310.15(A)(2) exception

[rbossler]

Question from one of my students. The exception reads. 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 (10ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.
When calculating the total circuit length is the section at the an elevated temperature included in the measurement? Example - if the run is 5 feed in a area of elevated temperature and 100 ft is in an area of normal temperature, 105 ft total, do you use 100 feet or 105 feet to calculate the 10%. The exception stated at the higher ampacity, I take that as the normal temperature section, thus using the 100 ft measurement to calculate the 10%. But I am not sure I am correct on this point.
rbossler

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[ October 04, 2004, 06:28 AM: Message edited by: charlie ]

 

[rbalex]

Re: 310.15(A)(2) exception

310.15(A)(2) Selection of Ampacity. Where more than one calculated or tabulated ampacity could apply 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.

Using your description, there are "two adjacent portions of a circuit." ?Portion 1? is 100? @ ampacity ?A? and the other (Portion 2) is 5? @ ampacity ?B.? As described, ?Portion 1? has the higher ampacity. (Ampacity ?A?>?B?)

The ?circuit length [of Portion 1] figured at the higher ampacity? is 100?. 10 percent of that is 10?. Since Portion 2 is less than 10? and it is less than 10 percent of the ?adjacent portion? it may use the ampacity of Portion 1.

Therefore the entire circuit is permitted to use Ampacity ?A.?

If either Portion 2 was over 10? or Portion 1 was less than 50?, the entire circuit would have to use Ampacity ?B? per the basic rule.

It may have been helpful if "given circuit length" in the basic rule, were replaced with "conductor", but a "given circuit length" does include the entire conductor.

[ October 04, 2004, 11:16 AM: Message edited by: rbalex ]