Expansion fittings on PVC conduit
PVC conduit grows and shrinks with temperature. The code requires an expansion fitting where a straight run will change length by ¼ in. or more and, new in 2023, above grade where buried PVC can be stressed by earth movement.
What the code requires
Expansion fittings must be provided where the expected change in length of a straight PVC run, from the table in 352.44, is ¼ in. (6 mm) or more. The run is measured between securely mounted items such as boxes, cabinets, elbows or other terminations.
The table is based on a coefficient of 3.38 × 10⁻⁵ in./in./°F. A 100°F temperature swing works out to about 4.06 in. of change per 100 ft of conduit.
New in 2023, where direct-buried PVC comes up out of the ground, expansion fittings must be provided above grade where needed to absorb earth settling or movement, including frost heave. This ties to the earth-movement rule in 300.5(J).
On the job
- Long PVC runs on sun-exposed walls or roofs without an expansion fitting are a common problem. Manufacturer guidance, such as Carlon's, adds 30°F to the high ambient temperature for sunlight exposure.
- The number of expansion fittings and how each one is set come from the manufacturer's instructions.
- Where buried PVC rises out of the ground, earth settling or frost heave can damage the conduit and the equipment it connects to, which is what the 2023 above-grade fitting rule addresses.
What changed in 2023
The 2023 NEC split the section into thermal expansion (A) and earth movement (B), adding the requirement for above-grade expansion fittings on buried PVC that emerges from the ground.
Related sections
- NEC 300.7(B)
- NEC 300.5(J)
- NEC 352.30
Where this comes from
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