Unit Label

[Jackz]

I have an inspector tell me my wiring is incorrect. Wiring a chiller, 480v 3 phase. The nameplate says FLA=96 amps. MOCP= 225 amps.
Since it is 1 compressor i add 25% to the entire 96 amps = 120 amps. At 75c i use [3] #1 copper THHN [130 amps]. Then since i am using a 400 amp source i full size tap [split]to 2 fused disconnects. [Their are 2 chillers.] So now i have 2- 200 amp disconnects each with 200 amp fuses. Now i am told the wire is to small for the fuse size. So can someone explain help me in detail including article numbers how this is correct. I know it is, i just can't explain it very well including how it is ok for overloads [built in] to protect the motor. Since the MOCP is 225 amps the factory must have concluded that over current protection can be raised from 175% to 225%. The chiller is UL listed. The wire feeding the disconnects is not in question, the question is the #1 wire from the disconnects to the chillers since the disconnects have 200 amp uses.

 

[infinity]

What size are your tap conductors? The conductors feeding the unit need to be sized according to the MCA of the unit and not the fuse size. The MCA already has the 125% factored in by the manufacturer so adding an additional 25% to your calculation is not necessary. Your conductors need to be a minimum of 96 amps and your OCPD can be up to 225 amps with the 96 amp conductors.

 

[Jackz]

The unit does not list the MCA, it lists FLA at 96 amps and MOCP at 225 amps.

 

[Jackz]

The tap conductors are 600MCM copper. Power starts out as 600 MCM from a 400 amp breaker. It is then split and stays full size to the line side of the 2 200 amp disconnects. From each of the 2 disconnects is feed to the chiller using #1 THHN copper with the disconnects having 200 amp fuses. The MCA is not listed. Just FLA listed at 96 amps and MOCP listed at 225 amps.

 

[augie47]

I stumble in the dark a bit on this so take it with a grain of salt....
Since you don't have a MCA, and based on 440.2 it would seem you should use the "branch circuit selection current".
440.4(C) states that if the current is in excess of that given in 440.52(B)(2) or (4) shall be marked with a branch circuit selection current.
Since yours is not marked, it would seem your current does not exceed 440.52(B)(2) which gives a maximum current of 156% of the rated load current.
I conclude, perhaps incorrectly, that your selection current should be 156% of the load current or 150 amps max.
I may be overlooking something, but, unlike Art 430, Art 440 does not seem to show a 125% for branch circuit conductors.
This corresponds to my being told that inrush on hermetic compressors is greater than equivalent sized standard motors.
Thats my 2 cents worth.

99% of the units I see have an MCA so it's not a problem I have tackled in some time.

 

[Jackz]

I have not looked at the tag in a while so i am sorry, it does show the MCA and it is shown at 120 amps. The entire question by the inspector is because i used #1 wire i should drop my fuse size to 130 amps.

 

[dkarst]

I have not looked at the tag in a while so i am sorry, it does show the MCA and it is shown at 120 amps. The entire question by the inspector is because i used #1 wire i should drop my fuse size to 130 amps.

Given what you have provided you appear to be correct in my opinion. The unit nameplate specs min circuit ampacity of 120 A and you have provided conductors suitable for 130A. The max overcurrent protection is specified by nameplate at 225 A and you are protected at 200A.

You mentioned fuses and I would be cure that the MOCP is spec'd with fuses or breakers but I think you're correct on this one.

This is one of those situations where it takes a bit of thinking to answer how does a 200A breaker protect my wire from overload? In this case the unit protection provides the OL protection for the wire and the fuse provides short circuit protection.

 

[infinity]

I have not looked at the tag in a while so i am sorry, it does show the MCA and it is shown at 120 amps. The entire question by the inspector is because i used #1 wire i should drop my fuse size to 130 amps.

Sorry I missed the FLA part but in this instance it doesn't change anything. Your inspector needs a refresher course if he doesn't know that the OCPD is there to provide short circuit and ground fault protection and is permitted to be larger than the conductor ampacity.