5.2 Thermal Movement: Expansion Fittings and Table 352.44

The General Rule — 300.7(B)

300.7(B) Expansion, Expansion-Deflection, and Deflection Fittings. Raceways shall be provided with expansion, expansion-deflection, or deflection fittings where necessary to compensate for thermal expansion, deflection, and contraction.

This applies to every raceway type, not just PVC. The specific numeric trigger and the table that quantifies movement live in the PVC article, but a 300-foot exposed aluminum rigid conduit run on the south wall of an Interior Alaska warehouse also needs an engineered provision for movement.

The PVC Rule — 352.44

352.44 Expansion Fittings. Expansion fittings for PVC conduit shall be provided to compensate for thermal expansion and contraction where the length change, in accordance with Table 352.44, is expected to be 6 mm (0.25 in.) or greater in a straight run between securely mounted items such as boxes, cabinets, elbows, or other conduit terminations.

Three elements of that sentence matter:

1. The threshold is 0.25 inch of calculated movement — a very small number.
2. The movement is measured in a straight run between securely mounted items. An elbow or a box acts as a boundary; the run you evaluate is the segment between two fixed points.
3. The value comes from Table 352.44, based on the anticipated temperature change over the life of the installation, not the temperature on the day of installation.

Reading Table 352.44 Correctly

The table's heading note gives the physical constant:

Coefficient of Thermal Expansion for PVC Rigid Nonmetallic Conduit = 6.084 x 10 to the minus 5 mm/mm/degree C (3.38 x 10 to the minus 5 in./in./degree F)

The body of the table converts that coefficient into inches of length change per 100 feet of conduit for each temperature change. This is where prep material most often goes wrong: 3.38 is the coefficient in inches per inch per degree F, not the number of inches of movement. Confusing the two understates the expansion by about 17 percent.

Do the arithmetic once so the relationship is clear:

Length change = coefficient x delta-T x length in inches
              = 0.0000338 x 100 degrees F x (100 ft x 12 in/ft)
              = 0.0000338 x 100 x 1200
              = 4.06 inches per 100 ft per 100 degrees F

Selected values from Table 352.44 (inches per 100 ft):

A useful mental shortcut: PVC moves about 4 inches per 100 feet per 100 degrees F, or roughly 0.4 inch per 100 feet for every 10 degrees F.

Worked Example 1 — Interior Alaska Exterior Run

A 120-foot straight run of Schedule 40 PVC conduit is mounted on an exterior wall in Fairbanks. Design temperatures range from minus 50 degrees F to plus 90 degrees F. Is an expansion fitting required, and how much movement must it accommodate?

Delta-T = 90 - (-50) = 140 degrees F
Table 352.44, 140 degrees F ......... 5.68 in per 100 ft
Run length ........................... 120 ft = 1.20 hundreds of feet
Total movement = 5.68 x 1.20 = 6.82 inches

6.82 inches vastly exceeds the 0.25-inch threshold, so expansion fittings are required. Standard PVC expansion couplings are commonly rated for 6 inches or 8 inches of movement; a single 8-inch fitting or two 4-inch fittings would cover this run. Always verify the specific fitting's rated travel from its listing under 110.3(B), and set the piston position at installation according to the manufacturer's temperature chart — a fitting installed fully compressed in July has no room to contract further in January.

Worked Example 2 — Short Interior Run

An interior 40-foot PVC run in a heated Anchorage building sees an operating swing of 20 degrees F.

Table 352.44, 20 degrees F ........... 0.81 in per 100 ft
Run length ........................... 40 ft = 0.40 hundreds of feet
Total movement = 0.81 x 0.40 = 0.32 inch

0.32 inch is greater than 0.25 inch, so a fitting is still required. This is the point that surprises people: the threshold is low enough that even modest interior runs can trigger it. Conversely, a 25-foot run at the same delta-T gives 0.20 inch and does not.

Worked Example 3 — Finding the Maximum Unfitted Length

What is the longest straight PVC run permitted without an expansion fitting where the temperature change is 140 degrees F?

0.25 in / 5.68 in per 100 ft = 0.044 hundreds of feet = about 4.4 feet

Under a 140 degrees F swing, straight PVC runs longer than roughly 4.4 feet between securely mounted items need thermal movement provisions. That is the honest answer to why exposed PVC is uncommon on Alaska exteriors — the practical alternative is a metallic raceway.

Other Raceway Materials

PVC is by far the worst offender, which is why the Code puts the trigger in Article 352. Approximate linear coefficients of thermal expansion:

So a 100-foot steel conduit run through the same 140 degrees F swing moves roughly 1.1 inches rather than 5.7 — significant on a long run, but far more manageable. Article 355 covers reinforced thermosetting resin conduit (RTRC) and contains its own Table 355.44 and expansion rule; 352.44 does not apply to RTRC, so look up the correct table for the material in front of you.

Vertical Movement and Frost

Expansion fittings solve thermal movement along the axis of the raceway. Frost heave is a different problem: ground movement perpendicular to a horizontal run or along a vertical riser. The governing rule is 300.5(J), not 352.44, and the Informational Note there recognizes expansion fittings in raceway risers, S loops at direct-burial-to-raceway transitions, and flexible connections to equipment. Some products are marketed as expansion-deflection fittings and handle both axial and angular movement; 300.7(B) explicitly names expansion, expansion-deflection, and deflection fittings as the three permitted types.

Do not conflate the two rules on the exam. A question about a long exposed exterior run is asking about 352.44 and thermal expansion. A question about a riser out of frost-susceptible ground is asking about 300.5(J) and earth movement.

Bonding Across Fittings

One field consequence worth remembering: metallic expansion fittings interrupt the metallic path. 250.98 requires expansion fittings and telescoping sections of metal raceways to be made electrically continuous by equipment bonding jumpers or other means. A PVC expansion coupling carrying a wire-type equipment grounding conductor inside the raceway does not have this problem, but a steel expansion fitting needs an external bonding jumper unless it is listed to maintain continuity.