My boss sent me to relocate 2 a/c feeds, rooftop units. The existing units have 90 amp breakers on #4 thhn. The new units have the need for 60 amp, so since the brekers for the original units are hard to get, and expensive, a 3 phase disconnect (100 amp) with 60 amp fuses, (mounted on the unit) will protect the unit. One of the mechanics at the shop thought you could get away with a 60 amp fused disconnect, but I say that you have to go by the rating of the highest amperage. I only have the handbook at home, but I don't see a reference to this. Also, if one of the fuses lets go, will the unit burn up? (becasue of running on single phase?) Thanks for any help on this.

You can go either way with this it is your choice.

In industrial applications it is not unusual to run a large feeder 400, 600, etc and have a group of disconnects tapped off of these of any size.

Bob

One thing I forgot to mention is if you do go with the 100 amp disconnect you will need fuse reducers to get 60 amp or smaller fuses to fit in a 100 amp switch.

I guess what I meant to ask, Should there be a 100 amp rated disconnect,with 60 amp fuses, or can you get away with using a 60 amp disconnect with 60 amp fuses. The # 4 thhn is protected by a 90 amp breaker in the main switch gear.

yes i got the reducers

The 60 amp disconnect is fine. The 90 amp breaker protects the wire to the line side of the disconnect. The 60 amps fuses protect the load side wire and the equipment.
Don

Richard,

I haven't poked my head inside an AC unit in years, so I don't know if it comes built-in now but you can get phase loss protectors and add them yourself if necessary. I think it's a good idea.

Bill

Thank you all for getting back so quickly. Have you noticed that on the rooftop units, the available space for the disconnects are very limited?

Nope their is plenty of space as long as you do not mind screwing the disconnect to the condenser coil.

Yes it is hard to find space.