By Blissom · Updated 2026-11-04
Layout values: check them against your own bender, on scrap first.
Formulas you need
R₁ is the inside conduit. Radii and spacing are to conduit centerlines, the same as the concentric bend chart.
Radius of each conduit
Rₙ = inside radius + (n − 1) × center spacing, all measured to conduit centerlines. Center spacing is center to center: the clear gap plus one OD when the conduits are the same size.
A concentric rack has a 15" inside radius on 3" centers. What is the radius of the fourth conduit?
- a. 27"
- b. 24"
- c. 18"
- d. 21"
Answer: b. 24".
- R₄ = 15 + (4 − 1) × 3 = 15 + 9 = 24".
- 27" multiplies by 4 instead of 3: the inside conduit is conduit 1 and adds no spacing.
Five 3/4" EMT (OD 0.922") run with a 1" clear gap between them and turn concentrically. The inside conduit has a 10" radius. What is the radius of the fifth conduit?
- a. 14"
- b. 17-11/16"
- c. 19-5/8"
- d. 13-11/16"
Answer: b. 17-11/16".
- Center spacing = gap + OD = 1 + 0.922 = 1.922".
- R₅ = 10 + 4 × 1.922 = 17.688" = 17-11/16".
- 14" uses the gap as the spacing, which pinches the outer conduits together through the turn.
Six conduits turn concentrically. The inside conduit has an 18" radius and the outside one a 33" radius. What is the center spacing?
Answer: 3".
- The outside conduit is five spacings out: 33 − 18 = 15".
- 15 ÷ 5 = 3". Dividing by 6 (the number of conduits) gives 2-1/2", which is wrong.
1-1/4" EMT (OD 1.51") is racked with 1/4" between conduits. What center spacing do you use for the concentric radii?
Answer: 1-3/4".
- 1.51 + 0.25 = 1.760" = 1-3/4".
- Using the trade size (1-1/4") instead of the actual OD understates the spacing.
Developed length of each conduit
Each conduit uses 1.571 × its own radius in the 90. Every step out adds 1.571 × the center spacing.
Three conduits turn concentrically on a 12" inside radius with 2-1/2" centers. What is the developed length of each 90?
Answer: 18-7/8", 22-3/4" and 26-11/16".
- Radii: 12", 14-1/2", 17".
- 1.571 × 12 = 18.850"; 1.571 × 14.5 = 22.777"; 1.571 × 17 = 26.704".
In a concentric rack on 4" centers, how much more conduit is in the curve of each conduit than in its inside neighbor?
- a. 4"
- b. 6-5/16"
- c. 12-9/16"
- d. 3-1/8"
Answer: b. 6-5/16".
- 1.571 × 4 = 6.283" = 6-5/16".
- 12-9/16" is π × 4, the step for a 180°, not a 90.
Four conduits turn concentrically on a 20" inside radius with 3" centers. How much conduit is in the curve of the outside conduit?
Answer: 45-9/16".
- R₄ = 20 + 3 × 3 = 29".
- 1.571 × 29 = 45.553" = 45-9/16".
Where each bend starts and ends
In a concentric 90 every conduit starts its curve on one line square to the rack, and every curve ends on one line square to the new direction. The outer conduits are only longer in the curve.
Four conduits with their ends cut flush turn concentrically on 2" centers. The inside conduit's start-of-curve mark is 40" from its end. Where is the start-of-curve mark on the third conduit?
- a. 40"
- b. 44"
- c. 46-5/16"
- d. 36"
Answer: a. 40".
- All the curves start on the same line square to the rack, so with flush ends every start mark is at the same distance.
- The outer conduits end their curves later, not start them later.
Same rack, inside radius 16" on 2" centers. Where does the third conduit's curve end, measured from its end?
Answer: 71-7/16".
- R₃ = 16 + 2 × 2 = 20". Developed length 1.571 × 20 = 31.416".
- 40 + 31.416 = 71-7/16". The inside conduit ends its curve at 65-1/8".
Each conduit in that rack has 40" of straight before the curve and 30" after it. What are the cut lengths of the inside and the third conduit?
Answer: Inside 95-1/8", third 101-7/16".
- Inside: 40 + 25.133 + 30 = 95-1/8".
- Third: 40 + 31.416 + 30 = 101-7/16".
- The difference, 6-5/16", is 1.571 × the 4" between their radii.
