Derating conductors on roof.

[nickelec]

Need some help understanding this . I have a 3.5 inch conduit from the roof going to the basement combiner panel.

There are 30#6 thhn conductrs installed on a 30 amp breaker.

The conduit is mounted on kindorf roof curbs about 5-6inches from surface.

Am I code compliant. Can someone help me with the formula.

I've attached the chart for my local ambient tmeps

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[wwhitney]

If you want to do what the NEC requires, what version of the NEC is applicable?

If you just want to use the highlighted line in the last chart, 131F would be a 0.76 temperature correction for 90C insulation, 30 CCCs is 0.45 adjustment, and #6 Cu has a 75A 90C ampacity. So the ampacity would be the product of those, or 25.65A. Just good enough for a 30A breaker under 240.4(B) (particularly as you could argue that the temperature 5" - 6" above the roof would be more like 127F from interpolation, so your temperature correction factor would be more like 0.78, which gives a 26.3A ampacity) as long as your calculated load is no more than 25.65A.

Cheers, Wayne

 

[nickelec]

If you want to do what the NEC requires, what version of the NEC is applicable?

If you just want to use the highlighted line in the last chart, 131F would be a 0.76 temperature correction for 90C insulation, 30 CCCs is 0.45 adjustment, and #6 Cu has a 75A 90C ampacity. So the ampacity would be the product of those, or 25.65A. Just good enough for a 30A breaker under 240.4(B) (particularly as you could argue that the temperature 5" - 6" above the roof would be more like 127F from interpolation, so your temperature correction factor would be more like 0.78, which gives a 26.3A ampacity) as long as your calculated load is no more than 25.65A.

Cheers, Wayne

I came up with the same thing. Here in NYC believe it or not we are still on 2008 code cycle.

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