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A question regarding "Free Air"

A question regarding "Free Air"

A question regarding "Free Air"

(OP)
I have a question for regarding the definition of "Free Air". I have attached a photo of an older installation. You will three (3) large disconnects. Inside each disconnect is an old Klockner Moeller 800AF/AT Breaker. On the line side of each breaker lands two (2) sets of 3-1/C 500MCM (insulation type unknown). The other end(s) terminate onto the ATS you see on the far left. I highly suspect the termination provisions on the KM breakers are not 90C rated; therefore, I would have to utilize the 75C column, yes? The problem is...

Do I use the "Raceway, Cable, or Earth" or "Free-Air"? I suspect "R, C, or E" as there is a fair amount of the cable protected/supported by conduit/weatherhead and enclosure.

Based on what I see (as explained above) it would appear the designer/installer based the wire size on either:
a) 90C column of "Raceway, Cable, or Earth" which would handle 860A, or
b) 75C column of "Free Air" which would handle 1240A (overkill)

Assuming the termination provisions are 75C and the cable can not be classified as being "Free Air", it would be safe to say this installation is not to code given that the cable would only be able to to support 760A (before temp correction factors, derating, etc) and the breaker is 800A.

I welcome any comments on my thinking/logic.

Thanks in advance for you time!

RE: A question regarding "Free Air"

See NEC 240.4. If you conclude the ampacity is 760 A, then an 800 A breaker is acceptable. The "next larger size" exception applies up to and including 800 A breakers. Above 800 A the "next larger size" exception cannot be applied.

See NEC 310.15 for help in determining ampacity for a circuit where more than one NEC ampacity could apply. Generally, you must use the lowest ampacity, but there are some exceptions.

RE: A question regarding "Free Air"

SparkOmatix,
As DPC stated, as long as the 800 Ampere breaker rating is the next larger standard trip rating, then you are code compliant with conductors with an ampacity that is slightly below that value. Those old breakers will most likely be the 80% rated type anyway, so this circuit should never see an overload condition that will harm those conductors.
Regards,
EEJaime

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