Chiller Breaker sizing

[Mumbaigirl]

Hello, I am very new to this:

I have a 440-ton quad compressor chiller unit we are renting and below are the RLA and FLA details:
460V/60HZ
System 1 : Compressor 1: RLA= 180A; Condensor 1 Fans FLA = 2.8A each for 6 fans
System 2 : Compressor 2: RLA= 180A; Condensor 2 Fans FLA = 2.8A each for 5 fans
System 3 : Compressor 3: RLA= 150A; Condensor 3 Fans FLA = 2.8A each for 5 fans
System 4 : Compressor 4: RLA= 150A; Condensor 4 Fans FLA = 2.8A each for 5 fans
System 1/3 : Control KVA = 2.4
System 2/4 : Control KVA = 1.8

Per the cutsheet, RLA for the chiller ONLY is 763, the MCA is 763A and the recommended fuse/circuit breaker rating is 1000A when you run the entire system.

I have an 800A service available (disconnect switch) for this chiller and I am trying to figure out if that will be enough if I run only 3 compressors. (they tell me they need about 300-350ton capacity and not the entire 440 ton):

Assuming I can only run compressor 1, 2 and 3 (they have individual disconnects) , can I calculate the maximum overcurrent protection as :
2.25*180 + 6*2.8 +180 + 5* 2.8 + 150 + 5* 2.8 = 780A + some control RLA = 800 approx. and use my 800A breaker to feed this?

 

[augie47]

Do the compressor start across-the-line or by a softstart of VFD ?

 

[Mumbaigirl]

Do the compressor start across-the-line or by a softstart of VFD ?

It has a VFD.

The cutsheet says the system inrush current is less than the RLA becuase of YORKS Variable speed drive technology. The typical starting currents per compressor are: 23A @ 460/60/3

 

[Eddie702]

Your MCA to run compressors 1, 2 and 3, 16 condenser fans and 2.4KVA for controls totals up to an MCA of 605 amps. 800 amp disconnect should be fine as they don't all start at one time and are staged on.

At least that is what I came up with

 

[Eddie702]

you have 16 condenser fans @ 2.8 amps=44.8 amps
1 compressor (largest load ) @ 180 amps x 1.25=225 amps
2 compressor @ 180 amps
3 compressor @150 amps
controls 8.75 amps

Total MCA 608.55

I misread the control requirements in my previous post.

Your load at 76% of the 800 amp disconnect should be fine.

 

[augie47]

Considering the VFD controls I would think the 800 would be adequate,

 

[Mumbaigirl]

Thank you so much for your help on this. I really appreciate it.

 

[LarryFine]

Your load at 76% of the 800 amp disconnect should be fine.

With all the info given, could the fourth unit be started, or would it trip the 800a main?

 

[zbang]

If the 4th compressor is likely to trip the feed, physically lock it out to keep some well-meaning person from trying to start it.

 

[wwhitney]

Per the cutsheet, RLA for the chiller ONLY is 763, the MCA is 763A and the recommended fuse/circuit breaker rating is 1000A when you run the entire system.

Not sure if this is an Article 440 application (hermetic refrigerant motor-compressors) or an Article 430 application (all other motors), but I don't see how the manufacturer gets to 1000A MOCP from the data you provided. I get a maximum OCPD size of 943A (= MCA + 100% largest RLA) if 440.22 applies, or 988A (= MCA + 125% largest RLA) if not. That's ignoring the controls, but they only add up to 9A.

That issue aside, if your currently available service is 800A conductors with 800A OCPD, then for motor loads you are allowed to upsize the service OCPD based on the motor MOCP while sticking with 800A conductors (as that is greater than the 763 MCA). See 230.90(A) Exception 1. In case changing the disconnect/OCPD size is possible.

Cheers, Wayne

 

[Mumbaigirl]

Not sure if this is an Article 440 application (hermetic refrigerant motor-compressors) or an Article 430 application (all other motors), but I don't see how the manufacturer gets to 1000A MOCP from the data you provided. I get a maximum OCPD size of 943A (= MCA + 100% largest RLA) if 440.22 applies, or 988A (= MCA + 125% largest RLA) if not. That's ignoring the controls, but they only add up to 9A.

That issue aside, if your currently available service is 800A conductors with 800A OCPD, then for motor loads you are allowed to upsize the service OCPD based on the motor MOCP while sticking with 800A conductors (as that is greater than the 763 MCA). See 230.90(A) Exception 1. In case changing the disconnect/OCPD size is possible.

Cheers, Wayne

Wayne, they also have a 50HP pump motor that we are not going to use as we use our own pumps. I am assuming that factors into the 1000A MOCP.
Thank you for explaining the OCPD exception. Unfortunately, the 800A disconnect in the field is all I have and changing it in the next week (when we need this to go in) is not an option.

