15KV Metalclad Switchgear Using 2 x 2000A Parallel Mains

kingpb

kingpb

Senior Member
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SE USA as far as you can go
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Engineer, Registered
So, I have heard of this, but never actually seen it, or has it ever been proposed to me by a manufacturer before, but honestly, never needed 4000A MV gear either. Always stayed within 3000A.

I need a 4000A 13.8KV bus. In lieu of using a single 4000A breaker, which requires fan cooling, the manufacturer has proposed using 2 x 2000A breakers in parallel that don't require fan cooling. Fault current will be controlled by the transformer impedance ahead of it, to keep it within short circuit ratings.

Looking for pros/cons.
 
I don't have my code book handy. But I believe there are very few instances in which breakers are allowed to be in parallel. I doubt this is one of them.

Can anyone confirm or refute my belief?
 
charlie b said:
I don't have my code book handy. But I believe there are very few instances in which breakers are allowed to be in parallel. I doubt this is one of them.

Can anyone confirm or refute my belief?
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NEC 240.8 says it's ok as long as it is a factory assembled and listed as a single unit. It is coming from the OEM this way.
 
kingpb said:
Good point, they are the latter. is an OEM not considered factory assembled?
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Kind of.

Unless the breakers have been tested in parallel, I would not accept them as a single unit. The listed assembly could then be installed in an enclosure with bus bars by an OEM.

I have seen OEMs who's mottos appear to be "if you can design it, we can build it"
 
Once you get into the MV world, things are different. It’s very common for custom switchgear manufacturers to build the gear itself, but using factory made breakers and such. At that size, it’s likely the there will be one trip unit looking at the totality of the current, so the only issue will be the impedance matching and the actuation time possibly being slightly different. But I imagine these will be vacuum breakers, so we are looking at milliseconds of possible differential, maybe less. They have to be matched and tested for this, but it’s actually a common practice recognized by IEEE and used extensively for high current applications like this. It is likely beyond the purview of UL and listing by them, but there are other more stringent design criteria that are used for this, such as ANSI/IEEE C37.20.2 (for metal-clad) and C37.20.3 (for metal-enclosed) MV gear.
 
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