Switchgear sizing

[DarbyD]

Hey engineers,

I was wondering if one or some of you might explain a rather lengthy theory that I have yet to get my head around, that is, sizing switchgear and the components thereof (I said it was lengthy).
Anyway I understand the whole square footage break down per floor depending on the building type.
So let’s say for the sake of round numbers that it’s a 2 story office building. Each floor has a 200A/MCB 480/277 panel feeding a transformer, feeding a120/208 200A MLO panel.
Also I’ll throw in a couple of 25 HP (27 FLA) AHUs and (2) 40 HP (52 FLA) Chillers.
What I would like to know is:
How to size the switchboard
The Switchboard main circuit breaker
The fault current (500 KVA pad mount transformer?)
The utility Transformer size.
Is there anything else that needs sizing?
If anyone can take time out from their work of sizing switchboards to answer this, that would be great. Thanks a lot.

 

[DanDel]

The size of the switchboard and main C/B is based on the total load it will serve, along with an 80% safety margin and any other increases for furure loads that you may envision.
The size of your individual panels doesn't mean too much.

The Utility will size their own transformer(typically much lower than you expect) based on your load calcs.

The fault current will again be based on the Utility contribution and the size of their transformer, but a good maximum estimate can be calulated from the transformer kVA and %Z.