HVAC ELECTRICAL RATINGS

T

Therealcrt

Senior Member
Location
Kansas City
Occupation
Electrician
I’m soon gonna be wiring a make up air unit ( HEAT ONLY) and the heat source is Gas. the nameplate provides the following ; 480v 3PH the MCA 27.1 and MOP is 45. I planned on using power from an existing make up air unit that will be demoed. And I traced it back to the panel and it’s 10 gauge wire on a 30 amp QMB branch switch fused so my question is on the new unit obviously I’m sizing the wire according to 27.1 A which I’m safe to use 10 gauge wire, but I’m curious if I can use that existing 30 amp QMB branch switch? I know I can’t go over 45 A but can I safely run it at 30 A otherwise I’d have to switch out the whole bucket to a 60 amp branch switch infuse it to 45.
 
45A is the maximum. The installation is code complaint.

The code does not compel you to install a bigger disconnect. The only reason you would do that is if the 30A fuses are not large enough to start the load.

The 30 amp fuses will probably start the fan in that make up air unit just fine.

If they don't, you will have to replace the disconnect. So plan for that.

But you probably won't have to.

The way to find out for sure is to look at the specs on the fan motor, look at the motor starting system, and then use that to calculate inrush. Compare the inrush to the time current curve of the installed fuses. Then you don't have to hope, if that's a problem.
 
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