N.Y.C. Admin. Code § 27-980: § 27-980 Power sources of smoke detecting devices.

Dwelling units shall be equipped with smoke detecting devices receiving their primary power from the building wiring and there shall be no switches in the circuit other than the over-current device protecting the branch circuit; provided, however, that dwelling units in existing buildings may, in the alternative, be equipped with battery-operated smoke detecting devices except where such buildings are substantially improved or altered on or after January first, nineteen hundred eighty-two.