Cable tray expansion joint method

Cable tray expansion joints are installed to accommodate thermal expansion and contraction, ensuring system integrity and preventing stress on cables and supports.

Purpose of Expansion Joints

Cable trays expand and contract due to temperature fluctuations, which can cause bending, misalignment, or damage if not properly managed. Expansion joints absorb these movements, maintaining the structural and electrical integrity of the system while allowing natural thermal movement without straining the cables or tray supports .

Placement and Support

• Location: Expansion joints should be installed at regular intervals along the cable tray, particularly in areas with significant temperature variations, such as rooftops or outdoor installations .
• Support: Place cable tray supports within 2 feet of each side of the expansion joint splice plates. The tray should not be clamped too tightly to these supports, allowing free movement .
• Anchoring: Anchor the tray at the midpoint between expansion joints using hold-down clamps, while other supports use expansion guides to allow sliding movement .

Gap Calculation

• Determine the temperature range the tray will experience (e.g., winter to summer extremes).
• Use the NEMA VE 1 standard nomograph to calculate the required expansion gap. For example, a steel tray with a 100°F differential may require an expansion joint every 128 feet, while aluminum may require one every 65 feet .
• Set the gap at installation based on the tray's metal temperature at that time. For instance, if the installation temperature is 50°F, the gap may be set to 3/8 inch .

Installation Steps

1. Prepare the tray ends at the expansion joint location.
2. Install expansion joint splice plates between tray sections.
3. Attach bonding jumpers around the splice plates to maintain electrical continuity; size them to match the tray's fault current capacity .
4. Secure the tray at the midpoint with hold-down clamps.
5. Install expansion guides at other supports to allow sliding movement.
6. Verify alignment and ensure the tray can expand and contract freely without obstruction .

Compliance and Standards

• Follow NEC Section 300-7(b), which requires cable trays to accommodate thermal expansion and contraction .
• Adhere to NEMA VE 1 guidelines for material-specific expansion intervals and gap settings .
• Consider environmental conditions, tray material, and cable type when determining joint placement and configuration .

Summary

Proper construction of cable tray expansion joints involves careful placement, support, gap calculation, and installation of splice plates and bonding jumpers. Following NEC and NEMA standards ensures long-term reliability, prevents structural stress, and maintains electrical continuity in cable tray systems subjected to thermal expansion and contraction .