RTRC Conduit Expansion Calculator

How RTRC conduit expansion is calculated

RTRC (fiberglass) expands and contracts with temperature, though far less than PVC. NEC Table 355.44 captures that behavior as a single coefficient of linear thermal expansion: 1.5 x 10^-5 inch of length change per inch of conduit for every degree Fahrenheit of temperature change. The length change of a run is that coefficient times the temperature swing times the length:

length change (in) = 0.000015 x temperature swing (degF) x length (ft) x 12

Per 100 feet that reduces to about 0.018 inch for each degree, so the table lists 0.09 inch per 100 feet at a 5 degF swing and 1.80 inches per 100 feet at a 100 degF swing. The temperature swing is the full range the conduit sees: the expected high temperature minus the expected low temperature.

Table 355.44 length change per 100 ft

Values computed from the NEC Table 355.44 coefficient (0.018 in per 100 ft per degF). The calculator uses the same coefficient for any length and swing, so its output matches these rows exactly.

RTRC vs PVC expansion

RTRC and PVC follow the same NEC method and the same 6 mm (1/4 inch) fitting threshold, but they expand at very different rates. RTRC (Table 355.44) has a coefficient of 1.5 x 10^-5 in/in/degF; PVC (Table 352.44) is 3.38 x 10^-5, more than twice as much. For an identical run and temperature swing, RTRC moves about 44 percent as far as PVC, so a run that needs an expansion fitting in PVC may not need one in RTRC. Use the PVC Conduit Expansion Calculator for Article 352 raceway, and this tool for Article 355 fiberglass.

Worked example: 100 ft run, 0 to 100 degF

1. Temperature swing: 100 degF high minus 0 degF low = 100 degF.
2. Length change: 0.000015 x 100 x 100 x 12 = 1.8 inches (45.7 mm).
3. That is well over the 1/4 inch threshold, so NEC 355.44 requires an expansion fitting.
4. Select a fitting whose travel covers 1.8 inches and set the piston for the temperature at the time of installation.

Why direct sunlight matters

Conduit in direct sunlight absorbs radiant heat and runs well above the air temperature. Conduit manufacturers recommend adding about 30 degF to the high end of the expected range for exposed runs. The direct-sunlight toggle applies that allowance. It is a design recommendation rather than an NEC Table 355.44 value, so it stays off by default; turning it on only increases the computed movement, which is the conservative choice when sizing a fitting.

Frequently asked questions

When is an expansion fitting required?

NEC 355.44 requires one wherever the anticipated length change in a straight run reaches 6 mm (1/4 inch). Below that the conduit and its supports absorb the movement.

How much does RTRC conduit expand?

About 0.018 inch per 100 feet for each degree F. A 100 foot run over a 100 degF swing moves roughly 1.8 inches, well under half of what PVC would move over the same run.

Does this size the expansion fitting itself?

No. The tool returns the movement the fitting must accommodate and whether one is required. Pick a listed fitting whose travel covers that movement, and set it per the manufacturer instructions for the install temperature.

What about PVC or metal conduit?

This calculator is for RTRC (reinforced thermosetting resin, fiberglass) conduit under Article 355. PVC rigid nonmetallic conduit (Article 352) expands more than twice as fast, so use the PVC Conduit Expansion Calculator for it; metal conduit expands so little that fittings are usually only needed at building expansion joints.

Related calculators

• PVC Conduit Expansion Calculator: the Article 352 sibling, thermal length change and fitting threshold for PVC.
• Conduit Support Spacing Calculator: maximum support spacing by conduit type and trade size.
• Conduit Fill Calculator: mixed conductors in EMT, RMC, IMC, or PVC against the Table 1 limits.
• Conduit Size Calculator: minimum trade size for a conductor bundle across all raceway types.