Load Calculations/Panel upgrade

[Chris Lovingood]

Hello all,

I have question in regards to load calculations. A customer wants to add a 1ea-125A and 1ea-225A circuit breakers on a existing 400amp panel that already has (3ea) 200amp circuit breakers.

The reason the customer wants to add the two circuit breakers to the 400amp is that they are on a back-up generator.

This 400amp power panel powers a computer lab with multiple computer servers. The transformer is rated 75kVA, 480volts primary & 120/208Y secondary.

My Questions:

I figured the best option is to do amperage reading throughout the business day since we do not have a amp recording meter. Is this the best way to do this?

Also, is the 75kVA transformer undersized? I did the calculations and its telling me I need a least a 112.5kVA transformer.

Any help, advice, & experience would be much apprieciated. Thank you.

v/r,

Chris

 

[beanland]

Just to make sure...

Just to make sure...

Let me see if I have this right:
480V feeder to a 75kVA 3-phase 480V-120/208V transformer
400A 120/208V panel (main breaker?)
Three 200/3 breakers existing
Want to add one 125/3 and one 225/3 breakers.
Something about a generator somewhere. 480V? 208V?
The 75kVA transformer must be protected by OCPD on the secondary at no more than 125% of nameplate (up to next common size).
125% on 75kVA is 260A so a 300A main breaker in the 400A panel.
You can have as many 200/3, 225/3, 125/3 as you want in the panel.
400A existing panel can support 144kVA of load.
Measure current on 480V legs feeding the transformer.
Determine existing load on transformer and panel.
What is the load owner wants to add?
See if there is enough spare capacity.

 

[Dennis Alwon]

What was the calculation for the building? I don't think you can record with a recording meter for just a day, nevermind a regular ampmeter.