How to count exit signs and emergency lights
Exit signs are counted by spacing and emergency lighting by illuminance, so the two use different rules. NFPA 101 section 7.10 requires that no point in an exit access corridor be more than 100 ft, or the sign rated viewing distance if that is shorter, from the nearest visible sign, plus a sign at every door or opening serving as a required exit and at stair landings where the path is not obvious. Emergency lighting under section 7.9 is a performance requirement, not a fixture count.
Luminaire quantity is estimated with the lumen method: number of units = target footcandles x area / (lumens per unit x coefficient of utilization x light loss factor). The tool uses the 1 footcandle initial average that NFPA 101 requires along the means of egress at floor level, with your CU and LLF. Battery capacity is sized for the 90 minute duration the same section requires.
The average is only half the requirement. NFPA 101 also sets a minimum of 0.1 footcandle at any point and a maximum to minimum uniformity ratio of 40 to 1, and allows decay to 0.6 fc average at the end of the 90 minutes, so real layouts usually need more units than the average alone suggests. IBC section 1013 adds sign illumination and lettering rules. Confirm the final layout with a photometric plan and the AHJ.
Frequently asked questions
No, it is a minimum. Signs are also needed wherever the direction of travel changes or a sign is not continuously visible, and emergency luminaires must be added to clear dark spots and meet the 0.1 footcandle minimum.
NFPA 101 requires emergency illumination for at least 1.5 hours after loss of normal power, with the average allowed to decay to 0.6 footcandle by the end of that period. Unit equipment and inverters are tested to that duration.