Concentric Conduit Bend Radii and Lengths
The radius, arc length, mark spacing and extra stock for each conduit in a rack bent concentrically around one corner.
Example
You enter
- Inside conduit centerline radius (in) 12
- Conduit outside diameter (in) 1.163
- Gap between conduits (in) 0.5
- Bend angle (deg, up to 180) 90
- Conduits in the rack (2 to 12) 4
- Shots per conduit (2 to 60) 9
You get
- Pitch (in) 1.663
- Rack width (in) 6.152
- Max extra (in) 7.8367
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
ScopeEvery conduit shares one center, so each radius is the inside one plus one pitch (outside diameter plus gap) per position, and the outer conduits need more arc. Four 1 in EMT (1.163 in OD) at a 0.5 in gap on a 12 in inside radius through a 90 sit on 12.000, 13.663, 15.326 and 16.989 in radii with 18.85, 21.46, 24.07 and 26.69 in of arc: the outer conduit needs 7.84 in more stock than the inner one, and the rack is 6.15 in wide. Bend each with the same number of shots and space each conduit's marks by its own arc / shots (2.09, 2.38, 2.67 and 2.97 in at nine shots) so the shots line up on radial lines and the rack stays evenly spaced; it also flags a rack whose straight runs would touch between shots. The Conduit Segment Bend Layout calculator lays out any one of them and checks the inside radius against NEC Chapter 9 Table 2; bender take-up and springback are not included.
Formula or tablepitch = OD + gap; R_i = R_1 + (i - 1) x pitch; DL_i = R_i x theta x pi / 180; mark spacing_i = DL_i / n; extra_i = DL_i - DL_1; rack width = (conduits - 1) x pitch + OD.
Source and editionCircular-arc geometry by name; the outside diameter is the reader's entry from the raceway listing or NFPA 70 (NEC) 2023 Chapter 9 Table 4, 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 namesinside_radius_in, od_in, gap_in, bend_angle_deg, conduits, shots, pitch_in, rack_width_in, max_extra_in
- One center, constant pitch every conduit the same OD and gap; a rack that changes pitch is not handledstated assumption
- Centerline layout bender take-up and springback are not includedstated assumption