Selection of ampacity 310.15(a)2 exception

[jwj956]

Could this exception apply to this scenario: A panelboard has multiple branch circuits leaving thru a single (properly sized for conductor fill) conduit to a junction box,(also adequately sized) located in the ceiling above. Mc cables then branch out from the ceiling mounted junction box. There are over 41 current carrying conductors in the conduit between panelboard and junction box, however the length of these conductors complies with this exception. Although a derating factor of 35 percent typically applies to the current carying conductors, it would seem to me that by complying with the conductor length given in the exception (10 feet or 10 percent of the circuit length whichever is less) that derating at 35 percent would not apply to these conductors. I have only seen examples of this exception applied where a conduit is exposed to different ambient temperatures, however this scenario may also apply. Any thoughts?

 

[charlie b]

I had never thought of applying that exception to this type of installation. But I don't see any words in the rule that would exclude it. So I would say it is acceptable.

Welcome to the forum.

 

[fmtjfw]

You need to pay attention to "10 feet or 10 percent of the circuit length whichever is less". This would appear to require an additional 90 feet for each of the circuits in a 10 foot common conduit. My guess is that the 90% additional circuit length is used as a heat sink to dissipate the excess heat in the conduit.

 

[haskindm]

It says whichever is less. So if you have a 3 foot conduit for which you are wanting to use this exception the total circuit length would need to be "at least" 30'. The maximum length to which this exception would apply is 10 feet regardless of the total circuit length.

 

[infinity]

Could this exception apply to this scenario: A panelboard has multiple branch circuits leaving thru a single (properly sized for conductor fill) conduit to a junction box,(also adequately sized) located in the ceiling above. Mc cables then branch out from the ceiling mounted junction box. There are over 41 current carrying conductors in the conduit between panelboard and junction box, however the length of these conductors complies with this exception. Although a derating factor of 35 percent typically applies to the current carying conductors, it would seem to me that by complying with the conductor length given in the exception (10 feet or 10 percent of the circuit length whichever is less) that derating at 35 percent would not apply to these conductors. I have only seen examples of this exception applied where a conduit is exposed to different ambient temperatures, however this scenario may also apply. Any thoughts?

I agree that the exception does apply here as long as you're within the 10% or 10' limitation of the rule. How long are the circuit conductors and the conduit?