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// CODE SNIPPET

M1506.2 Duct Length

2017 FBC - Residential, 6th edition > 15 Exhaust Systems > M1506 Exhaust Ducts and Exhaust Openings > M1506.2 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 M1506.2.

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.

TABLE M1506.2

DUCT LENGTH

DUCT TYPEFLEX DUCTSMOOTH-WALL DUCT
Fan airflow rating
(CFM @ 0.25 inch wca)
50801001251502002503005080100125150200250300
Diameterb
(inches)
Maximum lengthc, d, e
(feet)
3XXXXXXXX5XXXXXXX
4564XXXXXX1143110XXXXX
5NL8142162XXXNL1529151284XX
6NLNL1589155181XNLNLNL16811253259
7NLNLNLNL161784019NLNLNLNLNL1488854
8 and aboveNLNLNLNLNL18911169NLNLNLNLNLNL198133

For SI: 1 foot = 304.8 mm.

  1. Fan airflow rating shall be in acordance with ANSI/AMCA 210-ANSI/ASHRAE 51.
  2. For noncircular ducts, calculate the diameter as four times the cross-sectional area divided by the perimeter.
  3. 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.
  4. NL = no limit on duct length of this size.
  5. 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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