Sizing conductors and fuses for a chiller

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JdoubleU

Senior Member
The minimum circuit ampacity for the chiller is 485, the Maximum over current protection is 600 amps. I would probable want to size my conductors to 600 amps but according to the code would I go 125% of the minimum circuit ampacity?
 

iwire

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The minimum circuit ampacity for the chiller is 485, the Maximum over current protection is 600 amps. I would probable want to size my conductors to 600 amps but according to the code would I go 125% of the minimum circuit ampacity?

For this chiller the code allows you to a 600 amp breaker with conductors rated 485 amps or more.

The 125% is already included in the MCA.
 

JdoubleU

Senior Member
I have another question related to this. I understand I can size my conductors to the 485 from the disconnect to the chiller, but I am tapping off a transformer to the disconnect. That being said I believe the conductor from the transformer to the disconnect have to be rated for 600amps or more correct.


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don_resqcapt19

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I have another question related to this. I understand I can size my conductors to the 485 from the disconnect to the chiller, but I am tapping off a transformer to the disconnect. That being said I believe the conductor from the transformer to the disconnect have to be rated for 600amps or more correct.


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The conductors from the transformer to the disconnect must be protected per the rules in 240.21(C). That will require protection at or below the ampacity of the conductor and the "round-up" rule in 240.4 does not apply to the conductors between the transformer and the first OCPD. Also note that there is a maximum length of conductor that is permitted between the transformer terminals and the OCPD.
 

JdoubleU

Senior Member
Sizing conductors and raceways.

Sizing conductors and raceways.

What is your opinion on this design?

Electrical specs: 485 amps MCA, 600 amp MOCP, 460V/3PH

1. Outdoor transformer tap to 600 amp fused disconnect

1. 3 1/2 inch PVC raceway = 3.895 area inch squared

1. Six 400kcmil THHN copper conductors for service ungrounded conductors = .5863 area inch squared
2. One 2/0 THHN copper conductor for system bonding = .2223 area inch squared
3. Total area inch squared = 3.7401

2. 3 1/2 inch PVC raceway = 3.895 area inch squared

2. Chiller feeder from 600 amp fused disconnect

1. 3 1/2 inch PVC raceway = 3.895 area inch squared

1. Six 300kcmil THHN copper 3PH feeder conductors
2. One #1 THHN copper conductor for equipment ground
 

Carultch

Senior Member
Location
Massachusetts
What is your opinion on this design?

Electrical specs: 485 amps MCA, 600 amp MOCP, 460V/3PH

1. Outdoor transformer tap to 600 amp fused disconnect

1. 3 1/2 inch PVC raceway = 3.895 area inch squared

1. Six 400kcmil THHN copper conductors for service ungrounded conductors = .5863 area inch squared
2. One 2/0 THHN copper conductor for system bonding = .2223 area inch squared
3. Total area inch squared = 3.7401

2. 3 1/2 inch PVC raceway = 3.895 area inch squared

2. Chiller feeder from 600 amp fused disconnect

1. 3 1/2 inch PVC raceway = 3.895 area inch squared

1. Six 300kcmil THHN copper 3PH feeder conductors
2. One #1 THHN copper conductor for equipment ground

No issues with your conductor sizing.

Your conduit for the transformer secondarys is very tight. 38% fill.
I know it meets code, but in a practical sense, it is probably a lot tighter than you really want to go. Plus, if you do need to transition to schedule 80, for even just a part of it, you need a 4" conduit. This is common practice when conduit emerges from underground.

If it is a short and straight pull, go for it. If you got any complexity of the route, rethink it and upsize to 4".
The feeder conduit should be fine, both in sch40 and any local piece of sch80.
 

iwire

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Location
Massachusetts
I agree 100% with using 4" for this, no real cost savings in using 3.5" anyway. The only thing that would motivate me to use 3.5" is if the area I had to run it would not fit 4".
 

JdoubleU

Senior Member
I have a few question on the 2/0 that I am running. This is being sized from the table 250.66 because it is a tap off the transformer supply a service for a chiller that does not require a neutral. Would the 2/0 by definition be call the supply side bonding jumper. I have a ufer at the disconnect that will have a number 4 copper ran from it to the ground bar on the disconnect. I have a rigid sweep going to both the transformer and the disconnect. I would assume the 2/0 would have to connect to the rigid sweeps via a bond bushing. I just received the bond bushings and they are only big enough to take as high as a #4 conductor. What are your thoughts?
 

ActionDave

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Licensed Electrician
I have a few question on the 2/0 that I am running. This is being sized from the table 250.66 because it is a tap off the transformer supply a service for a chiller that does not require a neutral. Would the 2/0 by definition be call the supply side bonding jumper.
I say, yes.
I have a ufer at the disconnect that will have a number 4 copper ran from it to the ground bar on the disconnect.
Where is the XO in the transformer bonded? You describe your set up as a tap from the transformer. I'm intrigued because I have never done this and I'm thinking you don't need the connection to the Ufer if the bond is in the tranny.
I have a rigid sweep going to both the transformer and the disconnect. I would assume the 2/0 would have to connect to the rigid sweeps via a bond bushing.
Either the XO is bonded at the tranny or at your disconnect and an EGC is run from there, either way both of the rigid 90's shouldn't need a bond bushing.
I just received the bond bushings and they are only big enough to take as high as a #4 conductor. What are your thoughts?
You can change the lugs on the bushings to a larger size.
 
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