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ok.. bought a new Generac 15KW .. it came with one auto transfer 200@ unit.. I have a 400@Meter panel with two 200@ breaker boxes.. so, since both breaker boxes feed out of the 400, what or how do I hook up?.. the reason for the two breaker boxes was to divvy up as many circuits as I could without having two bedrooms per circuit and such.. gives me more individual control over lights, recept, etc.. do I need to buy another 200@ auto transfer, can I use two 100@ auto transfer boxes, or can I just use the one 200@ auto transfer? no way would this house ever use anywhere near 1/2 capacity, much less full.. thanks.. wayne

What size service do you have? You say 400A meter but what is the service size?
Service equipment must be rated for the service size, not undersized or in the event of a fault it can become the weak link rather than the over current protection fuse or breaker. The number of main breaker(s) determine the service protection. Two parallel 200A breakers are a 400A service load, and a single 200 A breaker is a 200 service load, regardless of how many panels they are split into. A 20A circuit must be configured using all 20A components and wiring regardless if you are drawing 19.9A or .1A. It is for the system protection in the event of a fault.

So, if the service load is 400A, a (200*2) then a 400A switch should be used to feed both panels.
Only one panel should be fed if it has a 200A breaker with a 200A switch and all the generator loads could be in this panel. Both panels could be fed if the second panel was fed from the load side of the 200A breaker in the first panel, so in effect it is only one panel. This second panel should be a MLO type panel (Main Lug Only).

I see no place in this scheme to use 100A transfer switch unless...
You feed it from a 100A breaker in one of the panels and install a third panel specifically for the generator loads.

Remember it is not possible to power the full system at the time of transfer if the load demand exceeds the generator capacity at the time of start up.
Loads like compressors used in heat pumps, air conditioning and refrigeration have large starting loads and likely will all want to start if they have been off for some time. Resistance heat may also come on if a heat pump at start up if the building is cool.
Your 15KW (60A) generator can easily be overloaded with damage to the generator and loads from a low voltage condition.

If this is not a 'permit' job and you have an unfortunate event, like a fire, the insurance company may take a dim view on 'improper' connections.

yes.. will be a permit job.. it is 400@service but no where near that for a load.. just a lot of breakers on individual circuits.. split into two 200@ panels.. 100@ feed over to the shop which isn't used in the winter months anyway and usually just lights in the summer..

from all I see and hear, I need to sell the 200amp auto transfer box and replace with a 400amp auto transfer box.. anyone have one for sale or trade?

NOW...
You're on the right track!

thanks.. I knew you would know the answer all along.. just making me figure it out huh...

Sometimes, we mortals will refuse to allow the facts to interfere with our theory!