RTU derate wiring

[Dyn]

My ? is, here's a example, Greensboro NC daily avg. 91degrees, change out of a RTU heat pump MCA is 116 , existing wire is #2 thhn/thwn using table 310.15 (B)(2)(a) correcting in 90 degree would be 124 amps, since feed comes thru roof into disconnect is there any other correcting.

 

[kwired]

Likely not.

310.15(A)(2) exception:

Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

Unless the distance above the roof is more then 10 percent of the circuit length, but if going through roof and almost immediately into the disconnect - that seldom will be more then 10 percent of circuit length, the beginning end of the circuit would have to be almost immediately below where it penetrates the roof

 

[Dyn]

dyn

dyn

So the 10ft does not matter if the penetration is vertical or horizontal, my case would be vertical straight into disconnect about 3ft.

what if out of side wall 4in off roof less than 10ft then into disconnect ? Thanks again for post

 

[infinity]

This might help from Mike:

 

[kwired]

So the 10ft does not matter if the penetration is vertical or horizontal, my case would be vertical straight into disconnect about 3ft.

what if out of side wall 4in off roof less than 10ft then into disconnect ? Thanks again for post

Correct but don't forget it says "(10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less". So unless the total circuit length is 100 feet or more that 10 feet will be reduced to 10% of total circuit length. If total circuit length is only 20 feet, then 10% is only 2 feet.

 

[GoldDigger]

It is not the total circuit length that you take 10% of, it is the length which is at the higher ampacity.
If you have a lower calculated ampacity on a 10 foot segment off a 100 foot total run it is a violation. The limit would be 10% of 90, not 10% of 100.

 

[infinity]

I always have found this calculation to be somewhat silly and typical of NEC confusive verbiage. Why not just say 100' circuit length and 10' segment would be code complaint? The original proposal for this code change had suggested 15', so the 100'/10' scenario would still be safe. :roll:

 

[kwired]

It is not the total circuit length that you take 10% of, it is the length which is at the higher ampacity.
If you have a lower calculated ampacity on a 10 foot segment off a 100 foot total run it is a violation. The limit would be 10% of 90, not 10% of 100.

Sorry but I'm not seeing it that way, you care to explain your view some more? The way I see it if total length of run is 100 feet you are allowed to use the higher calculated ampacity beyond point of transition (likely the roof line) for up to 10 percent of the total run or 10 feet maximum if the total run is over 100 feet. If the total run were only 10 feet then you would need to increase conductor size if more then one foot of conductor is above the roof.

(2) Selection of Ampacity.
Where more than one ampacity applies for a given circuit length, the lowest value shall be used.


Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

 

[Smart $]

Sorry but I'm not seeing it that way, you care to explain your view some more? The way I see it if total length of run is 100 feet you are allowed to use the higher calculated ampacity beyond point of transition (likely the roof line) for up to 10 percent of the total run or 10 feet maximum if the total run is over 100 feet. If the total run were only 10 feet then you would need to increase conductor size if more then one foot of conductor is above the roof.

What GD is saying is that 10% of the adjacent circuit length at higher ampacity can never be more than 1/11th (one eleventh) of the total circuit length.

10% of adjacent + adjacent = 100% circuit length
one eleventh + ten elevenths = 100% circuit length

 

[Dyn]

dyn

dyn

Found this video by Mike Holt to explain in depth the temp adjustment (not derating, as it is being eliminated from terminology)
https://www.youtube.com/watch?v=OLIHEiiY_Rs
Think I have it now, thanks guys, I really like this forum

 

[kwired]

What GD is saying is that 10% of the adjacent circuit length at higher ampacity can never be more than 1/11th (one eleventh) of the total circuit length.

10% of adjacent + adjacent = 100% circuit length
one eleventh + ten elevenths = 100% circuit length

I see how you guys are coming up with that, I also don't think that is the intent of the wording, I still think it is intended to mean 10% of total circuit length.

FWIW, if you want to go with 1/11th though you will never fail an inspection by an inspector that reads it as 1/10th.:happyyes:

 

[GoldDigger]

I see how you guys are coming up with that, I also don't think that is the intent of the wording, I still think it is intended to mean 10% of total circuit length.

FWIW, if you want to go with 1/11th though you will never fail an inspection by an inspector that reads it as 1/10th.:happyyes:

It specifically refers to the length of the circuit "figured at the higher ampacity". That wording is totally unnecessary and indeed contradictory if the intent was to use the total circuit length.