, no correction would be necessary for NM in the attic space itself. :D

Factor in the allowance in 310.15(A)(2), exception, and we can get pretty crazy in an attic space without having to worry about derating for ambient temperature.

IMO. :D
 

iwire

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georgestolz said:
If the NM is in the attic space, I think you take an average of the ambient temperature year-round. In my area, I would guess that an attic gets as low as 25?F in the winter, and as high as 140?F in the summer. So, a wild wild guess...
140 - 25 = 115
115 / 2 = 57.5
57.5 + 25 = 82.5

...would put the average attic temperature at 82?F.

Where in the NEC tells us we can use the average temperature? :?:

IMO we have to use the maximum expected temperature.

That goes against Charlie B's 'insulation life expectancy bank' :wink: which may work quite well, I just do not see anything in the NEC that allows averaging here.

When derating conductors for fill do you use the highest ampacity a conductor may carry or do you use the average ampacity the conductor may carry?

To me it is the same issue we are dealing with heat damaging the insulation.
 

George Stolz

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iwire said:
Where in the NEC tells us we can use the average temperature? :?:
It doesn't. Since you brought it up, I looked closer:
310.10, FPN No.1, (1) Ambient temperature ? ambient temperature may vary along the conductor length as well as from time to time.
Unfortunately, it doesn't provide much assistance in decision making. :?

IMO we have to use the maximum expected temperature.
If so, we're still okay. A 14-2 NM cable in a 140?F attic has an ampacity of 17.25 amps. The same cable in a 158? attic has an ampacity of 14.5 amps. Provided the load does not exceed 14.5 amps for less than three hours, or 11.6 amps for over three hours, then we're still okay.

When derating conductors for fill do you use the highest ampacity a conductor may carry or do you use the average ampacity the conductor may carry?
Neither. The expected load does not factor into derating, until you have finished the derating process. Once we're done derating, then we can turn our attention back to the load, and overcurrent protection.

To me it is the same issue we are dealing with heat damaging the insulation.
But the damage isn't a foregone conclusion. I think what's messing with us is that we're trying to determine mathematically if a conductor is going to be damaged under certain conditions. If we assume that the damage is guaranteed, then we would be inclined to follow the "max temperature" approach. If we're not sure that it's guaranteed, then we turn
our attention to what "the conductor can withstand over a prolonged time period without serious degradation." If we're looking to the big picture, we'd need to look at the environment over a prolonged period to get an accurate assessment, I think.

(But rest assured, I'm sitting on the fence on this one! :) )
 

pierre

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Take a look at the FPN after 310.10.

George
I agree with Bob about your calculations, I kind of got a chuckle of how you handled that.
Just think for a minute... If you have the cable going through the attic space and we have a heat wave (which in NY we seem to have each summer), and the attic temp is approximately 130 F, that can and most likely will affect the insulation integrity. Specifically how, I am not sure, but it cannot be good over time. Have this happen over a couple of summers and I am sure it is not good.
If we were to use your calculations, it would not even show up on the average as a problem at all, very misleading.
 
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