Generator Install Question

[Hs Electric Calif.]

Hey there.. first time asking a question so bear with me please...

Ive got a 400a Service Im adding a 26 kw Backup Generator to. Its a full 400 not split. ive got a 200A sub feed and a dozen branch circuits off the 400a buss. I need to back up the 200a sub because it has majority of the backup circuits the homeowner wants/ needs, unfortunatley theres 8 other circuits that are on the buss that need backing up as well, im racking my brain to engineer this job with the limited space for conduits. my plan is to use a 200ATS for the subpanel feed.. which is a no brainer, its the remaining loads i thought of running a simple 100a ATS with distribution to power the remaining branch circuits... heres the question.

3" PVC with
-line/load for 200ATS (3-4/0 thhn aluminum Line/2-4/0 ALu load back to feed 200a sub) (load goes back and junctioned with polaris lugs)
-line for 100A ATS with dist.(2 #4 thhn Line from 400 buss)
-8 branch circuits from new 100 ATS Distr. panel (8 #12 thhn to feed branch circuits from 400a buss) going back into main service panels and junctioned
-1-#2 ground

can all this be ran without derating?

im sure someones going to ask whynot just backup the whole house?... well theres no room on load side of main to rework with lugs... theres only 4" of room from load side to buss connection

 

[Birken Vogt]

Maybe use a 200 amp transfer switch with built in distribution. Tap the 200 amp sub feed off the transfer switch lugs direct. Put the extra circuits on the distribution.

Another way would be to use a subpanel with 100 amp (or less) main breaker for the extra circuits. Use the 10 foot tap rule to tap the 200 amp conductors after the transfer switch and feed the main of the small sub.