Underfloor duct derating

[sevlander]

Can anyone explain the procedure for derating circuits in underfloor duct? I have an installation requiring 25 120volt circuits in the same duct run. What column do I use to determine conductor amps? What is the step by step method needed to determine derating of conductors and code articles justifying it? When can 50% load diversity be used? Thanks!

 

[don_resqcapt19]

You start with the ampacity listed under the maximum temperature rating of the conductor. For example if you are using THHN, you start with the 90?C ampacity and using Table 310.15(B)(2)(a) you adjust for the number of current carrying conductors in the duct. As far as the diversity, the reason that information is in the Annex and not part of the actual code is that no one really knows how you are to apply it.

 

[sevlander]

Thanks.Where is it stated to start with the maximum temperature rating of the conductor? That is how I do it, but can't find where it says that in the NEC. I believe you start with the 90 degree column for THHN for example, even though you may be using the 75 degree for the actual connection. Someone must know about load diversity...someone put it in there...unless the guy who wrote died and nobody else will touch it! It would change the job considerably if I could actually use it.

 

[don_resqcapt19]

Look at 110.14(C) for the derating from the highest rating of the conductor.
As far as the diversity, it was in the main part of the code for one cycle and there was a lot of debate as to how you could use it. No one arrived at a workable answer so it was moved to the Annex. I guess if you can work it out with your AHJ you could use it.

 

[480sparky]

You start at 310.15, Ampacities for Conductors.

Table 310.13 will give you the temperature rating for a specific insulation type.