310.15 (a)2 exception for ampacity

[gfdbiker]

My question is can you use the exception for 310.15 (a) 2 for not using the derating factor for having multiple conductors in a conduit. In my example I have a circuit length of 150' of 4 sets of 3/c 500mcm each in its' own conduit except for a distance of about six feet where it has to transition between 2 manholes and I only have 2 spare conduits (5"). Since it is less than 10' and 10% of the circuit, can I use that exception? However, 310.15 (b) 3 states that for raceways greater than 24 inches you must derate for multiple conductors. Which is the correct interpetation?

 

[charlie b]

Which is the correct interpetation?

Both. For the 144 foot length, you have one ampacity (1520 amps, I believe). For the remaining 6 feet, you have a lower ampacity (1216 amps, if I understand the installation correctly). But you are allowed to use the higher of these two values for a distance of up to 10 feet, and you only need to use it for 6 feet. So the 1520 amps applies for the entire run.

Welcome to the forum.

 

[gfdbiker]

Why does the code say you must derate installations greater than 24"?

 

[charlie b]

Why does the code say you must derate installations greater than 24"?

It doesn't. Rather, it says you have to derate if you put more than three current-carrying conductors in a conduit, then it makes an exception if the run is shorter than 24". Reference 310.15(A)(2), Exception 3 (2008 NEC).

Welcome to the forum.

 

[rcwilson]

The exception is apparently based on the theory that temperature in a 24" or shorter raceway will be limited by heat conduction and convection out of the conduit. On longer raceways, the heat has to travel too far to get out.

A co-worker who spent many years as a forensic investigator strongly believes that this exception, along with excessive bundling of cables at panelboards, is the cause of many fires.

 

[electricguy61]

The exception is apparently based on the theory that temperature in a 24" or shorter raceway will be limited by heat conduction and convection out of the conduit. On longer raceways, the heat has to travel too far to get out.

A co-worker who spent many years as a forensic investigator strongly believes that this exception, along with excessive bundling of cables at panelboards, is the cause of many fires.

Has he (or anyone else) published something that supports his beliefs? I'd really like to read about it. Thanks.

 

[rcwilson]

He hasn't published anything yet. A lot of the data is not available for legal reasons. It's just personal opinion at this point.

 

[ActionDave]

The exception is apparently based on the theory that temperature in a 24" or shorter raceway will be limited by heat conduction and convection out of the conduit. On longer raceways, the heat has to travel too far to get out.

A co-worker who spent many years as a forensic investigator strongly believes that this exception, along with excessive bundling of cables at panelboards, is the cause of many fires.

I'd like to see a report on that too. What is the legal reason that the data is not available?

 

[rcwilson]

There were non-disclosure agreements on many of the investigations. The majority of their clients were lawyers so I was assuming there are attorney/client privelege issues involved.

I too wish he had full access to the data and the time to develop a paper. For now, we only have his impressions and his recommendations that we not use that exception in our designs.