Parallel 500's 1ka breaker, legal?

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Doug S.

Senior Member
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West Michigan
I've been scratching my head on this one, being that I want to believe this is "ok", but I'm not seeing it. So I differ to the masses, and the masters.

We have a couple of chillers, each fed with parallel 500kcmil, fed from 1000a breakers. The chillers are 760a name plate, with an internal 800a breaker, that also happens to be the disconnect.

So to my question, can any one think of a way this is legal?

Art. 240.4 leads me to believe not, and I can't find any thing in Art. 440 to lead me to believe it's OK. I have no doubt the breaker size is ok, it's the wire size that is the problem.

Anybody?

Also, are these feeders? or branch circuits? :roll: I did some digging on the forum and couldn't finish reading an '06 thread on the topic, so I'll re-open the can of worms.

Thanks,
Doug S.:-?
 
I was waiting to see Marc's reply as he is far more knowledgable than I, but I'll foolishly jump in anyway.

Also, are these feeders? or branch circuits? :rolleyes: I did some digging on the forum and couldn't finish reading an '06 thread on the topic, so I'll re-open the can of worms.

IMHO, your proverbal "can of worms" if the key, along with the namplate data on the chillers. If the nameplate, in addition to the MCA of 760, allows a MCOP of 1000 then the argument goes back to this being a feeder or branch circuit. If you consider the 800 amp "units" as "disconnects" then the 500's would be a branch circuit and could be sized by the namepate (or Art 440). If you consider the 800s as branch circuit protective devices, the the 500s are feeders and the 1000 might be more questionable.
Historically, Iv;e found it to eb an AHJ call. I'll be interested in Marcs and others response.



 
benaround,
The phase conductors, once in the CC, go directly to the disconnect / breaker.
After the disco, it's all factory. A "Marathon-block", Contactors, Ctrl Xfmr and the like...


And I since I've had the day to think about it, and at one point either the AHJ overlooked it, or approved of it, and since the current setup is shown in my building 1 lines. (AHJ and Engineer approved) ... ... ...

I am going to go out on a limb and say my chiller(s) are the "devices" being served, and that the contents of their control cabinet are, in this case, meaningless to the code.
If this is the case....

The parallel 500s are then part of a individual branch circuit feeding said 760a name-plate chiller(s), and are sized correctly according to Art. 440.6(A), 440.35, and Tbl 310.16 ('05 code book)

Furthermore the 1000a breaker that feeds the 500's and then the chiller, being treated as "Short-Circuit and Ground-Fault Protection", is sized correctly according to 440.22(B). It also correlates with 440.22(C) (Again '05 code book) ... ... ... If it were being treated as an OCPD (Art. 440 IV) it would have been over sized by 50amps and not compliant?

Whatchya think of that?

:confused:
Doug S.

___________________________________________________________
I'm not dumb. I'm just slow, just ask my wife...
err... uh ... maybe you should take my word on this one.
___________________________________________________________

Edit: Previously mentioned thread, as usual the last page sum's things up well.
http://forums.mikeholt.com/showthread.php?t=81310
 
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IMHO the parallel 500's complies with 440.35. The 1000A breaker complies with 440.22(A). I was always taught that a feeder terminates in an OCPD. A branch circuit terminates in a load.
 
Doug S. said:
...
The parallel 500s are then part of a individual branch circuit feeding said 760a name-plate chiller(s), and are sized correctly according to Art. 440.6(A), 440.35, and Tbl 310.16 ('05 code book)

Furthermore the 1000a breaker that feeds the 500's and then the chiller, being treated as "Short-Circuit and Ground-Fault Protection", is sized correctly according to 440.22(B). It also correlates with 440.22(C) (Again '05 code book) ... ... ... If it were being treated as an OCPD (Art. 440 IV) it would have been over sized by 50amps and not compliant?

Whatchya think of that?:confused:
Doug S.
Click to expand...
I am thinking that unless the 760 amps is the minimum circuit size, the conductors are too small. If the 760 amps is the actual load current then the conductors have to have an ampacity of 950 amps and parallel 500s do not have than ampacity.
We really need to know the actual wording on the nameplate before this question can be answered.
 
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