Subfeed Breaker 3

[Reesh14]

I have a 225A, 120/208V,3ph,4w panel that serves two boilers, two pumps, exhaust fan, receptacles, and other equipment in a boiler room. I want to add a 200A/3P breaker to it to serve a new rooftop unit. Total load is estimated at about 180A for the panel including the RTU. I was told by the panel manufacturer that I would have to replace it with a new panel in order to have a subfeed breaker for the new RTU. The feed for this panel from a MDP is 225A. I was told to replace the panel with a 400A panel rather than a 225A panel and use the same feed breaker from the MDP. The rep. from GE told me that the bus has to be rated for 400A because it was to be higher than the 200A subfeed breaker, but he was not sure. At the same site, there are 225A panels serving RTU's with 200A subfeed breakers along with other equipment which goes against what the rep told me. Is there an article in NEC that involves this? Thanks!

 

[wareagle]

"I have a 225A, 120/208V,3ph,4w panel that serves two boilers, two pumps, exhaust fan, receptacles, and other equipment in a boiler room. I want to add a 200A/3P breaker to it to serve a new rooftop unit. Total load is estimated at about 180A for the panel including the RTU. "

Panels have a maximum size breaker that can be installed.
The 225 amp panel should have the max size listed inside.
You are approaching a problem with 180 amps on the 225 amp breaker. The max continous load is 225 x 0.80 = 180 amps.

 

[alehman]

I agree with the above. Panelboards such as this commonly can accept branch breakers only up to 100 amps.

 

[rbulsara]

You generally can't add a subfeed breaker in existing panelboard. You need to order them when buying new. This is because of the physical space constraint. Panelboards with subfeed breakers are sized accordingly.

You may want to consider tapping the bus and providing a separate enclosed circuit breaker next to the panelboad (within 10 feet) for the new load. This of course assumes that your load calcs are correct.