In a 2x8 floor joist spanning 12ft what is the largest diameter hole you can drill through for conduit?

1/3 the size of the joist, so approximately 2 1/4 in.

Keep the hole a min. of 2" from the top of the joist. If it's 2" exactly, put a nail guard on the top.

Don't put the hole within 6" of another hole. don't put the hole within 4" of a notch.

This of course all depends on the joist spacing 12"/16"/24"? And what the live load is anticipated? If it's a regular floor it's one thing, a monster tub is another.