Disconnect sizing for chiller

acolella

Member
If you work for a manufacturer why don't you ask them? My understanding is they conduct "worst case" test and calculations on the equipment and that determines mca.
Absolutely correct. I work for a different group and am trying to get in contact with the appropriate person in the EE group for this particular line of product. I figured I’d throw the question at the internet just to see what others have to offer about the question.
 

acolella

Member
The MCA is only 6 amps more that the calculated MCA. It's the controls. Contactor relays etc will use 6 amps on that size of machine
Yes. I spoke with one of the other technicians today and we’re fairly certain that it is the control power. 5 kVA control power transformer. I believe it also feeds a convenience receptacle. Interesting that it’s not on the nameplate though. From what I understand, all loads over 1 amp are usually listed on the nameplate. Also interesting is that this unit has no contactors. Compressors and condenser fans are all on VFDs. Even with the convenience receptacle, 5 kVA seems a bit oversized to me. However, that additional 5-6 amps wouldn’t account for the full utilization of a 5 kVA 480-120 single phase transformer. If we tested it and determined that the control power loads are only 5-6 amps on the transformer’s primary then the listed MCA makes perfect sense. Appreciate the replies!
 

acolella

Member
Yes. I spoke with one of the other technicians today and we’re fairly certain that it is the control power. 5 kVA control power transformer. I believe it also feeds a convenience receptacle. Interesting that it’s not on the nameplate though. From what I understand, all loads over 1 amp are usually listed on the nameplate. Also interesting is that this unit has no contactors. Compressors and condenser fans are all on VFDs. Even with the convenience receptacle, 5 kVA seems a bit oversized to me. However, that additional 5-6 amps wouldn’t account for the full utilization of a 5 kVA 480-120 single phase transformer. If we tested it and determined that the control power loads are only 5-6 amps on the transformer’s primary then the listed MCA makes perfect sense. Appreciate the replies!
I’m forgetting oil/evap heaters. However, not all of those would be concurrent loads but it all still adds up for the most part. Still curious why it’s not on the nameplate though.
 

kwired

Electron manager
Location
NE Nebraska
Don't do a lot of work on large rooftop or other completely self contained units, but can't recall ever seeing data on nameplate for a crankcase heater, and don't see too much need for it to be there, they shouldn't need to run while the compressor is running.

5 amps of control power added to the supply volts is 1200 VA @ 240 volts 2400 @ 480 volts. That is a lot of contactor coils, I think if it is that high it probably includes things like damper motors, or motorized zone valves or something of that nature.
 

Mgraw

Senior Member
Don't do a lot of work on large rooftop or other completely self contained units, but can't recall ever seeing data on nameplate for a crankcase heater, and don't see too much need for it to be there, they shouldn't need to run while the compressor is running.

5 amps of control power added to the supply volts is 1200 VA @ 240 volts 2400 @ 480 volts. That is a lot of contactor coils, I think if it is that high it probably includes things like damper motors, or motorized zone valves or something of that nature.
The chillers I am familiar with have a separate 120v circuit for controls.
 

acolella

Member
Don't do a lot of work on large rooftop or other completely self contained units, but can't recall ever seeing data on nameplate for a crankcase heater, and don't see too much need for it to be there, they shouldn't need to run while the compressor is running.

5 amps of control power added to the supply volts is 1200 VA @ 240 volts 2400 @ 480 volts. That is a lot of contactor coils, I think if it is that high it probably includes things like damper motors, or motorized zone valves or something of that nature.
I have never seen crankcase heaters on a nameplate either. I'm saying control power as a whole. The heaters on this unit are fed from the control power transformer but, as I mentioned, and you did as well, they wouldn't be concurrent loads because they would only run when the compressor is not running.

This unit has no contactors, no dampers, no zone valves.

It certainly is a hefty control power transformer. The 20 amp convenience receptacle on top of all of the control boards and control box cooling fans is most likely the reason for the transformer being as large as it is. I'll need to take a closer look in the control box when I have a chance to see if there's something else in there that I'm missing.
 

topgone

Senior Member
IDK where you're at! The common disconnect switch sizes I knew of are 30, 60,100,200, and 400! I know there are bigger ones but just wait for others to chime in.
 

acolella

Member
You would use whatever MOCP the equipment manufacturer gives you as the maximum. It would be a code violation to use a larger size.
I understand that I can not exceed the MOCP on the nameplate and that it would be a violation if I did. I’m asking, would I need a 600 amp disconnect for a continuous load of 345 amps or would a 400 amp disconnect be compliant? I’m still not entirely clear on the loading of a fused disconnect. Can it only be loaded to 80% of its rating for a continuous load?
 

