Every common hall and stairway in residential occupancies,
other than in one- and two- family dwellings, shall be lighted at
all times with at least a sixty-watt standard incandescent light bulb
for each 200 square feet of floor area or equivalent illumination,
provided that the spacing between lights shall not be greater than
30 feet. In other than residential occupancies, means of egress, including
exterior means of egress, stairways shall be illuminated at all times
the building space served by the means of egress is occupied with
a minimum of one footcandle (11 lux) at floors, landings, and treads.