Sweeps vs Bends

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Isaiah

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Baton Rouge
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Electrical Inspector
Contractor is stating his underground duct bank contains 45 degree “sweeps” not bends and therefore the 360 degree max doesn’t apply. He’s currently got around 300 degrees if I count all as bends
Is there a legitimate difference ?


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The NEC makes no allowance for more than 360° when the bends are of a greater radius than the minimum permitted by the code.
 
infinity said:
The NEC makes no allowance for more than 360° when the bends are of a greater radius than the minimum permitted by the code.
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And, depending on the wire stiffness, the pulling tension may not even be significantly lower.

As a (poor, but serviceable) analogy, the total frictional force between a vehicle and the ground does not decrease as you increase the number of wheels so that he loading on each wheel is lower.
 
All: Thank you very much for this valuable feedback. I will be sure to relay this to the contractor!
 
The theory behind the bend rule, and tension calculations in conduit runs, is the Capstan equation. Notice a lack of dependence on the radius of the cylinder in this equation. The ratio of applied tension to held tension, only depends on the friction coefficient and the total angle of wrapping, provided that the rope or cable is wrapped around a simple cylinder.

The reason why you would use larger radius sweeps, rather than standard radius bends, is reducing side wall pressure, rather than reducing pulling tension. I.e. it allows distributing the radial force over a larger area as the wire is pulled around the bend, mitigating the risk of damaging the insulation or conduit.
 
Carultch said:
The theory behind the bend rule, and tension calculations in conduit runs, is the Capstan equation. Notice a lack of dependence on the radius of the cylinder in this equation. The ratio of applied tension to held tension, only depends on the friction coefficient and the total angle of wrapping, provided that the rope or cable is wrapped around a simple cylinder.

The reason why you would use larger radius sweeps, rather than standard radius bends, is reducing side wall pressure, rather than reducing pulling tension. I.e. it allows distributing the radial force over a larger area as the wire is pulled around the bend, mitigating the risk of damaging the insulation or conduit.
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They’re not using any factory radius 90 or even 45 degree bends. If they get closer to the 360 degree limit, I will request tension calculations to ensure side wall pressure is not exceeded on the cable


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Carultch said:
The theory behind the bend rule, and tension calculations in conduit runs, is the Capstan equation. Notice a lack of dependence on the radius of the cylinder in this equation. The ratio of applied tension to held tension, only depends on the friction coefficient and the total angle of wrapping, provided that the rope or cable is wrapped around a simple cylinder.

The reason why you would use larger radius sweeps, rather than standard radius bends, is reducing side wall pressure, rather than reducing pulling tension. I.e. it allows distributing the radial force over a larger area as the wire is pulled around the bend, mitigating the risk of damaging the insulation or conduit.
Click to expand...
I understand the theoretical perspective, and have heard pulling tension software does not differentiate for radius, but in practice I know a larger radius sweep makes a significant difference. What is missing from the textbook is the force required to continually form and unform the conductors to the radius.
 
electrofelon said:
I understand the theoretical perspective, and have heard pulling tension software does not differentiate for radius, but in practice I know a larger radius sweep makes a significant difference. What is missing from the textbook is the force required to continually form and unform the conductors to the radius.
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(y)
The problem is how to quantify that based on conductor size compared to radius and its stiffness properties.
 
Another aspect of the capstan equation is that bends are force multipliers, making pulling force around a bend exponential with the angle of the bend.

This makes the pulling force with lots of bends very sensitive to conditions, where small changes in things like pipe roughness or wire lubrication can make big changes in pulling force.

Jon
 
ActionDave said:
So do you count all the extra degrees if the bottom of the ditch is not perfectly flat, level, and smooth? What if the conduit is not perfectly straight? I have yet to see a piece of PVC that that is.
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See post #5.
 
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