Concentric bends are a set of conduit bends that have a common center point, but have different centerline radii.
Figure 1 shows three concentric bends produced from 2″ rigid conduit. Conduit 1 has a centerline radius of 20″. Conduit 2 has a centerline radius of 24 3/8″. Conduit 3 has a centerline radius of 28 3/4″.
The outside diameter of 2″ rigid conduit is approximately 2 3/8″. The conduits in this example have a 2″ separation between conduits.
The bends in this example are produced by making 15 evenly spaced bends of 6º each, to produce a 90º bend.
Figure 2 is a close-up of one of the 15 x 6º segment bends made to complete one of the three concentric bends.
Line ‘c’ represents one of the 15 segments used to bend Conduit 1.
The formula used to find the length of line ‘c’ is:
c = centerline radius of the bend x (2 x (tanØ / 2)), or 2 x (tanØ / 2) can be simplified to tanØ. Therefore,
c = centerline radius of bend x tanØ.
The centerline radius of Conduit 1 is 20″. Angle Ø is 6º. Then: c = 20 x tan(6º) Tan(6º) = .105 c = 20 x .105. c = 2.10″ = 2 1/8″.
The distance between bends for Conduit 1 = 2 1/8″.
Substitute the radii of Conduit 2 and Conduit 3 in the formula to find the center to center distance between bends for these conduits.
For Conduit 2: c = 24.375 x tan(6º). c = 24.375 x .105. c = 2.56″ = 2 1/2″.
The distance between bends for Conduit 2 = 2 1/2″.
For Conduit 3: c = 28.75 x tan(6º). c = 28.75 x .105. c = 3.02″ = 3″.
The distance between bends for Conduit 3 = 3″.
The length of conduit required to produce the bend is called the developed length.The developed length(DL) is equal to the centerline radius of the bend times the angle of the bend x 0.0175.
For Conduit 1: DL = 20 x 90 x 0.0175. dl = 31.5″.
For Conduit 2: DL = 24.375 x 90 x 0.0175. dl = 38.39″.
For Conduit 3: DL = 28.75 x 90 x 0.0175. dl = 45.28″.
Segment bends can be made from any number of bends, or segments. The closer together the bends are made, the smoother the total bend will look.
Notice, that as a matter of practicability, the distance between bends must be greater than the length of each bent segment. If the distance between bends is not greater than the length of each bent segment, select a lesser number of bends to make, and/or a different centerline radius.
Segment Bends contains more information about this type of bend, and also has a segment bending calculator for calculating the layout and the developed length of segment bends.