Section 1003.2.8.4 & 1003.2.8.5 says, Exit signs shall be internally or externally illuminated. To insure continued illumination for a duration of not less than 1 1/2 hours in case of primary power loss, the signs shall also be connected to an emergency electrical system provided from storage batteries, unit equipment, or an on site generator set.
To meet the code, we are planning to connect all exit lights to an emergency circuit powered through generator and automatic transfer switch.
A plan examiner in Los Angeles says:
Exit signs connected through a transfer switch are subject to failure in case of a transfer switch failure.
Provide two sources of power to all exit signs or battery operated exit sign.