How to Bend Back-to-Back 90s in Conduit

What a back-to-back 90 is

A back-to-back 90 is a U bent into one length of conduit: two 90° bends that turn the same way and leave two legs running parallel. The number that matters is the distance between the backs of the bends, the outside edges where the conduit turns. Get that right and both legs land where the job needs them.

You need one when a single stick has to:

• rise out of a slab or drop from a ceiling, run a short way, and turn the same way again to reach a second box or fitting;
• wrap over the end of a wall, a beam or a duct and come down the other side;
• tie two stubs together with a run that arrives square to both.

What you need before you mark

• A tape, a pencil or marker, a level, and a bender head made for your conduit type and trade size.
• The distance between the backs: the outside of one leg to the outside of the other. If the job gives you center to center, add one conduit OD to get it.
• The star on the head, often cast “BACK OF 90”, or the head’s take-up if you will mark for the arrow.

Method 1: bend the second 90 on the star

1. Bend the first 90 as an ordinary stub-up: stub length minus the take-up, mark on the arrow, hook toward the stub end. Check it with a level.
2. From the back of that first bend, measure the distance between the backs along the straight and mark the conduit.
3. Seat the conduit in the head with that mark on the star and the hook facing the free end, away from the first bend.
4. Make sure the first leg points the same way the second will, and that the conduit has not rolled, so both bends are in one plane.
5. Pull the second 90 until its leg is plumb or parallel to the first leg, then let off and check again.

Method 2: no star, use the arrow

Treat the first bend as the “end” of a new stub. Take the bender’s take-up off the distance between the backs, mark that far from the back of the first 90, put the mark on the arrow, and face the hook toward the first bend. Some manufacturers’ guides recommend this for tight spacings.

A worked example

Both marks make the same bend. If you would rather not do the sums, BendMate works out the mark for each 90 from your own bender’s take-up.

Checking it and what goes wrong

Lay the finished conduit flat on the floor. Both legs should point the same way, stand parallel, and sit square to the straight between them. Measure back to back with the tape and compare with the job.

• An S instead of a U: the conduit was rolled half a turn, or the hook faced the wrong way, so the second bend went the other way.
• Legs too close by the take-up: you used the star and also subtracted the take-up. Use one method, not both.
• Legs out of line with each other: the conduit twisted in the head. Draw a line along the top and keep it in the same plane for both bends.
• Measured from the end of the first leg, not from the back of the bend. The tape belongs on the outside of the first bend.