PVC Conduit Max Run Before an Expansion Fitting
The longest straight PVC run between anchors before an expansion fitting is required.
Example
You enter
- Temperature swing (°F) 50
- PVC coefficient (in/in/deg-F) 0.0000338
- Fitting trigger (in) 0.25
You get
- Max run before a fitting 12.3 ft
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeL_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 or tableL_max = trigger_in / (coefficient x 12 x |temperature_change|); the longest straight run whose length change stays below the 1/4-inch (0.25 in) expansion-fitting trigger (NEC 352.44 requires the fitting at 1/4 in or greater, so a run of exactly L_max already needs one), the inverse of conduit-thermal-expansion. PVC coefficient 3.38e-5 in/in/deg-F.
Source and editionNEC 2023 (NFPA 70). 352.44 (expansion fittings for rigid PVC conduit).
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namestemp_change_f, coeff_in_per_in_f, trigger_in, max_run_ft
- PVC coefficient the bundled 3.38e-5 in/in/deg-F coefficient is the public PVC physical property underlying NEC Table 352.44; the manufacturer's published coefficient governsNEC 2023 Table 352.44 / PVC manufacturer
- Temperature swing the run length varies inversely with the temperature swing; the magnitude of the max-minus-min swing is used, so the sign does not mattersite temperature extremes; the AHJ governs
- Trigger the 1/4-inch (0.25 in) default is the NEC 352.44 threshold at which a fitting is required; a longer run needs a fitting sized for the resulting travelNEC 2023 352.44