How to figure out sidewall pressure on conduits.

[aric.martens]

I understand that if you use longer radious fittings, you can greatly reduce sidewall pressure. I am not looking for a specific example, rather to learn the calculation required to determine the pressure. One other side note, if you are utilizing rigid sweeps for turns in ductbank and PVC everywhere else are you still required to bond the metallic fittings or could you simply suggest that they were not likely to get energized?

 

[infinity]

You could try this from Southwire:

http://www.southwire.com/support/SIMpullCablePullCalculator.htm

 

[don_resqcapt19]

As I recall the sidewall pressure is the pulling tension at the bend divided by the radius of the bend in feet.

As far as the rigid sweeps, they are required to be bonded unless permitted otherwise by Exception #3 to 250.86.

 

[Carultch]

As I recall the sidewall pressure is the pulling tension at the bend divided by the radius of the bend in feet.

As far as the rigid sweeps, they are required to be bonded unless permitted otherwise by Exception #3 to 250.86.

I would think that the degree angle of the bend would make a difference in sidewall pressure. Maybe in radians, because that unit is the more "mathematically natural" unit of angle measurement.

Also, pressure is supposed to be force per unit of area. Is this a fake pressure quantity, where they only divide it by feet?

The Southwire spreadsheet only indicates pulling tension in pounds.

 

[don_resqcapt19]

I would think that the degree angle of the bend would make a difference in sidewall pressure. Maybe in radians, because that unit is the more "mathematically natural" unit of angle measurement.

Also, pressure is supposed to be force per unit of area. Is this a fake pressure quantity, where they only divide it by feet?

The Southwire spreadsheet only indicates pulling tension in pounds.

The degrees of bend do not make any difference. The pressure is caused by the wire trying to take a straight line and the conduit forcing it to go around a radius. The sidewall pressure is force per unit of length, that is pounds per foot of radius. If you don't have a full 90, you just have the same sidewall pressure over a shorter length.

 

[Smart $]

The degrees of bend do not make any difference. The pressure is caused by the wire trying to take a straight line and the conduit forcing it to go around a radius. The sidewall pressure is force per unit of length, that is pounds per foot of radius. If you don't have a full 90, you just have the same sidewall pressure over a shorter length.

I'm inclined to agree with Carultch. Pressure is force per unit area... not length... and from a technically analytical perspective sidewall pressure would be lower for a 1° bend at practically any radius than for a 180° bend at the same radius.

But I can see where this has probably been evaluated and simplified for field use being wire/cable OD is always smaller than conduit ID, such that there is very little width to the area in which the maximum sidewall pressure exists through a radiused bend.

 

[don_resqcapt19]

I'm inclined to agree with Carultch. Pressure is force per unit area... not length... and from a technically analytical perspective sidewall pressure would be lower for a 1° bend at practically any radius than for a 180° bend at the same radius. ...

All of the definitions that I can find say that sidewall pressure is force per unit of length.

At the point of contact the sidewall pressure for that 1° bend would be identical to a same size point of contact on the 180° bend assuming the same pulling tension and bend radius. The biggest difference would be that the 1° bend would only require a very small increase in pulling tension as compared to straight conduit, while the 180° bend would a fairly large increase in the required pulling tension.

 

[Smart $]

All of the definitions that I can find say that sidewall pressure is force per unit of length.

At the point of contact the sidewall pressure for that 1° bend would be identical to a same size point of contact on the 180° bend assuming the same pulling tension and bend radius. The biggest difference would be that the 1° bend would only require a very small increase in pulling tension as compared to straight conduit, while the 180° bend would a fairly large increase in the required pulling tension.

Compare with standard definition of pressure. Not sure how sidewall pressure definition can deviate from the standard???

The change in pull tension for change in bend angle may be the part I was overlooking.... just not quite yet convinced.

I'm picturing a setup with a suspended sheave and a rope fixed at ground level to one side and pulled from other side at ground level. Sheave suspended from a weight scale (unit of pounds is force). Change distance between fixed and pull points with sheave in horizontal center to correlate with bend angle. If same tension is applied to free end of rope at various distances, does the scale suspending the sheave register the same force or does it vary?

 

[Carultch]

Compare with standard definition of pressure. Not sure how sidewall pressure definition can deviate from the standard???

In otherwords, sidewall pressure is not necessarily the same quantity as what you use PSI to measure.

I think I've figured it out. The sidewall pressure is probably ignoring the division by the dimension in the wire's diametral axis, and calculating force per length instead of force per area. Calculating the true contact mechanics pressure, is more than is needed in this application.

The change in pull tension for change in bend angle may be the part I was overlooking.... just not quite yet convinced.

OK. I get it. The calculation from pulling tension difference, as it translates to sidewall pressure, is what we are saying is not a function of bend angle. Yet, the pulling tension difference, is a function of bend angle.

I'm picturing a setup with a suspended sheave and a rope fixed at ground level to one side and pulled from other side at ground level. Sheave suspended from a weight scale (unit of pounds is force). Change distance between fixed and pull points with sheave in horizontal center to correlate with bend angle. If same tension is applied to free end of rope at various distances, does the scale suspending the sheave register the same force or does it vary?

Bends are a ratio of tension in and tension out. Straight sections are an increment of tension onto the cable.
The ratio of tension for a bend depends on the angle and the wire/conduit friction properties. Straight section tension depends on cable unit weight, length, friction, and verticalness. So given the option, it is strategic the pulling end of the feeder close to the extreme bends, and at low elevation.

 

[Julius Right]

According to IEEE Std 525-1992 IEEE Guide for the Design and Installation of Cable Systems in Substations ch.10.3.1.3 Maximum allowable sidewall pressure :
“The maximum allowable sidewall pressure is 500 lb per ft of radius for multiconductor power and control cables and single-conductor power cables #6 AWG and larger, subject to verification by the cable manufacturer.”
The following formula explains that :
P=To/r for single cable
P=To*(3*c-2)/3/r for three cables in cradle configuration.
where:
P=sidewall pressure [lb/ft]
To= the tension out of the bend, in pounds.
c=the weight correction factor (refer to 10.3.2.1).
r=the inside radius of bend, in feet
However if r is bigger a larger tension will produce the maximum permissible sidewall pressure.
By-the-way, To= depends -of course- upon the angle of all bends "out of the [considered] bend".

 

[GoldDigger]

Just a quick clarifying note from a physicist:
The sidewall tension has no direct relationship to the coefficient of friction between cable and wall, nor to the difference between Tension-in and Tension-out of the bend.
Increased friction will cause the pulling tension, To, to be higher everywhere in the length being pulled.
For a single wire or cable the sidewall force is simple geometry, it is the force required to change the direction of the pulling force as the raceway curves.
For three cables there is a wedging effect that causes the pressure against the sides of the raceway to be greater than the force against the inside of the curve. That is also just geometry, although not quite as simple.