As I live in a rural area, I installed a back-up 13 years ago when I built. From one of the 200A main panels a 40/2 feeds the normal side of the xfer. The diesel gen set feeds the emer side. The load side of the xfer feeds a 125A 12 circuit EM panel. The EM panel has various lighting, security system, freezer, refrigerators, kitchen counter outlets, sump pump, etc. Works well. Last outage I ran temp to neighbors home as they had just filled freezer and frig. 16 hours without a hitch.
