12/2 wires running under floor system and crossing I joist systems can you staple wires to I joist to support them from sagging down. Is there a code aganst this... thank you.

Can you staple wires under a I joist floor system. To keep them from sagging down.

It's preferred to place a furring strip perpendicular to the joists and nail to there. The NEC does have a particular nail spacing.

My house has wires running through holes in the center of the joist (50 yrs old). Again there is code as to how big and where these holes can be.

The weak points in the joist are the 1st and 3rd thirds. Drilling at the top or bottom weakens the joist.

Think about it. If I had a 2x10 and drilled a 2" hole at the edge, I'd have a 2x8 at that point.

Remember the nail plate uses. It shouldn't be possible to inadvertently drive a nail into the wire say, when someone removes and replaces a floor.

Assuming that you mean in an unfinished basement or a crawl space and romex (NM) cable.

See NEC 334.15 (C).

Cables smaller than 6-2 or 8-3 and perpendicular to the joist must pass through bored holes.