Compound 90 Calculator

A 90 degree turn made in two bends instead of one, so the conduit cuts across the corner on a diagonal and misses something sitting in it: a beam where the ceiling meets the wall, a duct in the corner, a pipe along the floor-wall junction. The two bends add up to 90, most often 45 and 45; 30 then 60 or 60 then 30 skew the diagonal when the obstruction is much wider than it is tall or the other way around.

The calculator measures both from the corner where the two legs' centerlines would meet. It places the chord between the bends so the pipe wall clears the nearest point of the obstruction by the clearance you enter, then reports how far back along the first leg the first bend sits and how far along the second leg the second one does. For a 4 inch by 4 inch beam with 1/2 inch clearance in 3/4 inch EMT at 45 and 45, the first bend is 8.44 inches back from that corner, the second 8.44 inches along the wall, and the marks are 11.93 inches apart.

It sits in the inside corner against both surfaces the conduit runs on: the first surface is the one the conduit arrives on, the second the one it leaves on. A rectangle is its size along the first leg (how far it sticks out from the second surface) by its size along the second leg (how far it sticks out from the first surface); a square has one side; a round obstruction is tucked into the corner touching both surfaces. The conduit lies against both surfaces too, half an OD off each on its centerline, which is what puts the obstruction in a one-shot 90's path.

To the pipe wall, from the nearest point of the obstruction along the chord. The calculator lays the bends out so the chord's centerline clears by the clearance plus half the OD, then measures the finished bend, with each bend an arc at the NEC Chapter 9, Table 2 radius for the size, and prints the clearance it actually draws. That figure is never less than the clearance entered; where the arcs pull the pipe farther from the obstruction it is more.

The chord between the two bends must keep straight conduit between their arcs, and each arc takes up conduit on both sides of its mark at the shoe's radius (4 inches for 1/2 inch EMT). A small obstruction with little clearance puts the two bends so close that the arcs overlap, and the calculator refuses, naming the smallest clearance that works at that split and any other split that answers. It also prints what a one-shot 90 in that size would clear the obstruction by: when that figure is positive, the plain 90 already misses it and no compound bend is needed for clearance.

From geometry, stated on this page. None of the bender manuals this site holds (Greenlee, Gardner Bender, Klein) describes a compound 90, so there is no table to cite and no rounding to inherit: the chord is placed by the clearance, the bends are placed by the chord, and the finished bend is measured with each arc at the NEC Chapter 9 Table 2 radius.