Conduit Bend Radius and Developed Length

The standard one-shot centerline bend radius for 3/4 inch EMT is 4.5 inches, the value tabulated in NEC Chapter 9, Table 2 that a hand bender is built to approximate. That radius is what the take-up and mark spacing come from (the field gain adds one conduit diameter on top of it). By EMT size the standard centerline radius is 4 inches for 1/2 inch, 4.5 inches for 3/4 inch, 5.75 inches for 1 inch, and 7.25 inches for 1-1/4 inch. An individual bender shoe can run slightly larger, so verify against your own bender with a test bend.

The developed length of a single bend is the length of conduit that travels through the curve: developed length = centerline radius x bend angle in radians, which is the same as (angle / 360) x 2 x pi x radius. For a 15 degree bend at a 25 inch centerline radius, developed length = 25 x (15 x pi / 180) = 6.54 inches. On a standard 3/4 inch EMT bender (4.5 inch radius) a 90 degree bend has a developed length of 7.07 inches.

They come from the same place. NEC Chapter 9, Table 2 lists the centerline radius of a one-shot or full-shoe bend by conduit size (4.5 inches for 3/4 inch EMT), which is the radius a hand bender is built to make; the same table sets the radius floor so a bend never damages the conductors inside. So the standard hand-bend radius is the NEC Table 2 one-shot value, and the take-up and cut lengths are figured from it. A specific bender shoe can run slightly larger than the tabulated figure, so verify against your own bender with a test bend.

Yes. A larger conduit uses a larger bender, so the centerline radius grows with size: 4 inches for 1/2 inch EMT, 4.5 inches for 3/4 inch, 5.75 inches for 1 inch, and 7.25 inches for 1-1/4 inch. Because developed length is radius times angle, a bigger radius also means a longer developed length for the same angle, which is why larger conduit saves more material (more gain) on a 90.

For a 90 degree bend, developed length = radius x (pi / 2), a quarter of the full circle. On standard EMT benders that is 6.28 inches for 1/2 inch (4 inch radius), 7.07 inches for 3/4 inch (4.5 inch radius), 9.03 inches for 1 inch (5.75 inch radius), and 11.39 inches for 1-1/4 inch (7.25 inch radius). Developed length is the arc through the bend itself; to size a whole piece with 90s you subtract gain from the summed legs instead.

The developed length is 6.54 inches. Developed length is the conduit that runs through the curve, and it equals the centerline radius times the bend angle in radians: 25 x (15 x pi / 180) = 6.54 inches. Only the radius and angle the problem gives are used, so the same formula answers any radius, including a large-radius factory bend.

Developed length = centerline radius times (pi / 2). A 90 degree bend is a quarter of a full circle, so its arc is the radius times pi/2 (1.5708 radians). On a standard 3/4 inch EMT bender the centerline radius is 4.5 inches, so the developed length is 4.5 x 1.5708 = 7.07 inches. The developed length chart above gives the value for every EMT size.