Requirements for Cable Tray Expansion Joint Installation

Cable tray expansion joints must be installed to accommodate thermal expansion and contraction, following NEC and NEMA VE 1 guidelines, with proper support, gap settings, and placement intervals.

Regulatory and Standard References

Cable tray expansion joints are governed by NEC Section 300-7(b), which requires raceways to include expansion joints where necessary to compensate for thermal expansion or contraction.

The NEMA Standards Publication V

E 1 provides detailed guidance on metallic cable tray systems, including expansion joint installation, support placement, and allowable tray lengths between joints.

Placement and Support

Support spacing: A cable tray support should be located within 2 feet of each side of the expansion joint splice plate. Anchoring: The tray must be anchored at the support closest to the midpoint between expansion joints using hold-down clamps. All other supports should use expansion guides to allow sliding movement during thermal expansion and contraction.

Avoid over-clamping: Supports must not clamp the tray so tightly that it cannot expand without distortion.

Expansion Joint Spacing

The distance between expansion joints depends on tray material and temperature differential: Steel trays: Require an expansion joint every 128 feet for a 100°F temperature differential. Aluminum trays: Require an expansion joint every 65 feet for the same differential. Temperature considerations: The installation temperature dictates the initial gap setting, which can be determined using the VE 1 nomograph to calculate the correct expansion gap.

Gap Setting Procedure

Plot the highest expected tray temperature on the maximum temperature axis. Plot the lowest expected tray temperature on the minimum temperature axis. Draw a line connecting these points.

Plot the installation temperature on the maximum axis, project to the line, and then down to the horizontal axis to determine the required gap.

Practical Considerations

Expansion joints should be installed at regular intervals, especially in areas with large temperature swings, such as rooftops. Incorrect placement or insufficient expansion allowance can lead to tray bending, bolt elongation, or detachment from supports. Each structure and tray material may require unique solutions due to differing thermal expansion coefficients.

Summary

Proper installation of cable tray expansion joints involves: Following NEC and NEMA VE 1 standardsCorrect support placement and anchoringCalculating joint spacing based on material and temperature differentialSetting the expansion gap according to installation temperatureEnsuring sliding capability at expansion guides to prevent stress on the tray and cables Adhering to these standards ensures long-term reliability, safety, and structural integrity of cable tray systems under varying temperature conditions.