Dryer Exhaust Duct Developed Length (IRC M1502.4.5)

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'S 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.

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).

The section requirements are cited by number. The code's fitting-equivalent TABLE is deliberately NOT reproduced: M1502.4.5.2 makes the manufacturer's values authoritative and the table a fallback, so both the ceiling and the per-fitting equivalents are user inputs here and no tabulated data is shipped.

Estimate. AHJ and licensed professional govern.

Field names used by the API: straight_run_ft, elbow_90_count, elbow_45_count, eq_len_90_ft, eq_len_45_ft, max_length_ft, transition_duct_ft, fitting_equivalent_ft, developed_length_ft, over_by_ft, remaining_straight_ft, fitting_share_pct

• Developed length straight run plus every fitting's equivalent lengthIRC M1502.4.5
• Source of equivalents the dryer manufacturer first; the code table only in its absence - so both are INPUTSIRC M1502.4.5.2
• Elbow radius dominates a tight mitered 90 can cost more than three times a wide smooth onethe spread within the code's own fitting table
• Label required over 35 ft of equivalent length, within 6 ft of the connectionIRC M1502.4.6
• Transition duct listed, 8 ft maximum, and NOT counted in the duct lengthIRC M1502.4.3
• Not checked diameter, material, supports, joints, termination, damper, screens, shield plates, booster fansstated scope limit