Mike Dolt Enterprises
Mike Dolt Enterprises
User: timm333, Location: Minneapolis, MN
Asked on: Thursday, July 3, 2025 at 6:07 am EDT
Answered on: Thursday, July 3, 2025 at 6:07 am EDT
Question: Just thought to ask. In the attached sketch, the width of the cable tray is 12". The cable is pulled at the center of this cable tray. How do we calculate the value of radius (R) of the circle in this attached sketch?Basically I am trying to prove that this cable can be pulled in this cable tray without the need of a 90 Deg elbow. The bending radius of the cable is 12.2". So if radius (R) is equal to or greater than 12.2” then this cable can be puled without the need of a 90-Deg elbow. Thanks
Okay, I can help you figure out how to calculate the radius (R) of the circle described in your sketch and determine if your cable's bending radius allows for pulling it through the tray without a 90-degree elbow.
Understanding the Geometry
The situation describes a circle tangent to both sides of the cable tray. The width of the cable tray (12") provides a key dimension. The center of the circle will lie on the centerline of the cable tray.
Calculation of the Radius (R)
The radius (R) is simply half the width of the cable tray. Since the circle is tangent to both sides of the tray: R = Width of Cable Tray / 2 R = 12 inches / 2 R = 6 inchesAnalysis
You want to determine if the bending radius of the cable (12.2") is greater than or equal to the radius of the turn (R = 6"). Cable Bending Radius (12.2") >= Calculated Radius (6")Conclusion
The bending radius of the cable (12.2 inches) is significantly larger than the radius of the turn in the cable tray (6 inches). Therefore, you cannot pull the cable through this section of the cable tray without exceeding the minimum bending radius of the cable. You will need to use a 90-degree elbow or a section of cable tray with a larger radius to avoid damaging the cable.
Important Considerations
Cable Type
Make sure the bending radius of 12.2" is correct for the specific type of cable you are using. Different cables have different minimum bending radius requirements. Check the manufacturer's specifications.
Installation Practices
Even if the radius calculation worked out marginally, forcing a cable to its minimum bending radius during installation can be difficult and increase the risk of damage. Aim for a larger bending radius whenever possible.
NEC/Code Requirements
Ensure compliance with the National Electrical Code (NEC) or any other applicable local codes regarding cable bending radii in cable trays.
Friction: When pulling the cable, friction will play a large role. It is always preferable to use rollers or lubricant to lower the coefficient of friction during cable pulling, to avoid insulation damage.In summary, while the geometry is simple, the result is definitive. Using a 12" cable tray to achieve a bend tight enough to avoid using an elbow in the orientation you described will violate the cable's minimum bending radius. You must use a wider radius bend. Flag for review