and shall be considered as operating at an ambient temperature of
50°C (122°F). The ampacity of conductors shall be calculated on the basis of the correction factors shown in the 0-2000 volt ampacity tables of Article 310. If this wiring is located above thermal insulation having a minimum thickness of 50 mm (2 in.), the wiring shall not require correction for temperature.
424.38 Area Restrictions.
(A) Shall Not Extend Beyond the Room or Area. Heating cables shall not extend beyond the room or area in which they originate.
(B) Uses Prohibited. Heating cables shall not be installed in the following:
(1) In closets
(2) Over walls
(3) Over partitions that extend to the ceiling, unless they are isolated single runs of embedded cable
(4) Over cabinets whose clearance from the ceiling is less than the minimum horizontal dimension of the cabinet to the nearest cabinet edge that is open to the room or area
(C) In Closet Ceilings as Low-Temperature Heat
Sources to Control Relative Humidity. The provisions of
424.38(B) shall not prevent the use of cable in closet ceil
ings as low-temperature heat sources to control relative hu
midity, provided they are used only in those portions of the
ceiling that are unobstructed to the floor by shelves or other
permanent luminaires.
424.39 Clearance from Other Objects and Openings.
Heating elements of cables shall be separated at least 200 mm (8 in.) from the edge of outlet boxes and junction boxes that are to be used for mounting surface luminaires. A clearance of not less than 50 mm (2 in.) shall be provided from recessed luminaires and their trims, ventilating openings, and other such openings in room surfaces. Sufficient area shall be pro- vided to ensure that no heating cable is covered by any surface-mounted units.
424.40 Splices. Embedded cables shall be spliced only where necessary and only by approved means, and in no case shall the length of the heating cable be altered.
424.41 Installation of Heating Cables on Dry Board, in
Plaster, and on Concrete Ceilings.
(A) In Walls. Cables shall not be installed in walls unless it is necessary for an isolated single run of cable to be installed
down a vertical surface to reach a dropped ceiling.
(B) Adjacent Runs. Adjacent runs of cable not exceeding 9 watts/m (23A watts/ft) shall not be installed less than 38 mm (l'/2 in.) on centers.
(C) Surfaces to Be Applied. Heating cables shall be applied only to gypsum board, plaster lath, or other fire-resistant material. With metal lath or other electrically conductive surfaces, a coat of plaster shall be applied to completely separate the metal lath or conductive surface from the cable.
FPN: See also 424.41(F).