Final answer:
When obtaining outdoor combustion air through horizontal ducts, each opening should have a minimum free area of not less than 1 square inch per BTU/h of the total input rating of all appliances in the enclosure.
Step-by-step explanation:
In the context of obtaining outdoor combustion air through horizontal ducts, each opening should have a minimum free area of not less than 1 square inch per BTU/h of the total input rating of all appliances in the enclosure.
For example, if the total input rating of all appliances in the enclosure is 60,000 BTU/h, the minimum free area for each opening should be 60,000 square inches.