Thoughts: I see plans above on a home-built transfer system.
A well designed and proper MANUAL transfer panel runs about 300 dollars. Plus install, Frankly, I doubt you could build the same panel for that amount, The panel I had could easily transfer a dozen 120 volt circuits (I forget the exact amount) a few 240 volt (you could split them so they trnasfered 120 volt) only switched the HOT lead (this is 100% safe) and had monitors to show you how much power you actually burned (1700 watts, times 2 at peak, generator was 5500 so not even working hard).
Considering the neutral carrying capacity... Normally on most houses it will be 100-200 amps, my generator can at most crank out 50 though some folks can hit 100.. just. Still that is within the Neutral wire capability of my box.
The EXTENSION cord however is another issue, but since it is in air the capacity is higher (mine was rated for 50 amps).
If you want AUTOMATIC transfer (I did not) then the unit that goes between meter and box is the item for you. Wish I'd saved a link to it.
Well, cancel that wish, Link saved, Turns out I have this good friend, Mr. Google. he found it in ONE TRY.
Generator Transfer Switch Installation | MJM Electric Cooperative