How does one calculate Table 310(B)(2)(a)?

[NorCalC10]

My crew installed 8"X8"X10' "Gutters" above the area where our panel back cans were to be installed(at a later date). The conductors were already derated to 70% but the local inspector said we had derate to 50% because of the cross section area inside the "gutter".

My question is do you derate based on the maximum allowable amps/watts on a conductor or is it based on the OCPD? I do not have a single circuit that will draw greater than 50% of the total wattage available per 310.16. My Inspector told me he has heard of people calculating this based on the conductor ampacity but it is not correct.(In his view)

Now the weird part. The inspector said we could use panel board back cans turned horizontal instead of the "gutters" and we would be fine. It is a matter of semantics I guess based on the classification of a gutter, wireway, or J-box can.

 

[don_resqcapt19]

The derating is for the ampacity of the conductor and it is based on the maximum permitted conductor ampacity as shown in Table 310.16 and the applicable correction and adjustment factors based on the conditions of the install. The number you get after you apply the adjustment and correction factors is the maximum permitted ampacity of the conductors and the conductors must be protected at or below their ampacity.