Conduit Bend Calculation

[FaradayFF]

Greetings,

I need to size a conduit bend for a 3 phase Medium Voltage cable circuit. My procedure would be to calculate the overall diameter of the cable bundle(triangular configuration) and then multiply by applicable factor as mandated by NEC. The radius of conduit bend would be equal or larger than my calculated number in the previous step. Am I on the right rack with this?

Thanks,
EE

 

[topgone]

IMO, the radius of steel elbows, bends, and 90-degree sweeps are fixed depending on the size of the conduit, if you buy it off-the-shelf. Please refer to RSC/GRC manufacturers' catalogues.

 

[suemarkp]

Look at Table 2 in chapter 9 of the NEC. There is a bend radius table right there with 2 columns -- one for one shot benders or full shoe, and one for other types. Table goes from 1/2" conduit to 6".

 

[infinity]

Greetings,

I need to size a conduit bend for a 3 phase Medium Voltage cable circuit. My procedure would be to calculate the overall diameter of the cable bundle(triangular configuration) and then multiply by applicable factor as mandated by NEC. The radius of conduit bend would be equal or larger than my calculated number in the previous step. Am I on the right rack with this?

Thanks,
EE

That sounds like it would work. If you have a calculation post it so we can see what you came up with.

 

[Julius Right]

One has to check sidewall bearing pressure since it depends on the bend radius in the cable pulling process.

From: General Cable Cable Installation Manual for Power and Control Cables.

3.9.2 Allowable Sidewall Bearing Pressure

Sidewall bearing pressure (SWBP), or sidewall loading, is the radial force exerted on a

cable being pulled around a conduit bend or sheave. Excessive SWBP can crush a cable

and is, therefore, one of the most restrictive factors in installations having bends

The maximum tension that can safely be applied to the cable during installation can be calculated

using the maximum SWBP for the cable and the radius of the bend it is traversing.

For example, a cable having a maximum SWBP of 300 lb/ft that is being pulled around

a bend having a radius of 2 ft should have no more than 300 lb/ft x 2 ft or 600 lb tension

applied to it as the cable exits the bend.

Sidewall bearing pressure (SWBP) may be calculated using the following equations:

1/C in a raceway: SWBP = T/R

3-1/C cradled: SWBP = [3wc–2 ]/3 T/R

3-1/C triangular: SWBP = [wc]/2 T/R

Where: T = tension (lb) out of the bend

R = radius (ft) of the bend

wc = weight correction factor

See:

Table 3.9.2 Sidewall Bearing Pressure (Sidewall Loading)

 

[don_resqcapt19]

Often the standard factory elbows or the minimum radii shown in Chapter 9 are not large enough for some cables, or those radii need to be increased because of the side wall pressure mentioned in the previous comment. Larger radius manufactured elbows are available, or you can bend your own using multiple 5° bends.