The intent of 210.21(A) is to restrict a fluorescent lighting branch-circuit rating to not
more than 20 amperes because most lampholders manufactured for use with
fluorescent lights have a rating less than that required for heavy-duty lampholders (660
for admedium type or 750 watts for all other types).
Branch-circuit conductors for fluorescent electric-discharge lighting are usually
connected to ballasts rather than to lampholders, and, by specifying a wattage rating for
these lampholders, a limit of 20 amperes is applied to ballast circuits.
Only the admedium-base lampholder is recognized as heavy duty at the rating of 660
watts. Other lampholders are required to have a rating of not less than 750 watts to be
recognized as heavy duty. The requirement of 210.21(A) prohibits the use of mediumbase
screw shell lampholders on branch circuits that are in excess of 20 amperes.