Concentric Bend Calculator

Related tools: Segment Bend Calculator, Saddle Bend Calculator, and Offset Bend Calculator.

A concentric bend keeps parallel conduits equally spaced as they change direction through the same total bend angle. The inside conduit follows the smallest centerline radius; each conduit farther out requires a larger radius so the raceways remain parallel through the bend.

The Concentric Bend Calculator produces the layout dimensions needed to mark and bend one conduit within a parallel conduit group:

• Target centerline radius
• Radius offset from inside conduit
• Developed arc length
• Angle per segment
• Mark spacing along conduit
• Straight chord between marks

These values support segmented conduit bending when a full-radius bend is not made in a single operation. The calculated mark spacing establishes where equal bends are placed along the conduit, while the angle per segment establishes the bend increment at each mark.

Conduit Centerline Radius and Spacing

The calculator starts with the inside conduit and moves outward by the entered centerline spacing.

Inside centerline radius (in) is the centerline radius of the innermost conduit. It is not the inside surface radius of the conduit and not the radius measured from the outer edge of the bend.

Spacing between conduit centerlines (in) is the distance from the centerline of one conduit to the centerline of the next parallel conduit. This spacing must match the intended finished conduit arrangement, including any clearance required for fittings, supports, adjacent raceways, or installation constraints.

Conduit number from inside identifies which parallel conduit is being laid out:

• 1 represents the inside conduit.
• 2 represents the next conduit outward.
• 3 represents the third conduit outward.

The calculator determines the added radius as:

\(\displaystyle \text{Radius offset from inside conduit} = (\text{Conduit number from inside} - 1) \times \text{Spacing between conduit centerlines}\)

Then it calculates the conduit’s required centerline radius:

\(\displaystyle \text{Target centerline radius} = \text{Inside centerline radius} + \text{Radius offset from inside conduit}\)

A conduit farther from the center of the bend travels a longer path. Giving every conduit the same bend marks or the same developed length will cause the group to fan out, crowd together, or lose alignment at the end of the turn.

Segmented Bend Marks

Total bend angle (deg) is the complete directional change required through the bend, such as 90 degrees.

Segment count is the number of equal bend segments used to form that total angle. The calculator divides the total angle evenly:

\(\displaystyle \text{Angle per segment} = \frac{\text{Total bend angle}}{\text{Segment count}}\)

For a segmented 90-degree concentric bend with six segments, each bend is 15 degrees. The number of segments affects both the bend increment and the distance between marks:

• More segments create smaller individual bend angles and closer marks.
• Fewer segments create larger individual bend angles and wider marks.
• The selected segment count must be practical for the conduit type, bender, available straight length, and the finished bend quality required.

The calculator provides two mark-spacing dimensions because field layout may be referenced either along the conduit or as a straight-line distance between marks.

\(\displaystyle \text{Developed arc length} = R \times \theta\)

where R is the Target centerline radius and \theta is the Total bend angle converted to radians.

\(\displaystyle \text{Mark spacing along conduit} = \frac{\text{Developed arc length}}{\text{Segment count}}\)

\(\displaystyle \text{Straight chord between marks} = 2R \sin\left(\frac{\theta}{2 \times \text{Segment count}}\right)\)

The developed mark spacing follows the curved centerline path. The chord value is the straight-line distance between adjacent points on the arc. For field marking on straight conduit before bending, use the calculator’s stated layout method consistently rather than mixing arc distances and chord distances.

Calculation Example

Use the following entered values:

Input	Value
Inside centerline radius (in)	10
Spacing between conduit centerlines (in)	2
Conduit number from inside	2
Total bend angle (deg)	90
Segment count	6

The second conduit is one spacing outward from the inside conduit:

\(\displaystyle (2 - 1) \times 2 = 2 \text{ in}\)

\(\displaystyle 10 \text{ in} + 2 \text{ in} = 12 \text{ in}\)

The calculated results are:

Result	Value
Target centerline radius	12 in
Radius offset from inside conduit	2 in
Developed arc length	18.8496 in
Angle per segment	15 deg
Mark spacing along conduit	3.1416 in
Straight chord between marks	3.1326 in

For this layout, make six equal 15-degree bend segments to produce the 90-degree change in direction. The second conduit uses a 12-inch centerline radius and requires 3.1416 inches of developed spacing per segment. The inside conduit would use the entered 10-inch centerline radius; each additional conduit outward increases its target radius by the entered 2-inch centerline spacing.

Field Layout Verification

This calculation is a concentric bend geometry estimate. It establishes centerline radii, equal bend angles, and theoretical mark spacing; it does not establish the final bender setup or verify the finished raceway installation.

Confirm the following before bending:

• Conduit size and actual centerline spacing.
• Bender setup, shoe geometry, and bend sequence.
• Springback for the conduit material and bending method.
• Available straight length before the first bend and after the final bend.
• Clearance between parallel conduits, fittings, supports, structural elements, and adjacent raceways.
• Alignment of the conduits at both ends of the finished bend.

A segmented bend can accumulate small errors at every mark. Maintain the same reference point, bend orientation, and bend angle for each conduit. Check the first completed conduit against the intended centerline path before repeating the layout across the remaining parallel conduits.