While large DPDT transfer switches are expensive; it's not hard to
get DPDT surplus relays with 30A contact ratings.


to line CB1 NC NO CB2 to genset

---------o o----|/|-----O---------||---------o o-------
|
|
|
|
\/ to load.


You could repeat the above on each circuit. The hard part will be a
2nd breaker box for the generator, and how to feed each leg of
CB2 from a separate source.

Use a small relay with a NO pushbutton to control it:

NO
[/\/\/\/\]--.-| |-.-----to gen
/ | |
| -| |-
| NO PB

The relay will latch up through its own contact, until the generator
runs out of fuel or otherwise shuts down. Use a 2nd contact on this
to run the mains contactors.

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433