Disconnect sizing for chiller

Mgraw

Senior Member
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.
RLA is 63 -70% of maximum compressor running amps before it trips on overload. It is much lower than the old FLA ratings they used in the past.
 

acolella

Member
Occupation
Engineering Technician
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.
I asked an additional question earlier in the thread.

"Let’s say, for example, that the continuous load was 345 amps. Minimum disconnect size would be 396.75 amps. However, I would need a 600 amp disconnect since I would be above 80% of 400 amps?"

Bigger machine going in later.
 

acolella

Member
Occupation
Engineering Technician
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.
The VFDs will limit the current or trip entirely. That RLA is determined at max. load conditions. Anything beyond that and the VFDs will start to back off.
 

acolella

Member
Occupation
Engineering Technician
I should also make it clear that the nameplate RLA is VFD input current. You could look at the VFD and compressor as one entity. The compressor itself doesn't even have a FLA/RLA, etc. listed on it. It is specifically designed to be applied with that VFD and will never be ran without it.
 

acolella

Member
Occupation
Engineering Technician
I should also make it clear that the nameplate RLA is VFD input current. You could look at the VFD and compressor as one entity. The compressor itself doesn't even have a FLA/RLA, etc. listed on it. It is specifically designed to be applied with that VFD and will never be ran without it.
To clear up any confusion this may cause. It is not nameplate input current of the VFD itself. The VFDs are 200 HP with FLA of 231 amps. The RLA on the machine’s nameplate is VFD input current when the machine is ran at max load. This one VFD may be applied with several compressor sizes so the RLA for its particular application is listed on the nameplate, not the VFDs actual listed FLA.
 

kwired

Electron manager
Location
NE Nebraska
Since you have a VFD, the max OCPD is likely less than would be required if this were started across the line.

RLA, FLA, MCA - all probably take any built in limitations that will result in less current into consideration even if an individual motor/compressor could draw more without damaging it.

Even a simple single motor circuit - say you have a motor with rating of 10 amps. You size the conductor at 125% minimum and overload protection to no more than 125%. That motor may draw 150% for short time occasionally but never trips anything, but let it do that long enough the overload protection still limits by shutting it down, or with advanced controls it is possible to reduce the loading automatically, and set it up that it never draws more than a certain amount. With straight art 430 applying you still need to use min circuit ampacity conductors and other equiment per code, but this HVAC unit is a listed unit and they can put whatever they want for rated load and MCA if they design to operate at no more than those levels.
 

acolella

Member
Occupation
Engineering Technician
Since you have a VFD, the max OCPD is likely less than would be required if this were started across the line.

RLA, FLA, MCA - all probably take any built in limitations that will result in less current into consideration even if an individual motor/compressor could draw more without damaging it.

Even a simple single motor circuit - say you have a motor with rating of 10 amps. You size the conductor at 125% minimum and overload protection to no more than 125%. That motor may draw 150% for short time occasionally but never trips anything, but let it do that long enough the overload protection still limits by shutting it down, or with advanced controls it is possible to reduce the loading automatically, and set it up that it never draws more than a certain amount. With straight art 430 applying you still need to use min circuit ampacity conductors and other equiment per code, but this HVAC unit is a listed unit and they can put whatever they want for rated load and MCA if they design to operate at no more than those levels.
With all of the units I have worked with, max OCPD is 225% of largest load plus all other loads then next size down if not a standard OCPD rating. I'm fairly certain that we do this for across the line and VFDs. I'm not entirely sure if this changes if the machine is designed with wye/delta start, etc. I don't think that it does but I could be wrong. I've only worked with a few machines in the last few years with wye/delta start and can't remember how the OCPD was determined for those. Now that I think of this, I'm curious and will find out. Yes, they never draw more than MCA at max load. Appreciate the replies.
 

acolella

Member
Occupation
Engineering Technician
I just took a look at a chiller with an optional wye/delta starter installed. Single compressor machine.

RLA: 283.3 amps
LRA: 578 amps wye/1800 amps delta
MCA: 354.1 amps
MOCP: 600 amps

MCA is RLA times 1.25 (283.3 x 1.25 = 354.125). MOCP is RLA times 2.25 then next size down (283.3 x 2.25 = 637.425). Next standard size down is 600 amps.

MOCP is the same whether starting across the line or wye/delta. It appears that MOCP is determined from RLA regardless of starting method/motor control. I'm pretty sure this is how it's done whether ATL, Y/D, VFD, etc. At least this is how it is with a listed unit. Outside of a listed unit, if I understand correctly, going by art 430, with a standard motor circuit, you can go higher than 225% with MOCP to permit starting if needed, correct? Forgive me, I've been in the HVAC world in a testing lab for a long time and I'm just trying to get a better understanding of the code.
 

kwired

Electron manager
Location
NE Nebraska
I just took a look at a chiller with an optional wye/delta starter installed. Single compressor machine.

RLA: 283.3 amps
LRA: 578 amps wye/1800 amps delta
MCA: 354.1 amps
MOCP: 600 amps

MCA is RLA times 1.25 (283.3 x 1.25 = 354.125). MOCP is RLA times 2.25 then next size down (283.3 x 2.25 = 637.425). Next standard size down is 600 amps.

MOCP is the same whether starting across the line or wye/delta. It appears that MOCP is determined from RLA regardless of starting method/motor control. I'm pretty sure this is how it's done whether ATL, Y/D, VFD, etc. At least this is how it is with a listed unit. Outside of a listed unit, if I understand correctly, going by art 430, with a standard motor circuit, you can go higher than 225% with MOCP to permit starting if needed, correct? Forgive me, I've been in the HVAC world in a testing lab for a long time and I'm just trying to get a better understanding of the code.
Keep in mind it is a max value. It very likely can be lesser value and never trip during starting, as less instantaneous demand is what wye/delta, part winding, autotransformer starting methods are all about. Often they are required more so by POCO when over a certain HP motor is being started than they are a necessity for the premises wiring.
 

topgone

Senior Member
RLA is 63 -70% of maximum compressor running amps before it trips on overload. It is much lower than the old FLA ratings they used in the past.
Around that figure, I think! Basically, RLA is a calculated value needed for UL approval. Manufacturers must run tests on their products to learn about the maximum current until the overloads trips, known as the maximum continuous current (MCC)! UL says they should divide the value by 1.56 and use it to stamp their unit's RLA. But Copeland and Carlyle use a different divider of 1.44!

I suspect the difference is on the choice of the overload factor of their controllers, 115% or 125%. 115% TOR setting X 125% conductor oversizing requirement = 1.44, while others use 125% TOR setting X 125% conductor oversize = 1.56!
 
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