800A CB in 3000A Swbd

[WoodrowJWeen]

I recently designed a building addition that required an 800A molded case circuit breaker to be added to an existing (less than 5 year old) 3000A GE switchboard. There appeared to be plenty of space in the board, a whole section as a matter of fact. When the electrical contractor went to install the breaker, he was told that a 600A breaker was physically the largest that could fit and that the engineer (me) should have checked with the manufacturer beforehand. They ended up adding the breaker externally in a separate enclosure at considerable extra cost. I've never come across a large switchboard that wouldn't take an 800A breaker and thought this to be exceedingly unusual. Anyone else ever encountered this problem.

 

[DRWeig]

Hi WoodrowJWeen,

I've found that it's always best to consult with the manufacturer before adding something to an existing switchboard, no matter how new the board or how obvious-looking the the solution is. Big boards are designed individually, and if in a competitive bid situation, sometimes money can be saved by reducing the max breaker frame size to the size of the largest one to be used in the initial installation...

Sorry you learned the hard way. I did too, long ago.

Old Dave

 

[mc5w]

The problem is wire bending space. If you look in the Squared Digest, a lot of their I-Line industrial panelboards are only rated for say 200 amp breakers because fo this issue. In some others only 1 side can take an 800 amp circuit breaker.

The wire bending space requirements were increased in 1987 with the result that older 200 amp busway plugs MUST use class J fuses. The intent was to get after electricians who were recycling used fuseboxes in new electrical services especially with aluminum wire. Since a lot of fuseboxes were rated for 100 amp or 120 amp wire it was possible to do a service upgrade by just replacing the service cable and meter socket. This also outlawed round die cast meter sockets on anything larger than a small sign or traffic light.

 

[jsldesign]

I just recently (within 60 days) ran into this exact same issue on a 2000A switchboard. Drawings showed an 800A breaker installed in this unit - I happened to be looking at the manufacturers web site for a part number to place in some bid documents and discovered that an 800A SWITCH was available, but the largest BREAKER that was available was 600A. Lucky catch on that one... no construction had started at that time.