Conduit Segment Bend Layout
How to lay out a large-radius conduit sweep as a segment bend: many small shots, equally spaced along the arc.
Example
You enter
- Centerline bend radius (in) 36
- Total bend angle (deg, up to 180) 90
- Number of shots (2 to 60) 9
- Minimum bending radius, Table 2 Other Bends or listing (in, 0 to skip) 20
You get
- Developed length of the arc 56.55 in
- Mark spacing (in) 6.28318
- First mark (in) 3.14159
- Effective radius (in) 35.9086
- Gain against the tangents 15.27 in
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeThe developed length is the centerline radius times the angle in radians; dividing it into n equal pieces and bending angle / n at the middle of each gives the marks, DL / n apart with the first one HALF an interval in, which is the detail most often got wrong. A 90 at a 36 in radius in nine shots has 56.55 in of arc, 10 degree shots, and marks 6.28 in apart starting 3.14 in in. Because the tube between shots keeps its length, the shots make an effective radius a little under the layout radius, 35.91 in here, and that is the radius it checks against the minimum the reader enters from the Other Bends column of NEC Chapter 9 Table 2 or the raceway's listing; it also gives the gain against running the two tangents, 15.27 in. The Conduit 90 Stub calculator's segmented mode counts shots from a per-shot angle for a 90; this lays out any angle up to 180 from a shot count. Bender take-up and springback are not included.
Formula or tableDL = R x theta x pi / 180; n shots of theta / n; marks DL / n apart, the first DL / (2 n) from the start of the arc; effective radius of the shot polygon r = (DL / n) / (2 tan(theta / (2 n))); gain = 2 r tan(theta / 2) - DL (theta < 180); radius check r >= R_min when R_min is entered.
Source and editionCircular-arc geometry by name; the minimum bending radius is the reader's entry from NFPA 70 (NEC) 2023 Chapter 9 Table 2 or the raceway's listing, not reproduced here.
Free accessFree read-only access at nfpa.org/freeaccess.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesradius_in, bend_angle_deg, shots, min_radius_in, developed_in, mark_spacing_in, first_mark_in, effective_radius_in, gain_in
- Centerline layout radius, arc and marks on the conduit centerline; bender take-up, shoe radius and springback are not includedstated assumption
- Equal shots every shot bends the same angle at the middle of its own intervalsegment-bending practice