How long should a cable tray be before adding an expansion joint
The spacing of cable tray expansion joints depends on the tray material and expected temperature variations, with steel trays typically requiring joints every 128 feet and aluminum trays every 65 feet for a 100°F temperature differential.
Key Considerations
Material Type: Different materials expand at different rates. Steel has a lower coefficient of thermal expansion than aluminum, so steel trays can span longer distances before requiring an expansion joint, while aluminum requires closer spacing to accommodate movement . Temperature Differential: The greater the temperature change the tray will experience, the closer the expansion joints should be placed. For example, a steel tray exposed to a 100°F differential may need an expansion joint every 128 feet, whereas an aluminum tray under the same conditions may need one every 65 feet . Standards and Guidelines: NEMA VE 1 provides tables and nomographs to calculate allowable tray lengths and expansion gap settings based on material and temperature differential. NEC Section 300-7(b) also emphasizes the need for expansion joints to prevent stress on the tray and cables . Support and Anchoring: Expansion joints should be supported within 2 feet of each side of the splice plate. The tray should be anchored at the midpoint between expansion joints, with expansion guides at other supports to allow free movement .
Practical Example
- Steel tray: For a 100°F temperature change, install an expansion joint approximately every 128 feet.
- Aluminum tray: For the same temperature change, install an expansion joint approximately every 65 feet.
- Installation tip: Always consider the temperature at the time of installation to set the correct expansion gap, and ensure cables are not tightly clamped to allow for tray movement .
Summary
The distance between expansion joints is not fixed but depends on the tray material, expected temperature variations, and installation environment. Following NEMA and NEC guidelines ensures that the cable tray can expand and contract safely without causing structural stress or damage to the cables .
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