Rooftop unit disconnect sizing

[deetz]

So what's the compliant sizing of a 3 phase industrial rooftop heat/cool unit disconnect. The label gives us a 53 amp MCA (minimum circuit amps) and 70 MOP (maximum overcurrent protection). Can I use a 60A fusible disconnect? My understanding is the MCA is already calculated out with 125% above all the loads. Then you could size the fusing above the MCA but not more than the MOP. Is that correct?

 

[jraef]

You CAN, but if you use a 60A switch you cannot use an MOP above 60A. So then you run the risk of starting current clearing your fuses or tripping the mag trips on your breaker. If it were me, I would use a 100A switch.


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