CRAC Unit wire & Breaker Size

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Alwayslearningelec

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I have a computer room AC that's 25.4 FLA. It has a 30A disconnect, a 50A breaker with 3 #6's. Sizes seem screwy.

I come up with:

1st step - For conductors 25.4 x 125% = 31.75A. So #10 THHN would be fine
2nd step - Breaker size - Not sure of calc. I believe it should be 125% of continuous load. 40A breaker would work.
3rd step - Disconnect size would be 115% of FLA. 30A disconnect.

Correct?
 
horsegoer said:
I have a computer room AC that's 25.4 FLA. It has a 30A disconnect, a 50A breaker with 3 #6's. Sizes seem screwy.

I come up with:

1st step - For conductors 25.4 x 125% = 31.75A. So #10 THHN would be fine
2nd step - Breaker size - Not sure of calc. I believe it should be 125% of continuous load. 40A breaker would work.
3rd step - Disconnect size would be 115% of FLA. 30A disconnect.

Correct?
Click to expand...
Nameplate should give all data needed. Minimum circuit ampacity, max fuse size.
 
I agree with Dsg the nameplate will tell you everything that you need to know. Also you do not add 25% to the MCA because it's already calculated in. So the minimum conductor size if 25.4 amps or as you've stated #10's. If it were 25.0 amps you could use #12's.
 
infinity said:
I agree with Dsg the nameplate will tell you everything that you need to know. Also you do not add 25% to the MCA because it's already calculated in. So the minimum conductor size if 25.4 amps or as you've stated #10's. If it were 25.0 amps you could use #12's.
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Obviously don't have nameplate as I'm not in the field. What is MCA?
 
horsegoer said:
Obviously don't have nameplate as I'm not in the field. What is MCA?
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Sorry I read your post incorrectly so disregard my first answer. You said FLA and I saw MCA (minimum circuit ampacity).
I agree the #6 conductors appear to be oversized.
 
horsegoer said:
Obviously don't have nameplate as I'm not in the field. What is MCA?
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MCA = minimum circuit ampacity, and is required to be on the nameplate for equipment that uses hermetic refrigerant motor-compressors. The nameplate will also list the maximum permitted size of the OCPD. It is code compliant to install conductors sized to the MCA with an OCPD sized to the maximum permitted OCPD.
 
horsegoer said:
I have a computer room AC that's 25.4 FLA. It has a 30A disconnect, a 50A breaker with 3 #6's. Sizes seem screwy.

I come up with:

1st step - For conductors 25.4 x 125% = 31.75A. So #10 THHN would be fine
2nd step - Breaker size - Not sure of calc. I believe it should be 125% of continuous load. 40A breaker would work.
3rd step - Disconnect size would be 115% of FLA. 30A disconnect.

Correct?
Click to expand...
Just look for the figures called MCA and MOCPD (MOP) in the nameplate. You size your supply conductors using MCA figure and your breaker size should be that what is labeled as MOCP (MOP). No calculations needed!
 
topgone said:
Just look for the figures called MCA and MOCPD (MOP) in the nameplate. You size your supply conductors using MCA figure and your breaker size should be that what is labeled as MOCP (MOP). No calculations needed!
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I agree but he said that he didn't have access to the nameplate.
 
horsegoer said:
Obviously don't have nameplate as I'm not in the field. What is MCA?
Click to expand...
Where did you get "FLA" from. That isn't normally a term used on those nameplates, at least not for the compressor, those usually marked as RLA.

But a self contained listed unit almost always has MCA and MOCPD which is all you need for selecting the supply conductors and OCPD.
 
kwired said:
Where did you get "FLA" from. That isn't normally a term used on those nameplates, at least not for the compressor, those usually marked as RLA.

But a self contained listed unit almost always has MCA and MOCPD which is all you need for selecting the supply conductors and OCPD.
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mechanical drawing equipment schedule
 
kwired said:
Where did you get "FLA" from. That isn't normally a term used on those nameplates, at least not for the compressor, those usually marked as RLA.
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A CRAC unit doesn't necessarily include a compressor. But I would think a CRAC unit with a compressor would have fan motor(s) in addition to the compressor(s). The complete unit would have a full load current, as rated load current applies only to the compressor(s).
 
david luchini said:
A CRAC unit doesn't necessarily include a compressor. But I would think a CRAC unit with a compressor would have fan motor(s) in addition to the compressor(s). The complete unit would have a full load current, as rated load current applies only to the compressor(s).
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If it has compressor(s) it would generally also have a MCA marked on it, unless it is a stand alone compressor - which would be more common for larger stand alone compressor units in a commercial/industrial refrigeration application that has evaporator and condenser both in separate location from the compressor unit.
 
I've only seen Liebert CRACs. They have nameplates on them. You may have to lift the lid on the unit to find it if you get the chance. Are any model numbers available?
 
david luchini said:
A CRAC unit doesn't necessarily include a compressor. But I would think a CRAC unit with a compressor would have fan motor(s) in addition to the compressor(s). The complete unit would have a full load current, as rated load current applies only to the compressor(s).
Click to expand...
Here's a web picture of a Liebert unit nameplate.
20-Ton-Liebert-AC-data-tag.jpg


It does say "Input amps" and Minimum Supply Circuit Ampacity" as opposed to the normal MCA and MOCP (MOP).
 
topgone said:
Here's a web picture of a Liebert unit nameplate.
20-Ton-Liebert-AC-data-tag.jpg


It does say "Input amps" and Minimum Supply Circuit Ampacity" as opposed to the normal MCA and MOCP (MOP).
Click to expand...
other than the word "supply" being in there that is what MCA per NEC is, and MOCP sometimes does have variants as well but is still usually fairly clear what it means.
 
kwired said:
OK I had to look up exactly what a CRAC is, can't say I ever ran into one.

If it falls under art 440 though, I'd still expect it should have a MCA if it is a multi load unit containing a hermetic refrigerant compressor.
Click to expand...
A crac is usually on a raised floor hot mess...
 
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