I am confused

[Pierre C Belarge]

I read this in a magazine:

"
Derating conductors on rooftops
What is the corrected ampacity of three 6 AWG copper conductors with Type THW insulation run exposed to the sun and within 4 inches of the rooftop of a manufacturing building? The wiring method is rigid metal conduit.
Because the person asking the question is from Miami, we will use the outdoor temperature of 91?F as shown in the Rooftop Ampacity Adjustment Tables provided by the Copper Development Association (CDA) with ambient temperatures for various cities taken from the ASHRAE Handbook. The Fine Print Note following Section 310.16(B)(2)(6) recognizes these ambient temperatures. For an ambient temperature of 91?F with the conduit located 4 inches above the roof, the temperature in the conduit is listed at 121?F.
With a CDA slide rule, the table should be set on the 114?F?122?F scale. Read down the 75?C column to 6 AWG copper for a corrected ampacity of 49 amperes. For increased ampacity without increasing conductor size, use Type THHW insulation. The 90?F insulation increases the ampacity of 6 AWG copper conductors to 62."


The blue portion confuses me.

 

[480sparky]

Are you meaning the 90?F reference? If so, it's probably a typo, and should be 90?C.

 

[ActionDave]

I saw the same article. Gotta' be a typo.

 

[Cavie]

I read this in a magazine:

"
Derating conductors on rooftops
What is the corrected ampacity of three 6 AWG copper conductors with Type THW insulation run exposed to the sun and within 4 inches of the rooftop of a manufacturing building? The wiring method is rigid metal conduit.
Because the person asking the question is from Miami, we will use the outdoor temperature of 91?F as shown in the Rooftop Ampacity Adjustment Tables provided by the Copper Development Association (CDA) with ambient temperatures for various cities taken from the ASHRAE Handbook. The Fine Print Note following Section 310.16(B)(2)(6) recognizes these ambient temperatures. For an ambient temperature of 91?F with the conduit located 4 inches above the roof, the temperature in the conduit is listed at 121?F.
With a CDA slide rule, the table should be set on the 114?F?122?F scale. Read down the 75?C column to 6 AWG copper for a corrected ampacity of 49 amperes. For increased ampacity without increasing conductor size, use Type THHW insulation. The 90?F insulation increases the ampacity of 6 AWG copper conductors to 62."


The blue portion confuses me.

The ambient air will be well over 91 degrees in Fl in the summer on the roof. The air alone will be 96/98 in the shade!