Emergency Lights on a Generator and Article 700.17

[JoeStillman]

If I read article 700.17 correctly, we can't put all the lights in an exit corridor or stairwell on the same emergency branch circuit, or even the same ATS. It doesn't jump right out at you when you read the article, but the explanation in the handbook is pretty definite...

Section 700.17(2) requires emergency lighting to be supplied by
a minimum of two branch circuits from separate systems with different
power sources. Where a failure of the normal lighting branch circuit
activates the emergency lighting supply, an area supplied by only one
lighting branch circuit will be in total darkness if that branch circuit
fails. For example, if a single branch circuit, supplied by an emergency
circuit panelboard, supplies the lighting in a stairwell (means of
egress), a failure of that branch circuit leaves the stairwell in total darkness.
If two branch circuits from separate systems are run to the stairwell,
it is unlikely that both circuits to the stairway would fail
simultaneously; therefore, the risk to occupants created by total darkness
is minimized.

Since the need for a generator instead of battery lighting is usually (for me) driven by a large standby load like an elevator or fire pump, we almost always have separate Life Safety and Standby transfer switches. I think going forward I'm going to design egress emergency lighting with a branch circuit fed off of each ATS. And, since 700.5(D) forbids me to put non-emergency lights on the Life Safety ATS, any lights in spaces that don't require emergency lighting will have to be on Standby Power.

If I read the code right, this would even apply to lights on the outside of the exit doors.

I used to design buildings with generators a lot more often than I do now. I don't remember the code being this way. But also, we were trying to minimize the generator size too, and I thought we were saving generator capacity by only putting 1/2 the lights on EM. Now with LED lighting requiring so little power it's tempting to just put them all on Emergency.

What's everybody else doing these days?