In a concentric 90 on 3" centers with a 12" inside radius, compared with the inside conduit, the corner point of the third conduit (where its straight legs would meet) is:
- a. at the same spot
- b. 6" farther out along each leg
- c. 3" farther out along each leg
- d. 9-7/16" farther out along each leg
Answer: b. 6" farther out along each leg.
- Each corner point sits one radius past the start of its curve along both legs. R₃ − R₁ = 18 − 12 = 6", so the third corner point is 6" farther out along each leg.
- Check with gain: both legs are 6" longer (12" more), but the gain also grows by 2-9/16" (5-1/8" to 7-3/4"), so the cut length grows by 9-7/16", the same as 1.571 × 6".
Segmented concentric bends
Every conduit gets the same number of shots and the same degrees per shot; only the spacing changes, because each conduit has its own developed length.
A rack on an 18" inside radius with 3" centers is bent in 10 shots per 90. What is the shot spacing on the inside conduit and on the third conduit?
Answer: Inside 2-13/16", third 3-3/4".
- Inside: 1.571 × 18 = 28.274" ÷ 10 = 2-13/16".
- Third: R = 24", 1.571 × 24 = 37.699" ÷ 10 = 3-3/4".
- Both are bent 9° per shot.
On a 20" inside radius with 2-1/2" centers, the third conduit is bent in 12 shots. How far past the start-of-curve line is its first mark?
Answer: 1-5/8".
- R₃ = 25". Developed length 39.270". Spacing ÷ 12 = 3.272".
- First mark half a spacing past the start line: 1.636" = 1-5/8".
Five conduits are bent concentrically with 9 shots on each. How many degrees is each shot on the outside conduit?
- a. 10°
- b. 18°
- c. 2°
- d. It depends on the radius
Answer: a. 10°.
- 90 ÷ 9 = 10° on every conduit. The radius changes the spacing between shots, not the degrees.
A rack of four conduits turns a concentric 90, each conduit in 9 shots. How much does the turn count toward the 360° limit for each conduit?
- a. 90°
- b. 360°
- c. 9 bends
- d. 810°
Answer: a. 90°.
- The limit applies to each raceway between its own pull points. Each conduit turns 90°, so each counts 90°.
Common traps
- Counting the inside conduit as a step. Conduit 4 is three spacings out, not four: R₄ = R₁ + 3 × spacing.
- Gap instead of center spacing. A 1" clear gap between 3/4" EMT is a 1.922" center spacing. Using the gap pinches the rack in the turn.
- Trade size instead of OD. 1-1/4" EMT is 1.51" across, not 1.25".
- Staggering the start marks. All curves start on the same line; the outer ones end later.
- Using the inside spacing on every conduit. Same degrees per shot, different spacing for each conduit.
- Cutting every conduit the same length. Each step out needs 1.571 × the spacing more conduit in the curve.
- Ignoring the minimum radius. These are layout numbers. The inside radius still has to meet the bender maker's instructions and the NEC minimum for the raceway and size.
The shot spacing on every conduit follows the same rule as a single sweep; if it is unclear, start with the segmented bend problems.
Sources: NEC 2023: Chapter 9 Tables 1, 4 and 5 (fill), 314.16 (box fill), 358.24 and Chapter 9 Table 2 (bends); Answers checked against the calculation engine used by the Conduit Bending Calculator app. Method.
Questions electricians ask
Is the radius of a concentric bend measured to the centerline?
Yes. The radius of each conduit is to its centerline, and the spacing between conduits is center to center. If you know the clear gap, add one outside diameter to get the center spacing.
Do the outer conduits start their bends later?
No. In a concentric 90 every curve starts on one line square to the rack and ends on one line square to the new direction. The outer conduits only have longer curves, so they need more conduit.
How much longer is each outer conduit?
1.571 × the center spacing for a 90. On 2" centers each conduit uses about 3-1/8" more than its inside neighbor; on 3" centers about 4-11/16".
Do all the conduits in the rack get the same number of shots?
Usually yes, so every shot on every conduit is the same number of degrees. Only the spacing changes, because each conduit has its own developed length.
Every conduit on the rack, checked exactly
- Concentric 90s: radius, developed length and gain per conduit
- Shot spacing and the first mark for each conduit
- Steps written out for every bend type on the exam
- Works offline, plus NEC conduit fill and box fill
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