How to size Over current Protection Mechanical Chillers

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Im_Butters

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Hi All, this is my first post.

I've asked this question on another forum, but was told to post my question here.

I'm working on a project where I need to size a 460V chiller from a switchboard that is rated at 208V. I'll need to provide a step up transformer, but there's a few items that I'd like to clarify. The chiller has the following characteristics at 460V:

  • Motor FLA = 551A
  • LRA = 3500A
  • Inrush Amps = 0
  • MCA = 690A
  • Max Breaker = 1200A
  • KW = 386.6kW
Converting the kW load to 208V gives me: 386600/(208*1.73) = 1073A.
According to the Canadian Electrical Code, I will need to account for inrush current at no more than 250% for the overcurrent Protection. Therefore, 1073A * 2.5 = 2682A. My question is, the cutsheets says there is 0 in rush. Can I get away with using a 1600A breaker that has an adjustable trip setting?
Note that my transformer is sized at 450KVA. For my wire size, I have sized it based on the MCA but at 208V. Should I be sizing it from the transformer? And for my own knowledge, can transformers handle inrush current from motors, or do you have to typically size it to account for it?
Thanks
 
With the NEC you would size the conductors according to the MCA which is 690 amps. And the OCPD between 690 and 1200 amps. No calculation needed. I'll wait for an engineer to comment on the transformer.
 
infinity said:
With the NEC you would size the conductors according to the MCA which is 690 amps. And the OCPD between 690 and 1200 amps. No calculation needed. I'll wait for an engineer to comment on the transformer.
Click to expand...

Thanks for the response. I believe calculations are required because the values given in the cutsheet were at 460, whereas my distribution system is at 208V.

Cheers,
 
Im_Butters said:
Hi All, this is my first post.

I've asked this question on another forum, but was told to post my question here.

I'm working on a project where I need to size a 460V chiller from a switchboard that is rated at 208V. I'll need to provide a step up transformer, but there's a few items that I'd like to clarify. The chiller has the following characteristics at 460V:

  • Motor FLA = 551A
  • LRA = 3500A
  • Inrush Amps = 0
  • MCA = 690A
  • Max Breaker = 1200A
  • KW = 386.6kW
Converting the kW load to 208V gives me: 386600/(208*1.73) = 1073A.
According to the Canadian Electrical Code, I will need to account for inrush current at no more than 250% for the overcurrent Protection. Therefore, 1073A * 2.5 = 2682A. My question is, the cutsheets says there is 0 in rush. Can I get away with using a 1600A breaker that has an adjustable trip setting?
Note that my transformer is sized at 450KVA. For my wire size, I have sized it based on the MCA but at 208V. Should I be sizing it from the transformer? And for my own knowledge, can transformers handle inrush current from motors, or do you have to typically size it to account for it?
Thanks
Click to expand...
I don't know CEC details, but is possible that rule only applies to a single motor with "across the line starting method"?

A load the size you have likely is multiple smaller loads in one assembly or uses reduced voltage starting/soft starters, or even uses VFD's and doesn't have the same inrush as an across the line individual load would have.

Just something to consider.
 
Im_Butters said:
Thanks for the response. I believe calculations are required because the values given in the cutsheet were at 460, whereas my distribution system is at 208V.

Cheers,
Click to expand...

My comment was for the load side of the transformer which is 460 volts, I'll let one of the engineers size the transformer based on the 208 volt primary.
 
kva = sqrt3 x 551 x 460 /1000 = 440 kva
is 450 a std size? I would use a 500

for the rest
mca = 460/208 x 690 = 1525
fla = 460/208 x 551 = 1220
cb 2500

imo a 1600 frame with adj trip would be ok

pf ~ 387/440 ~0.88
 
I agree with Infinity's advice and let an engineer confirm your transformer.
In you original post you stated a 450kva transformer... the closet likely available transformer is a 500 kva which would have a input of 1387 amps and call for a primary beaker in the 2000-3000 amp range but it is definitely an area where professional input is warranted.
 
norcal said:
OP is in Canada, 600V would be the prevalent voltage.
Click to expand...

why a 480 chiller?
looks like 480 is available too
if not a dedicated dist v: 480 xfmr makes more sense than 208:480
I doubt the existing 208 service has 500 kv/1200 A of excess capacity
 
augie47 said:
I agree with Infinity's advice and let an engineer confirm your transformer.
In you original post you stated a 450kva transformer... the closet likely available transformer is a 500 kva which would have a input of 1387 amps and call for a primary beaker in the 2000-3000 amp range but it is definitely an area where professional input is warranted.
Click to expand...

Thanks. I've used a 450kVA transformer in the past, but based on Infinity's calcs, seems like a 500kVA would be safe. If I size the breaker based on the transformer, I get the following:

500kVA/(208*sqrt3) = 1,387.86A rated primary current.

Breaker = 1,387.86A * 1.25 = 1734.82A ~ 2000A breaker.

Also, the reason why we have this set up is because the building is very old (50 years old). Typically, commercial buildings here are all at 600V, but for older buildings it usually runs on 208V. Changing the service size is unfortunately not an option.

Thanks
 
Im_Butters said:
Thanks. I've used a 450kVA transformer in the past, but based on Infinity's calcs, seems like a 500kVA would be safe. If I size the breaker based on the transformer, I get the following:

500kVA/(208*sqrt3) = 1,387.86A rated primary current.

Breaker = 1,387.86A * 1.25 = 1734.82A ~ 2000A breaker.

Also, the reason why we have this set up is because the building is very old (50 years old). Typically, commercial buildings here are all at 600V, but for older buildings it usually runs on 208V. Changing the service size is unfortunately not an option.

Thanks
Click to expand...

your mca is 1525, your fla 1220, a 1600 frame will do
an adjustable cb set at 460/208 x 2500 = 5200 corresponds to the mfg's max value

2500/551 ~ 450%
but you are sizing for a xfmr not a motor (or ss drive)
using nec guidance
I would use a setting of 250% x xfmr fla or 3500 assuming the chiller has a cb that can be set ~125% x xfmr sec fla
if not by 125% or 1800
a fixed 2000 may cause issues, not only the chiller but transformer inrush
whatever you do get a cb with adjustable settings & frame min 1600

your ampacity can be 1525 (not 1735)
with a 2000 frame do you required 2000 ampacity?
 
FLA = 551A w/ Inrush = 0 means its on a VFD.

XFmr = 440KVA, next size up is 500KVA.

MCA = FLA x 1.25 = 690A (This is used to determine cable size)

You do not have to size breakers for transformer, you can always downsize; thus you just won't get full capacity out of transformer.

Max Breaker @480V is 1200A; use 1200A. The cable does not have to carry anything more than the MCA

Select a breaker using the transformer damage and inrush on the 208V side of the transformer that is no greater than 125% of rated, which means you do not need another breaker on the 480V side, just feed the 1200A.
 
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