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While evaluating a property for a spec reno, Sparky opened the meter combo panel. 400A residential w/ two 200A main breakers. 2/0 Cu from each of the main breakers approx 60 feet to separate 200A distribution panels.

Why would the mfg feed the 200A main breakers and the neutral bus w/ 6 parallel 3/0 Cu when the load side is only 2/0 Cu? Sparky did not know the answer, said it would pass code as is. Just asking out of curosity.
 

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Ooops

I'm not 100% sure. But, pretty sure you need 3/0 Cu Min (or 4/0 Al). Uh oh...that could be a big mistake..I hope they ran 4 wire from mains..even a more uh oh..
 

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If it's a factory install, it falls outside the scope of the NEC and is more of a NEMA thing.
 
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99.9

that's why I prefaced it with not 100% sure. It's funny, I've run 4/0 Al for 200 amps as long as I can remember, so I didn't really need to know the exceptions. I wonder how many actual rules there are in the code book, and then, how many exceptions there are? It's got to be staggering
 
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