Cable tray expansion joint
Cable tray expansion joints are installed to accommodate thermal expansion and contraction, preventing structural stress and maintaining system integrity.
Purpose of Expansion Joints
Cable trays, typically made of metal such as steel or aluminum, expand and contract with temperature changes. Without proper expansion joints, these movements can cause bending, misalignment, bolt elongation, or even detachment from supports, especially in outdoor or high-temperature environments . Expansion joints absorb these movements, allowing the tray to slide or expand without damaging the cables or tray structure .
Standards and Guidelines
- NEC Section 300-7(b) requires raceways, including cable trays, to have expansion joints where necessary to compensate for thermal expansion or contraction .
- NEMA VE 1 provides detailed recommendations for metallic cable tray systems, including allowable lengths between expansion joints based on material and temperature differential. For example, a steel tray may require an expansion joint every 128 feet for a 100°F temperature change, while aluminum may require one every 65 feet .
- Expansion joints should be installed near structural expansion points but also consider the tray's own thermal movement, as tray and structure materials expand differently .
Installation Considerations
- Support Placement: Supports should be located within 2 feet of each side of the expansion joint splice plate. Trays should not be clamped too tightly to allow free movement .
- Anchoring and Guides: Anchor the tray at the midpoint between expansion joints and use expansion guides at other supports to allow sliding movement .
- Gap Setting: The expansion gap should be calculated based on the expected temperature range at installation, using nomographs or formulas provided in NEMA VE 1 .
- Material Selection: Aluminum expands more than steel, so the number and spacing of expansion joints must account for the tray material .
Practical Tips
- Always calculate the required expansion gap rather than relying on fixed distances.
- Use splice plates and bonding jumpers designed for expansion joints to maintain electrical continuity.
- Consider environmental conditions, such as rooftop installations or areas with large seasonal temperature swings, to prevent tray deformation or stress . Properly installed cable tray expansion joints ensure long-term reliability, safety, and structural integrity of electrical systems in varying temperature conditions.
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