Proper disconnect size?

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JES2727

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When providing a disconnect for a chiller, is it sized according to the nameplate minimum ampacity or the max OCPD? For example, the min ampacity is 47.5, the max OCPD is 90. (It's a replacement unit. The existing wire is #6, the existing CB is 50A.)
60 amp disconnect, or 100?
Edit: I just looked it up before submitting this. It looks like I'm good with a 60A disconnect, according to 440.12(A)(1). I'll submit the question anyway, in case anyone has some thoughts on it. Maybe I'm missing something.
 
You are correct. An unfused 60A disconnect is good as long as the supply circuit is not greater than 60A.
 
Smart $ said:
You are correct. An unfused 60A disconnect is good as long as the supply circuit is not greater than 60A.
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I agree with the 60 amp disconnect but in this case with #6 conductors, a 47.5 MCA, and a 90 amp OCPD how would you define the ampacity of the "supply circuit"?
 
JES2727 said:
No. I'm replacing it with a 90. I'm sizing the disconnect based on 115 percent of the nameplate rated-load current.
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That would work. The MCA already has a 125% factor added in so as a rule of thumb if your disconnecting means is the same size or larger than the MCA you're good.
 
Smart $ said:
You are correct. An unfused 60A disconnect is good as long as the supply circuit is not greater than 60A.
Click to expand...

.. and the available fault current is less than 10kA. If the fault current is higher, then it needs to be a fused disconnect.
 
JES2727 said:
No. I'm replacing it with a 90. I'm sizing the disconnect based on 115 percent of the nameplate rated-load current.
Click to expand...
While many believe a switch must be rated not less than the supply protection device, it technically only has to be rated for the circuit voltage and the maximum rated-load current (though Code requires 115% for this case) or motor horsepower.
 
Smart $ said:
You are correct. An unfused 60A disconnect is good as long as the supply circuit is not greater than 60A.
Click to expand...

Smart $ said:
While many believe a switch must be rated not less than the supply protection device, it technically only has to be rated for the circuit voltage and the maximum rated-load current (though Code requires 115% for this case) or motor horsepower.
Click to expand...

I'm not sure if you're contradicting yourself here or not. Is this a 90A supply circuit? What exactly is meant by 'branch-circuit selection current' as it states in 440.12(A)(1)?
 
mayanees
JES2727 said:
I'm not sure if you're contradicting yourself here or not. Is this a 90A supply circuit? What exactly is meant by 'branch-circuit selection current' as it states in 440.12(A)(1)?
Click to expand...
That I did. :ashamed1:

First post was my personal preference.

Second post is Code minimum. Keep mayanees post in mind regardless.

I've never really understood the "branch-circuit selection current" term (BCSC). It is not the MCA marked on the nameplate. I don't recall seeing any HRMC equipment nameplate ever having a BCSC marked... but I seldom work with HRMC equipment.

See page 13 of this UL publication:

http://www.ul.com/wp-content/uploads/2014/04/EHCMG_AG.pdf
 
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