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All air from outdoors. The confined space shall communicate with the outdoors in accordance with methods 1 or 2, which follow. The minimum dimension of air openings shall be not less than 3 in. (80 mm). If ducts are used, the ducts shall be of the same cross- sectional area as the free area of the openings to which they connect.
1. Two permanent openings, one commencing within 12 in. (300 mm) of the top of the enclosure and one commencing within 12 in. (300 mm) of the bottom of the enclosure shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or with attic spaces that freely communicate with the outdoors.
(a) If directly communicating with the outdoors or if communicating to the outdoors through vertical ducts, each opening shall have a minimum free area of 1 in.2 per 4000 Btu/hr (550 mm 2/kW) of the total input rating of all equipment in the enclosure.
(b) If communicating with the outdoors through horizontal ducts, such opening shall have a minimum free area of not less than 1 in.2 per 2000 Btu/hr (1100 mm
2
/kW) of the total input rating of all equipment in the enclosure.
2. One permanent opening commencing within 12 in. (300 mm) of the top of the enclosure, shall be permitted if the equipment has clearances of at least 1 in. (25 mm) from the sides and back and 6 in. (160 mm) from the front of the appliance. The opening shall directly communicate with the outdoors or shall communicate with the outdoors through a vertical or horizontal duct or space that freely communicate with the outdoors. The opening shall have a minimum free area as follows:
(a) 1 in.2/3000 Btu/hr (700 mm
2
per kW) of the total input rating of all equipment located in the enclosure, and
(b) Not less than the sum of the areas of all vent connectors in the confined spaces.