Addtional panel

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Bill Marino

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West Haven Ct.
I have an existing 600A-480/277V panel. From the 600A feeder breaker two sets of 500Kcmil feed an ATS(600A), and from there terminate on the main lugs of the above mentioned panel. There are spare lugs in the panel and I want to use them(tap off) to feed an additional panel so that existing equipment will have power supplied by the ATS during a power failure. There are no spare spaces in the 600A panel. There is 1-125 3 pole CB.(load XF 112.5), 2-100A 3 pole CB.(load Dist. PNL), 1-225A 3 pole CB.(load Dist. PNL), and 2-200A 3 pole CB.(load 2 Elevators) . The existing panels I want to add are 100A and 200A. They feed varied loads, one is for a sewage ejector pump system.
My question is how can I determine what my total load will be? I plan on taking actual current readings on all the panels.
I figure that I can use the actual current readings to determine the size of the main in the new panel, and if I'll be exceeding the current 600A ocpd. 125% of cont. plus 100% of non-cont.
Will it be a violation for me to use the spare lugs for the additional panel(taps)? I was reading Art. 240 on tap rules. The two panels(600A and new one) would be with in 10-15ft of each other, I plan on using a main breaker, and the tap conductors would be sized to the main.
Also this is a Hospital, (VA) are there any special considerations here as well?

Any help would be great.
 
Bill Marino said:
Will it be a violation for me to use the spare lugs for the additional panel(taps)? I was reading Art. 240 on tap rules. The two panels(600A and new one) would be with in 10-15ft of each other, I plan on using a main breaker, and the tap conductors would be sized to the main.
Also this is a Hospital, (VA) are there any special considerations here as well?

Any help would be great.
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You can use the lugs. Load calc is the only way to know if you are above 600A when you use them. What I mean is using current ratings won't give you a blessing from the NEC even though you likely won't be over 600A in real life.

When you say, "I plan on using a main breaker, and the tap conductors would be sized to the main", do you mean the 600A main, or a main in the new panel?
 
The tap conductors need a minimum size based on the size of the OCPD ahead of the tap. Either 10% or 33% based on the tap length.
 
I was reading that in a situation such as mine I would need to take a 30 day reading of the actual current the existing 600A panel, than take that number, I would assume it would be an average of the high 3, and multiply that figure by 125%. If the new load I want to add to my existing feeder is below that amount than it would okay.
 
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