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M1504.2 Duct Length
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The length of exhaust and supply ducts used with ventilating equipment shall not exceed the lengths determined in accordance with Table M1504.2.
Exception: Duct length shall not be limited where the duct system complies with the manufacturer's design criteria or where the flow rate of the installed ventilating equipment is verified by the installer or approved third party using a flow hood, flow grid or other airflow measuring device.
DUCT LENGTH
| DUCT TYPE | FLEX DUCT | SMOOTH-WALL DUCT | ||||||||||||||
| Fan airflow rating (CFM @ 0.25 inch wca) | 50 | 80 | 100 | 125 | 150 | 200 | 250 | 300 | 50 | 80 | 100 | 125 | 150 | 200 | 250 | 300 |
| Diameterb (inches) | Maximum lengthc, d, e (feet) | |||||||||||||||
| 3 | X | X | X | X | X | X | X | X | 5 | X | X | X | X | X | X | X |
| 4 | 56 | 4 | X | X | X | X | X | X | 114 | 31 | 10 | X | X | X | X | X |
| 5 | NL | 81 | 42 | 16 | 2 | X | X | X | NL | 152 | 91 | 51 | 28 | 4 | X | X |
| 6 | NL | NL | 158 | 91 | 55 | 18 | 1 | X | NL | NL | NL | 168 | 112 | 53 | 25 | 9 |
| 7 | NL | NL | NL | NL | 161 | 78 | 40 | 19 | NL | NL | NL | NL | NL | 148 | 88 | 54 |
| 8 and above | NL | NL | NL | NL | NL | 189 | 111 | 69 | NL | NL | NL | NL | NL | NL | 198 | 133 |
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.
- Fan airflow rating shall be in accordance with ANSI/AMCA 210-ANSI/ASHRAE 51.
- For noncircular ducts, calculate the diameter as four times the cross-sectional area divided by the perimeter.
- This table assumes that elbows are not used. Fifteen feet of allowable duct length shall be deducted for each elbow installed in the duct run.
- NL = no limit on duct length of this size.
- X = not allowed. Any length of duct of this size with assumed turns and fittings will exceed the rated pressure drop.
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