What size overcurrent device for #4 (copper) in Industrial Installation

[mxlplx00]

I have a job at an auto wrecking yard. There are 100 amp fuses from the main panel (240v single phase) run 100 feet to a small 100 amp 8 space panel. From there we have #4 - 3 wires in EMT (240V single phase) run 100 feet to a 12 space 100 amp main breaker.

The 1st Sub-panel has a 5 hp air compressor, a 50 amp welder, some car battery chargers and a 20 receptacle circuit. The 2nd panel has a couple of 20 amp circuits and a 50 amp welder.

So from what I have read I'm only allowed to use the 60 degree column (table 310.16) for anything under 100 amps but have also read somewhere that if it's in a raceway that I could use the 75 degree column (85A for #4) in which case I could go the next standard size to a 90 amp breaker in the first sub-panel to feed the second sub-panel. Is this actually the case?

It seems to me it would be preferable if I had #4 all the way and tap the 2 sub-panels.

Pleas point out what's wrong with my thinking here.

Thanks
Marc

 

[Dennis Alwon]

You can use the 75C column as long as your wire is rated 75C. Of course your panel would also have to be rated 75C and in most situations that is the case.

Now #4 is only good for 85 amps at 75C however if the calculated load is 85 amps or less then you can use section 240.4(B) which allows you to use a 90 amp breaker. You cannot use a 100 amp breaker.

With welders and car charger you better check and make sure that 85 amps is enough

 

[mxlplx00]

Thanks Dennis.
Also I meant to say "it would be preferable if I had #2 all the way and tap the 2 sub-panels"