De-rating

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The problem is more complicated if the raceway it is an open top but solid bottom cable tray.
NEC 2017[or 2020] art.392.80 refers you to Article 310.14 A which refers you to point C:
Exception to (2)(3)(c): For solid bottom cable trays the ampacity of
single conductor cables shall be determined by 310.15(C) [310.14(C) in 2020]:
C) Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general equation:
I=√[(Tc-Ta)/(Rdc*(1+Yc)/Rca]
where:
Tc = conductor temperature in degrees Celsius (°C)
Ta = ambient temperature in degrees Celsius (°C)
Rdc = dc resistance of 305 mm (1 ft) of conductor in microohms at temperature, Tc
Yc = component ac resistance resulting from skin effect and proximity effect
Rca = effective thermal resistance between conductor and surrounding ambient
Nor Neher and McGrath theory neither IEEE 835 does not indicate any method to calculate Rca for cable trays.
The only standard I know it is ANSI/NEMA WC51 /ICEA 54-440 [ based on Stolpe theory] AMPACITIES OF CABLE INSTALLED IN CABLE TRAYS.
However, in the introduction it is written:
This standard is intended primarily for use by the utility industry. It is not intended for use where
compliance with the National Electrical Code or other regulations is mandatory
No indication about cable exposed to sun [no solar heat absorption].





 
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