Parallel Bends
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 13/4" 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
11/2", 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 much 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 aligned, 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', to determine the amount to shift one conduit so that the bends will align
center
to center.
Given: The angle formed by line 'ab' = 15º.
The length of line 'a' = 2". Line 'a' is the center to center distance of the
two conduits. It is also the center to center distance of the bends before one conduit is
shifted.
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/a,
or c = Tan(Ø) x a. Tan(15) = .27, and a = 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 Trig Tables if you need to
look up the tangent of other angles.
