Best way to fix this code violation?

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donw

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Went on a site visit to an industrial plant the other day and saw a configuration like this:
600A fused disc.---2 sets 350MCM---600A disc. w/175A fuses---150KVA xfrmer---3/0---600A main breaker panel. There's a second tap on the transformer with another set of 3/0 and a second 600A main breaker panel. The 175A fuses would protect the secondary windings of the transformer, but the secondary tap conductors are not protected by the 600A main breakers. Would the least expensive way to fix this be to replace the secondary conductors with 2 sets of 350MCM (600A wire)? This appears to meet 240.21(C)(2) or (3). (Secondary conductors are less than 25' and may be less than 10').
 
coulter, 240.21(c)(4)(2) requires that the conductors terminate into a single circuit breaker that limits the load to the ampacity of the conductors. The 600A main breaker does not limit the current of the existing 3/0 conductors to 200A.
 
donw said:
coulter, 240.21(c)(4)(2) requires that the conductors terminate into a single circuit breaker that limits the load to the ampacity of the conductors. ...
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Yes, I think I knew that. However, you said:
Went on a site visit to an industrial plant the other day and saw a configuration like this: ...
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(emphasis is mine)
My work is all in industrial locations. All of the industrial locations I have worked are supervised. If this is your scenerio, then (2005) 240.92.D may apply to this case. You would have to decide that, I can't tell from my side of the keyboard.

Just curious, did you happen to read the suggested code section before you replied?

carl
 
coulter, you are right I read the wrong section! That would appear to allow for the present configuration.
 
Now that I look at it closer, there still appears to be a problem. The present configuration has 200A wire on the secondary, where it really needs to be a minimum of 400A wire (due to the 175A primary fuses and the transformer ratio)...even if it is a supervised industrial installation.
 
I would expect that the replacement of the 600a main breaker with a 200a breaker would be cheaper than replacing the 3/0 with parallel 350s.(assuming secondary conductors are 25' or less in length)
 
Don, the existing 600A breakers are Squ-D L-frame 600A. If we stayed with L-frame, a replacement retails for $3079 (maybe $1700+ steet?) I guess they could go for a K-frame at $1733 retail. But what if they simply added another 3/0 in parallel with the existing? It's probably less than 20 feet long.
 
donw said:
Don, the existing 600A breakers are Squ-D L-frame 600A. If we stayed with L-frame, a replacement retails for $3079 (maybe $1700+ steet?) ...
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I'm not familiar with SQD L-frame. No adjustable trip unit? Bummer.

carl
 
Coulter, I'm not that familiar with the breaker, either, but it has something called an adjustable AC Magnetic trip from 3000A to 4200A. Maybe that's an instantaneous trip setting, but not THE trip setting.
 
Don,
I thought you were talking about installing parallel 350s to get to 600 amps. How would a second set of 3/0s be compliant with the code rules?
 
What would the cost of a 200a fusible disconnect be compared with the cost of the additional run of 3/0?
It appears that the use of 240.92(D) will require some engineering calculations.
 
That may be a good solution. They don't want to spend a lot on this fix, and I am the engineer, so any calculations would be up to me. They may be afraid that, on occasion, either of the panels may go over 200A. Speaking of calculations, (480/208)x175A = 403.8A, which is greater than the 400A rating of 2 sets of 3/0, so 2 sets of 4/0 may be required.
 
The one I looked at carefully had 11 30A/3 breakers - 6 labeled machine, 3 labeled welder, and 2 labeled plasma.
 
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