MCC Vertical Buss Loading

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jarrodp

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I am looking at installing a new circuit in an existing MCC. The MCC has a horizontal buss rating of 600A and a vertical rating of 300A. I understand that the main breaker to the panel is sized to protect the horizontal buss and should be 600A. However, all I can come up with for protecting the vertical buss is common sense must be used. Is there anything written that shows good judgment in sizing the breakers installed off of the vertical buss?

Can you place a 300A breaker on the vertical 300A rated buss that already has 2 150A breakers installed? I can't find anything that says you can't. I am afraid to check the main breaker rating, because it is not tripping and I can't see us not being very close to 600A.

Any guidance?
 
How is the vertical bus fed? In the center or from the top? If it's center fed just put the two 150A breakers above the feed and the 300A below. If it's top fed you need to replace the section or add a section.

Martin
 
jarrodp --

Generally you want to check the mfg specifications. Specifically, you want to check the mfg specifications :)

I am familiar with SQD and AB 2100, and both have good design guides. My recommendation is to follow the design guides.

I don't know what you mean about being afraid of checking the main CB rating cause it's not tripping.

1. Are you saying the running load is in excess of the trip setting of the main cb? Yeah that would be a problem.

2. Are you saying the connected load is in excess of the trip rating of the main? That might not be a problem.


Guidance:
1. Get your plant one-line out and a clamp-on ammeter and collect data for a load study.

2. No one line? Get started collecting data and draw one. Just start with the section you need first.

3. Get the trip curves and trip unit settings for the CBs and put together a coordination study. You can do a small section with just colored pens and a straight edge. You don't have to have a computer program.

Good luck, keep us informed on how it goes - it sounds interesting.
 
Thank you everyone. I did start investigating the panel this week. The horizontal bus feeds the verticals from the top. One vetical section was rated at 600A with the other 7 rated at 300A. This made me feel better about the 400A break I knew that was installed in the panel.

The main was verified to have a 600A breaker installed and the load on the panel when I checked was about 200A averaged. The main loads on this panel however include three 20t cranes with magnet spreader beams and I only caught on running.

I will calculate the loads this week, and due to timing on getting approval for this project, I will just budget a new feeder panel be run. This way, if I later find out I can use the existing gear, I'll be good on the budget.

Thanks!
 
in a typical high rise office building we find 4000 amp vertical bus duct maybe feeding one third of the building -- and the bus is reduced maybe twice as it reaches the upper floors that it serves. the main will still be rated at 4000 amps. it is sized for the load at that point in the system like stepping down to 3000amp and then 2000 amp... also when calculating loads "on site" you must consider "connected load" not only actual load... a few years ago we were called to a large building because the emergency power generator's output breaker would trip from time to time. they had added emergency power loads to their system calculated on actual generator load during normal operation --- the thing they missed was the smoke evacuation fans during a fire or fire alarm. the fans doubled the load they had considered full load. they thought back and the times the breaker tripped -- the fans were running from a false alarm caused by the power fluxuation before the generator came "on line".. they had to add an additional generator for the smoke fans............
 
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