Help with 310.15(B)(3)(a)

[MNSparky]

Running five 120v circuits with individual neutrals in wiremold 3000 for receptacle loads (battery chargers) using THHN. Looking at the table, I need to derate 50% for the ten CCC. According to the engineered plans, the connected load for each circuit is 1500va (12.5a). The circuits will be breakered at 20amps and run no further than 50' from the panel so voltage drop isn't an issue.

From the 90 degree column, I'm under the impression that I can still use #12 since it's rated at 30a so 50% of that would be 15a, which satisfies my 12.5a load. I can still put them on a 20a breaker as this demand doesn't effect the OCPD amapcity selection right? Am I doing the calculation correctly? Does the fact that they are all receptacles factor in?

The thing that is giving me pause is the plans call for all these circuits to be run in #8 (no type indicated) and it specifically says due to 310.15(B)(3). Does anyone know why this would be or what the engineer is including that I am not? Any help is greatly appreciated.

 

[infinity]

How big is Wiremold 3000? If it complies with this then derating applies after you hit 30 CCC's.

386.22 Number of Conductors or Cables. The number of
conductors or cables installed in surface metal raceway shall
not be greater than the number for which the raceway is
designed. Cables shall be permitted to be installed where such
use is not prohibited by the respective cable articles.
The adjustment factors of 310.15(B)(3)(a) shall not apply to
conductors installed in surface metal raceways where all of the
following conditions are met:
(1) The cross-sectional area of the raceway exceeds 2500 mm 2
(4 in. 2 ).
(2) The current-carrying conductors do not exceed 30 in
number.
(3) The sum of the cross-sectional areas of all contained
conductors does not exceed 20 percent of the interior
cross-sectional area of the surface metal raceway.

 

[MNSparky]

Wiremold 3000 is 1-15/32" x 2-3/4" (37mm x 70mm). So if I take the easy metric math and do 37 x 70 = 2,590mm2. So I don't need to derate after all. Thanks infinity. Any ideas how the engineer came up with #8s? was my math above correct (for next time)?

 

[infinity]

Wiremold 3000 is 1-15/32" x 2-3/4" (37mm x 70mm). So if I take the easy metric math and do 37 x 70 = 2,590mm2. So I don't need to derate after all. Thanks infinity. Any ideas how the engineer came up with #8s? was my math above correct (for next time)?

You're welcome. Your math was correct but with #12 AWG conductors you would be limited to a 15 amp OCPD since that's your conductor ampacity after derating and it's a standard size. So if you wanted to use 20 amp OCPD's you would need #10 AWG conductors.

As far as how someone came up with #8's my guess is that they derated from something other than the 90° C column in the ampacity table.