PVC Conduit Max Run Before an Expansion Fitting
The inverse of the conduit-thermal-expansion tile: the longest straight PVC run between anchors before an expansion fitting is required, L_max = trigger / (coefficient x 12 x temperature swing) (NEC 352.44). At a 50 deg F swing a run over about 12.3 ft reaches the quarter-inch trigger; at 100 deg F it is about 6.2 ft. Answers 'how long a run can I make before I need a fitting' instead of checking one length. The AHJ and the conduit manufacturer govern.
Formula and source
L_max = trigger_in / (coefficient x 12 x |temperature_change|); the longest straight run whose length change stays at or below the 1/4-inch (0.25 in) expansion-fitting trigger, the inverse of conduit-thermal-expansion. PVC coefficient 3.38e-5 in/in/deg-F.
NEC 2023 (NFPA 70). 352.44 (expansion fittings for rigid PVC conduit).
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