310.15(b) (2)

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tower

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I have a question that was on a test that I would like to run by you. word for word. an installation consisting of 7 non- jacketed 12 awg type mc cables, each with 3 current carrying conductors, bundled together longer than 24" shall have an ajustment factor of. the way I read it, there is 21 current carrying conductors and acording to 310.15 (b)(2) it exceeds the 20 conductors and can be adjusted to 60% instead of using table 310.15 (b)(2)(a) which states 45%. which one is the right answer to the question?
 
Re: 310.15(b) (2)

I agree with you. The answer is "60%." The reference is Exception 5 to 310.15(B)(2)(a).
 
Re: 310.15(b) (2)

why would you use the 60%. is`nt 45% in your favor. what prevents you from using the tables. is there two right answers here. is it a error in the code book.
 
Re: 310.15(b) (2)

Tower, there is not an error, it is simply giving special consideration for AC and MC per exception 5 which allows us a high conductor ampacity after derating.

Why would you think we would want to reduce to 45% if we were not required to?

Roger
 
Re: 310.15(b) (2)

I think I may have a misinterpintation of the percentages.If you reduce something by 45% its not as bad as reducing it by 60%.or less is more in this case,is it not
 
Re: 310.15(b) (2)

Tower, notice the wording in table 310.15(B)(2)(a) "Percent of values in Tables 310.16 through 310.19 ....." this would mean if we were to look at 310.16 and take a #12 rated at 30 amps in the 90 deg column and calculate 45% of its ampacity, it would reduce the ampacity to 13.5 whereas 60% would be 18 amps.

It is not reducing by the percentage, it is reducing to the percentage.


Roger

[ February 10, 2005, 06:51 PM: Message edited by: roger ]
 
Re: 310.15(b) (2)

jim, as long as the calculated or computed load does not exceed 18 amps after all is considered, yes it can be protected by an OCPD of 20 amps if all the conditions of 240.4(B) are met.

This is the exact reason for exception 5 to 310.15(B)(2) IMO.

Roger
 
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