Figure 1 displays a sample conduit run of two parallel offsets of 1/2″ EMT. Each offset has been bent using 30º bends. Conduit 2 is spaced 2″ center to center from Conduit 1. The red line ‘a’ connects the center of the bend of Conduit 1 with the center of the bend of Conduit 2, and represents the center to center distance between the two parallel conduits. Conduit 1 and Conduit 2 are displayed with their ends aligned.
Although this may look acceptable in this example, notice that Conduit 1 and Conduit 2 have only 1-3/4″ of separation at the center section of the offsets, as compared to the 2″ separation on the ends. This condition becomes worse as the degree of the bend used to make the offset is increased.
Figure 2 represents similar parallel offset bends, except that these offsets were produced using 45º bends. In this example, the amount of separation between the center sections of Conduit 1 and Conduit 2 has decreased to 1-1/2″, and the conduits are less acceptable visually. The foreshortening of the distance between conduits can also start to cause installation problems. With larger sizes of conduit, there may not be sufficient room to strap the conduit, and can even result in conduits that cannot be installed at the desired separation, because the conduits actually contact each other.
Figure 3 corrects this problem. Conduit 1 has been shifted so that the center of the bend of Conduit 1 is aligned with the center of the bend of Conduit 2. In this example, the center section of Conduit 2 is equally spaced from Conduit 1 by the same amount as the separation at the ends of the conduit.
How does the electrician determine how must each successive parallel conduit must be shifted in relation to the previous conduit? The method to determine the spacing is illustrated in figure 4. Line ‘a’ represents the original alignment between the center of the bends of Conduit 1 and Conduit 2, when the ends of the conduits were alligned, as in figure 1. Line ‘b’ represents the line connecting the center of the bends of Conduit 1 and Conduit 2 after Conduit 1 has been shifted, as in figure 3. Line ‘b’ is the perpendicular bisector of the 30º bends of Conduit 1 and Conduit 2. Since line ‘b’ bisects, or divides in two, the 30º bend, it is 15º in relation to line ‘a’. Line ‘c’ represents the difference in the centerline of the bends of Conduit 1 and Conduit 2, or the amount that Conduit 1 has to be shifted in relation to Conduit 2.
We wish to find the length of line ‘c’.
Given: The angle represented by line ab = 15º. The length of line ‘b’ = 2″. The tangent function is used to find the length of line ‘c’. The tangent of an angle is the opposite side divided by the adjacent side. Therefore, the Tan(Ø) = c/b, or c = Tan(Ø) x b. Tan(15) = .27, and b = 2, therefore, c = .27 x 2 = .54. The amount that Conduit 1 is shifted in relation to Conduit 2 is .54″.
Simplified: The distance to shift one conduit in relation to the other is the tangent of 1/2 of the angle used to bend the offset times the center to center distance between the conduits. In this example, for offsets bent using a 30º angle, the distance is equal to the tangent of 15º time 2″, or .27 x 2 = .54″.
If you make most of your bends using 30º angles, it is fairly easy to remember that the tangent of 15º is .27. For most conduit runs where the conduits are spaced fairly close together, you will find that using 1/4″ works fine. Its an easy number to remember, and will be accurate enough in most cases. Refer to the TrigTables if you need to look up the tangent of other angles.