How to Bend Concentric 90s for a Rack of Conduit

What concentric bends are

When a rack of conduits turns a corner, there are two ways to keep it tidy. Both leave the legs parallel and evenly spaced. Only one is truly concentric.

• Staggered 90s: every conduit is bent on the same shoe. The corners step along like stairs, and each conduit outside the first has a longer stub and leg by the center-to-center spacing.
• Concentric 90s: every conduit has its own radius, larger than the one inside it, so all the bends share one center. The conduits follow each other round the corner like lanes on a track.

Concentric bends are what you see on an exposed rack, on strut or a trapeze, where the sweeps are meant to be looked at. A hand bender’s shoe only has one radius, so they are usually built from segmented bends, which are several small bends spaced along each sweep.

The rules that make them concentric

The arcs start along one straight line across the rack, and the leg ends line up. The inner conduit has the shortest sweep and the outer conduit the longest, which is why the outer conduit is always cut longest.

Marking a segmented concentric 90

1. Choose the inner radius and measure the center-to-center spacing between neighboring conduits.
2. Choose the number of shots, the same for every conduit. More shots make a smoother sweep. Each shot is 90° ÷ shots.
3. Work out each conduit’s radius, sweep length and spacing between shots with the formulas above.
4. Lay out each conduit from its own end. Start every one from the line where the arcs begin, using the same layout you would for a single segmented 90.
5. Bend each shot to the same angle, in the same plane, and sight along the conduit as you go.
6. Lay each finished conduit beside the one before it. A drift in angle shows long before the last conduit.

A worked example

The sweep grows by the same amount for every conduit, 3" × π ÷ 2, which is about 4 11/16". That is why the outer runs of a rack of sweeps take extra conduit.

The quick version: staggered 90s

If the corner will be hidden, or you only need the legs to line up, bend the same-size conduits on the same shoe and stagger them. Every conduit has the same take-up and gain, so each next conduit’s stub and leg are longer by the spacing, and its cut length by twice that.

Mistakes to avoid

• Putting the conduits in the wrong order. The one on the outside of the turn needs the biggest radius.
• Using different numbers of shots on different conduits, which makes the sweeps drift apart.
• Using the gap between conduits instead of center to center. The radius grows by the center distance.
• Mixing sizes without allowing for them. For a different size, add half the difference in OD to the center distance.