Sample method to bend 3-1/2″ rigid conduit around a storage tank with a diameter of 160 feet.
Segment bending is a method of bending conduit by making several small bends to produce one larger bend. The method on this page illustrates a technique for bending a 3-1/2″ rigid conduit around a tank with a 160′ diameter.
The steps for calculating a large radius segment bend are:
- Determine the radius of the desired bend.
The following procedure illustrates the procedure to bend a length of 3-1/2″ rigid conduit around a storage tank with an 80′ radius, or a 160′ diameter.
First, find the circumference of the tank.
Diameter in inches, with 2 strut, 1 on each side, plus 1 conduit diameter, or 1/2 for each side, = 160 x 12 + 1-5/8 x 2 + 2 x 2 = 1925.625″
Circumference = Pi x Diameter = 3.14159265 x 1925.625 = 6,049.53″. This is the total length of conduit needed to complete the bending angle of 360 degrees to go all the way around the tank. A 10′ length of 3-1/2″ rigid conduit without the coupling is 9′ 10-1/4″ = 118-1/4″. The number of conduit required to go all the way around the tank, or 360 degrees, is 6,049.53 / 118.25 = 51.16.
Divide the total angle to go around the tank, 360 by the number of conduits required, 51.16, to get the angle required to bend each length of conduit. 360 / 51.16 = 7.04 degrees for each length of conduit.
- Determine the number of segments desired or required for the completed bend.If a length of conduit is going to be bent to such a small angle, it is practical to only make 2 or 3 bends on each length of conduit.
- Determine the Developed Length of the bend.Developed length (DL) is the length of conduit that is actually bent. The formula to calculate the developed length of a bend is: Developed length (DL) is equal to the centerline radius (R) of the bend times the angle (A) of the bend times 0.01745. DL = R x A x 0.01745The radius of the tank plus the strut and conduit is 962.8125″ The developed length for a 7.04 degree bend with a 962.8125″ radius is 7.04 x 962.8125 x 0.01745 = 118.28″.
DL = 7.04 x 962.8125 x 0.01745 = 118.28″. You can see that this is one length of 3-1/2 rigid conduit.
If you want to make the bend with 1 bend, you can make on 7 degree bend in the center of the conduit.
More realistically, partly for purposes of strapping the conduit, and NEC requirements for distance from any junction boxes to straps, I would make 2 bends of about 3.5 degrees at about 30″ and 90″ on the length of conduit, at approximately the 1/3 and 2/3 distances on the conduit.
You could select a larger number of bends, but I think it would be more difficult to bend to a smaller angle, although the bends will look smoother.