Compound 90° around a round obstruction

A compound 90 turns a corner with two 45° bends instead of one 90, cutting across the corner in front of something that sits in it. This screen is for a round obstruction, such as a pipe or tank touching both walls.

What to measure

Stub length: from the end of the conduit to the back of the other leg, as for a 90.
Obstruction diameter: across the round obstruction, which is taken to sit in the corner touching both walls.
Other leg (optional): from the other end to the back of the stub. With it you get the Cut length.
First bend or Between bends (optional): when the first bend has to go somewhere in particular, or the chamfer has to be a set length, type it in and the other follows. Clearance then shows whether the conduit still clears the obstruction; leave the obstruction blank if there isn’t one.

How to bend it

Mark the First mark from the stub end, and the second mark Between bends further along.
Put the first mark on the arrow with the hook facing the stub end, and bend 45°.
Slide to the second mark and bend another 45° the same way, with the hook still facing the stub end, so the two make 90°.
Check that the stub and leg are square to each other and both bends are in one plane.

Where the numbers come from

The two 45s are joined by a straight chamfer that passes just outside the obstruction. For a circle touching both walls, the chamfer’s length from corner to corner (where the straight lines would meet) is 2.4 × the diameter. The rest is the same radius math as any other bend.

chamfer = 2.4 × diameter

Corner to corner of the two 45s. The exact factor is 2.414.

between bends = chamfer − g(45°)

first mark = stub − OD/2 − chamfer ÷ √2 − setback(45°) − start of bend

For the arrow. Chamfer ÷ √2 is how far each 45° corner sits back along its leg from where a plain 90’s corner would be.

A 12" round obstruction with a 36" stub and leg, on a 1/2" EMT hand bender

Stub length: 36"
Obstruction diameter: 12"
Other leg: 36"
First mark: 13 1/16" (15 5/8" by multiplier)
Between bends: 28 5/8" (28 13/16" by multiplier)
Cut length: 59"

The chamfer doesn’t allow for the conduit’s own size, but because the stub, leg and obstruction are all measured to the walls, the conduit still clears by about a fifth of its OD. The 2.4 is rounded down from 2.414, which eats into that: with 1/2" EMT the gap is about 1/16" on a 12" obstruction and gone on a 24" one. Clearance shows the gap, and the bend turns orange if the conduit would touch. Adding about 1% to the diameter you enter opens it up again.

If it turns orange or red

Red means the stub or the leg is too short for the chamfer, so a bend would run off the end of the conduit. Orange or red can also mean the two 45s are too close together for the shoe, which only happens with a very small obstruction. Orange also shows when the conduit would touch the obstruction (a negative Clearance), or when the cut length is longer than one stick.

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