I appreciate this. Thank you!

 

[suemarkp]

Per the cutsheet, RLA for the chiller ONLY is 763, the MCA is 763A and the recommended fuse/circuit breaker rating is 1000A when you run the entire system.

How can this ever be? Isn't MCA, by definition at least 25% larger than the the largest one RLA? Maybe the total RLAs are 660A and not 763?

 

[Mumbaigirl]

 

[Eddie702]

I don't see the confusion. 608.55 is the MCA I calculated. Add the 4th compressor and you get 608.55 + 150 amps + 14 amps for the additional condenser fan motors=772 amps MFG says 763 only 9 amp difference.

Pretty close

I may have use 480 volt instead of 460

You just add the running amps for controls, fans and compressors + 25% of the largest compressor to get mca.

MCA is minimum wire size.

MOCP is max breaker or fuse you don't have to use the max the equipment just needs to start and it will because all the compressors and fans are staged on and off by the controls.

 

[Mumbaigirl]

I heard that the vendor for the chiller was saying that instead of running 3 out of 4 units they would prefer that we run all 4 units at 70% capacity - would that change anything from a MCA standpoint? Will the power (ampere draw) will be proportionately lower?

70% (763 RLA for all 4 compressors) = 534.51 and so using the 800A disconnect for all 4 compressors is still an option? Am I wonrg in saying this?

 

[Eddie702]

Most modern chillers have the ability of limiting their maximum load so you can easily do that through the programming. You can set the chiller to whatever percentage you like so as to not overload the conductors or the disconnect.

If it is not inspected as it is temporary that that is the solution.

If it is being inspected the AHJ may not approve it with all the equipment connected

 

[Mumbaigirl]

Thank you! This helps. So if they set it at 70% maximum load, would that mean that the RLA would not be greater than 70% of 763? Would underloading the compressor continously jhave an affect on that number?

 

[suemarkp]

I heard that the vendor for the chiller was saying that instead of running 3 out of 4 units they would prefer that we run all 4 units at 70% capacity - would that change anything from a MCA standpoint? Will the power (ampere draw) will be proportionately lower?

70% (763 RLA for all 4 compressors) = 534.51 and so using the 800A disconnect for all 4 compressors is still an option? Am I wonrg in saying this?

I still don't see where you are getting 763 for RLA. According to the sheet you posted, RLA is 180+180+150+150 = 660A. If you're adding all the fan FLA's to that, this add another 6*2.8 + 15*2.8 = 58.8 If you add 25% of the largest compressor, you are basically computing the MCA. That is 660 + 58.8 + 45 = 764 but they have done this for you and said it is 763A.

And I don't see what your issue is. Your MCA is 763, the running loads are about 720A, the service is 800A. 800 > 763 so code wise you are fine. Will the main trip on start up? Probably not, but it could, especially if ambient temperatures are hot. Max breaker is just that, a max limit. It will probably work fine on a 900 and maybe even the 800. Running the system at 70% helps, ramping up the speed with VFDs helps. Staging the compressors so they don't all start at once helps. And it sounds like there is a code solution to increase the main if it really becomes a problem since you have a motor load.

 

[Eddie702]

Modern chillers with multiple compressors all stage on in sequence.

@Mumbaigirl

I am not saying that setting the chiller to 70% capacity will only draw 70% of rated current. We have no way of knowing that. On most modern chillers you can go in the setting and limit the chiller capacity based on % of capacity.

Some of them allow you to limit the capacity based on current draw.

 

[Mumbaigirl]

I am still struggling with this:
We now have the 440-ton rental chiller on site. Although the minimum circuit ampacity for this equipment is 763A, the rental distributor has confirmed with his technical team that the chiller wont draw more than 600A if we run the system at 75% capacity. We have set the capacity at 75% max.

The chiller upto this point was being fed off an 800A disconnect and the TCC curve is shown in the attached picture. We now have to switch the feeds becuase we need the 800A disconnect for a different service and the new scenario is below:

A 1600A breaker feeds an 800A power distribution panel out in the field via 2 parallel-3/C-500MCM cables. This 800A breaker in the PDP will now feed this chiller.

The Ir on the 1600A breaker in the substation is set at 640A and I understand that this breaker needs to protect the cable downstream which is limited to 760A @ 75decC (2 parallel conductors). Is it right that I need to set the 800A PDP breaker (dials on the breaker) in a way that it trips at or less than 640A?
I see three dials on the 800A breaker. Longtime (Ir and Tr) and Instantaneous (li).

For the 800A breaker Ir = 0.8xIn = 0.8*800 = 640A.

If I set it to 0.8 on the dial, will it mean that this breaker and the upstream breaker will trip at the same time? Or should I increase the Ir on the 1600A breaker to 800A. But then that will not protect my cable.

Thank you,