Cable tray bending coefficient

The coefficient for cable tray bends is generally considered as 1 for well-maintained sheaves, meaning bends do not multiply pulling force, but fill capacity increases by 10–20% due to cable crowding on the inside of the bend.

Understanding the Coefficient

In cable tray systems, bends affect both cable fill and pulling forces: Pulling Force Coefficient: For bends with properly maintained sheaves, the coefficient is assumed to be 1, indicating no additional multiplying effect on the pulling force compared to a straight run. If sheaves are not well-maintained, the bend can increase the pulling force, and standard conduit bend equations may be applied to calculate the required force. Fill Adjustment Coefficient: When cables pass through a bend, the inside of the bend becomes more crowded, effectively increasing the fill percentage.

It is common practice to allow a 10–20% growth buffer for bends. For example, a tray filled to 40% in a straight section may reach 50–60% at a bend due to crowding.

Practical Considerations

Tray Type: Ladder trays, ventilated troughs, wire mesh baskets, and channel trays all behave differently in bends. Ladder trays allow easier access, while wire mesh and channel trays may reduce usable fill. Bend Radius: The inside radius of the bend should be at least equal to the diameter of the largest cable in the tray to prevent excessive crowding and maintain serviceability.

Bend Angle and Count: Multiple bends or offsets increase the total arc length and cumulative fill penalty. Two smaller bends may clear obstacles but double the arc and stack the fill penalty. Installation State: Pulling cables through bends requires extra radius allowance if the cables are being pulled, laid temporarily, or installed with service loops.

Summary

Pulling coefficient: 1 (no multiplying effect if sheaves are well-maintained)Fill increase at bends: 10–20% over straight section fillMinimum bend radius: At least the diameter of the largest cableConsiderations: Tray type, bend angle, number of bends, and installation state all influence the effective coefficient and fill capacity. Using these guidelines ensures safe cable installation, prevents overcrowding, and maintains serviceability in cable tray systems.