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Every duct or pipe shall be constructed of galvanized metal sheets in accordance with the provisions of Table 57.2404.7.9.1.
Duct Cross-Sectional Dimension in Inches | Gauge of Duct Metal |
Up to 8 | 24 |
Over 8 inches to 18 | 22 |
Over 18 inches to 30 | 20 |
Over 30 | 18 |
Exhaust ducts shall have a clearance from unprotected combustible construction or other combustible material of not less than 18 inches. This distance may be reduced if protection is provided for all combustible surfaces within 18 inches according to the following schedule:
1. 28 gauge sheet metal on 1/4-inch insulating millboard 12 inches.
2. 28 gauge sheet metal on 1/8-inch insulating millboard spaced out 1 inch on noncombustible spacers 9 inches.
3. 22 gauge sheet metal on 1-inch rockwool bats reinforced with wire mesh or the equivalent 3 inches.
4. Any other approved material of equal of greater fire-resistance.
Ducts or pipes may be separated from combustible material by a ventilated collar which provides a clear air space equal to one-half the diameter of the duct, but in no event shall such collar provide less than 3-inch air space, and such collar shall extend not less than 12 inches above and below such combustible material.
Every branch duct or pipe shall be connected to the main duct or pipe at an angle not exceeding 45 degrees measured from the centerline of the main duct or pipe. Every bend, turn or elbow in any duct or pipe shall be constructed so as not to reduce the cross sectional area of the duct or pipe. Every change in cross section of any such duct or pipe shall be by means of a transition fitting having a slope or taper of not more than 1 inch for each 4 inches in length of the said duct or pipe.
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