disconnect sizing

[JH13]

hello, i have a compressor that requires a 225A circuit breaker. disconnect sizing has nothing between 200A and 400A. manufactrurer is calling for 200A class J fuses to be installed at the disconnect. question is: because 200A fuses are being used can i use a 200A disconnect instead of jumping up to the 400A or do i need to upsize because of the breaker?

****EDIT***
also, if i already have a 400 amp disconnect can i add a knife blade fuse block inside or after the disconnect in its own enclosure?

 

[augie47]

If the manufacturer is calling for 200 amp fuses then it seems a 200 amp fusible disconnect would be all that you need,
It sounds as if they are calling for a 225 amp feeder to supply that disconnect which makes some sense based on 430.52. How do you intend to feed the disconnect ??

 

[JH13]

If the manufacturer is calling for 200 amp fuses then it seems a 200 amp fusible disconnect would be all that you need,
It sounds as if they are calling for a 225 amp feeder to supply that disconnect which makes some sense based on 430.52. How do you intend to feed the disconnect ??

the disconnect is fed off of a 480v panel, 225A breaker. for cost reasons of using things already on site, the question came up if i could add a fuse block in a seperate enclosure between a non-fusible disconnect(already on site) and the compressor. i was asking about a 200 amp fused disconnect as an option to replace the current one if adding an enclosure isnt cost effective.

 

[Jraef]

The disconnect switch must have a HP rating equal to or greater than the motor HP, regardless of the amp rating.
Typically, 200A 480V disconnects are rated for a maximum of HP, so if your motor is 150HP or less, you are good to go.

Side issue relating to your other question about the fuse blocks:
You ALSO need for the disconnect to have an SCCR (withstand) rating equal to or greater than the Available Fault Current. Non-fused disconnects are all rated for 10kA at best, some less than that. If you have more than 10kA at that point, it may not pass inspection.

FUSED disconnects are usually rated to whatever the fuses are rated to. If you mount the fuses separately you could make an argument that it is the same, but an AHJ can also take the position that the Non-fused disconnect has a LABEL on it that will say 10kA and reject it. It's a chance you can take, but I tend to not like to have to do things twice...