To me, "open at the top or bottom on the same floor level" means, for example, a stud space that is not sealed, such as 8' drywall on 9' studs with the gap accessible above a suspended ceiling, at least on one side.
I think we are saying the same thing. I agree. I also made a proposal awhile back to delete this exception as it makes no sense. What good does it do if the neutral for that circuit is not available as would be if it were a 3 way dead end switch for a stair light where said switch is in an unfinished basement.
Considering that 3-ways travel in pairs (all puns intended), the rule does, or should apply to both (or all if 4-way(s), too.)
If you mean a fire-block or other framing member, then the wall cavity is not open.
As long as the box is available through one of the openings.
That'll work. The rule applies to each switch.
A national electrical code site with no book - hope you don't cross over to electrical engineering with this mindset, I'm guessing you?re a curious mechanical?
Physics with electromotive force is one perspective and the use of electricity another, I think the NEC proves how complicated it can be to use electricity correctly and or safely.
Nope, it has to do with how difficult the english language can be. :grin: If one thinks about it the NEC is quite an amazing feat. They have had years but trying to cover every situation is a nightmare.
Communicating sure adds to it and we're ever using electricity differantly