Mgraw

Senior Member
I understand that I can not exceed the MOCP on the nameplate and that it would be a violation if I did. I’m asking, would I need a 600 amp disconnect for a continuous load of 345 amps or would a 400 amp disconnect be compliant? I’m still not entirely clear on the loading of a fused disconnect. Can it only be loaded to 80% of its rating for a continuous load?
Appears to me you are working at a manufacturer that is testing equipment. They are going to push that equipment to the limits. Listen to the engineers and do what they say. The NEC in irrelevant in that situation.
 

acolella

Member
Appears to me you are working at a manufacturer that is testing equipment. They are going to push that equipment to the limits. Listen to the engineers and do what they say. The NEC in irrelevant in that situation.
Yes, this is true. We are testing and we are very much pushing the limits. I am responsible for the installation and I’m curious as to how it would be done in the field. Even though NEC is irrelevant and this will not be officially inspected, I’d still like to know what would be acceptable in the field. In the past and even today, many would figure 345 amps and just say that the 400 amp disconnect is good. I really just want to know if a 345 amp continuous load on a 400 amp fused disconnect would pass an inspection in the field. That’s really all I’m asking.
 

Greentagger

Senior Member
Location
Texas
Occupation
Master Electrician, Electrical Inspector
Disconnecting means to be sized 115% per 440.12(A)(1).


I understand that I can not exceed the MOCP on the nameplate and that it would be a violation if I did. I’m asking, would I need a 600 amp disconnect for a continuous load of 345 amps or would a 400 amp disconnect be compliant? I’m still not entirely clear on the loading of a fused disconnect. Can it only be loaded to 80% of its rating for a continuous load?
 

acolella

Member
Disconnecting means to be sized 115% per 440.12(A)(1).
Right, so that would put me at 396.75 amps for disconnecting means. 400 amp disconnect, right? But what about 80%? Running amps will be 345 amps which is greater than 80% of 400 amps. Do I need to consider that I would be higher than 80% of 400 amps in this situation since the disconnect is also the OCPD? Just trying to grasp a better understanding of this.
 

Greentagger

Senior Member
Location
Texas
Occupation
Master Electrician, Electrical Inspector
Air conditioning equipment and motors are different animals. There rules are different from the norm. Their conductors, disconnecting means, etc. can be sized smaller than short circuit protection , ground fault ,etc. because of the assumption conductors will be protected by overload protection often in the form of motor overloads etc.
 

kwired

Electron manager
Location
NE Nebraska
I understand that I can not exceed the MOCP on the nameplate and that it would be a violation if I did. I’m asking, would I need a 600 amp disconnect for a continuous load of 345 amps or would a 400 amp disconnect be compliant? I’m still not entirely clear on the loading of a fused disconnect. Can it only be loaded to 80% of its rating for a continuous load?
Going back to your OP the MCA was only 316 IIRC and MOCP was 400.

How can you have a continuous load that is more than the MCA?

I don't see you needing more than a 400 amp disconnect if the MOCP is 400 either.
 

Mgraw

Senior Member
Going back to your OP the MCA was only 316 IIRC and MOCP was 400.

How can you have a continuous load that is more than the MCA?

I don't see you needing more than a 400 amp disconnect if the MOCP is 400 either.
If they are testing equipment they manufactured a compressor with a 119 RLA could run 170 - 185 without tripping the overload. Times two compressors and you are over the MCA.
 

kwired

Electron manager
Location
NE Nebraska
If they are testing equipment they manufactured a compressor with a 119 RLA could run 170 - 185 without tripping the overload. Times two compressors and you are over the MCA.
Sounds to me like either the RLA stated is lower than what it can do, or the overload isn't going to protect it if loaded that much. If the latter, a 400 amp OCPD is only going to trip once one of them develops a short circuit or ground fault due to overloading.
 
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