Dryer Exhaust Duct Developed Length (IRC M1502.4.5)
The 35 ft everyone quotes is not the number that matters, because fittings eat it. Developed length is the straight run PLUS the equivalent length of every elbow, and on a short interior run the fittings are often more than half the total. The pair that makes the point: 22 ft of duct with three 90s and two 45s comes to 42 ft on tight mitered elbows -- 7 ft OVER -- and 28 ft on wide smooth ones, 7 ft to spare. Same run, same elbow count, compliance flipped by elbow selection alone, which is usually cheaper than rerouting. Per M1502.4.5.2 the DRYER MAKER\u0027S instructions determine size and maximum length and the code table is only a fallback, so the ceiling and the per-fitting equivalents are inputs and no code table is reproduced. Flags the M1502.4.6 label over 35 ft and the 8 ft transition-duct limit, which is not counted in the length.
Formula and source
fitting equivalent = (90-elbow count x equivalent length per 90) + (45-elbow count x equivalent length per 45); developed length = straight run + fitting equivalent; compliant when developed length <= the maximum allowable length; remaining straight run = max - developed; label required when developed length exceeds 35 ft.
IRC M1502.4.5 (maximum exhaust duct length measured from the connection to the transition duct to the outlet terminal, reduced by fitting equivalent lengths, and not including the transition duct); M1502.4.5.2 (the dryer manufacturer's installation instructions determine the size and maximum length, with the code's fitting table used only in the absence of manufacturer equivalent lengths); M1502.4.6 (equivalent length identified on a permanent label or tag within 6 ft of the duct connection where it exceeds 35 ft); M1502.4.3 (listed transition duct limited to 8 ft and not part of the duct length).
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