There once was a practice that exit and emergency lighting not only had their own circuit- the circuit originated at the service entrance, before the panel. They wanted to ensure that no one ever turned it off!
That practice has gone away, mainly because of the economy of battery back-up units. You'll stikk encounter it though.
In this area and I imagine most Exit signs are required to be illuminated.
To that end they are either suppled from an emergency generator circuit and will likely be tied into the local emergency lighting circuit.
They have battery back up either self contained or remote.
Once an exit sign is backed up with a battery it becomes "Unit Equipment" and the rules of 700.12(E) apply.
700.12(E) is long but requires unit equipment to be supplied by the local lighting circuit.
part of 700.12(E)
The branch circuit feeding the unit equipment shall be the same branch circuit as that serving the normal lighting in the area and connected ahead of any local switches.
If you read all the way to the end of 700.12(E) you will see this exception that sometimes allows a dedicated circuit to supply unit equipment.
Exception: In a separate and uninterrupted area supplied by a minimum of three normal lighting circuits, a separate branch circuit for unit equipment shall be permitted if it originates from the same panelboard as that of the normal lighting circuits and is provided with a lock-on feature.
Bob Badger Construction & Maintenance Electrician Massachusetts
Bob, We have been down this road before and I still don't agree that the exit light has to be on the general lighting circuit. It is my opinion that the general illumination in the area of the exit light is provided by the exit light itself and can be on any unswitched circuit. I'm also not convinced that an exit light meets the definition of unit equipment at shown in 700.12(F). Don
If I am doing a new installation, I will put the emergency lights and exit signs on a lighting circuit. It seems to make sense to have the light go "on" when power fails to the lighting circuit. Of course, if they control the lights by flipping breakers, this idea won't work.
In that case, or in a retrofit where circuits are a mess, I tie the exit signs and bugeyes into and convenient unswitched circuit. This, at least, ensures lighting in the event of a PoCo failure.
The only joker in this deck is that we have at least one Fire Inspector who can't be bothered to use the 'test' buttons; rather, he wants to flip breakers. I mark the assorted breakers with red tape just for him!
Two things...the definition of unit equiment covers emergency lights very cleary, but doesn't really cover illuminated signs. Second there is no safety reason to have the exit lights on the area lighting circuit...it doesn't change anything...the exits lights do their job either way.