Concentric Bend Formula & Spacing Chart for Electricians

A rack of parallel conduit looks great on a straight run. Then you hit a corner or an obstacle, and suddenly every pipe needs a different radius to keep the spacing uniform through the bend. That's a concentric bend. Get the math wrong and the conduit bunches together in the curve or fans apart on the far side. Either way, it looks sloppy and it's a pain to strap.

This guide covers the concentric bend formula, a ready-to-use spacing chart for EMT, IMC, and Rigid conduit, the segment bending method to actually execute these bends, and worked examples you can follow on the job. If you're already comfortable with single-conduit 90-degree bends and offsets, you have the foundation. Concentric bending just layers spacing math on top.

What Is a Concentric Bend?

A concentric bend is a bend in two or more parallel conduits where every pipe curves around the same center point but at progressively larger radii. The innermost conduit has the tightest radius, and each successive pipe adds enough radius to preserve the gap between them.

Think of it like lanes on a highway curve. The inside lane has a shorter, tighter arc. The outside lane sweeps wider. But the lane width stays the same throughout the turn. Same principle applies to a conduit rack making a 90, an offset, or any other bend.

When You Need Concentric Bends

• Exposed conduit racks in commercial and industrial buildings where appearance matters
• Panel feeds with multiple conduits entering the top or bottom of a panel
• Motor control centers with parallel conduit runs turning corners
• Any parallel run where a foreman or spec requires uniform spacing through bends

Not every job demands concentric bends. If the conduit is hidden above a ceiling or buried in a wall, standard bends with adjustable spacing are usually fine. But on exposed work, especially on union jobs or high-spec commercial projects, concentric bends are the expectation. As the EC&M guide on multiple conduit runs notes, maintaining uniform spacing through bends is a mark of professional craftsmanship.

The Concentric Bend Formula

The entire concentric bend calculation rests on two formulas. The first determines the spacing. The second determines the radius for each pipe.

Formula 1: Center-to-Center Spacing

C-C = OD + Gap

• OD = outside diameter of the conduit (see the spacing chart below)
• Gap = desired clear space between conduit faces (typically 1/2" for same-size conduit)

For same-size conduit, this is straightforward. For mixed sizes (say a 1" and a 3/4" running side by side), use the average of the two ODs: C-C = (OD1/2 + OD2/2) + Gap.

Formula 2: Radius for Each Conduit

R(n) = R(1) + (n - 1) x C-C

• R(1) = centerline bend radius of the innermost conduit
• n = conduit number, counting outward (1 = innermost, 2 = next, etc.)
• C-C = center-to-center spacing from Formula 1

The innermost radius must meet or exceed the minimum bend radius from NEC (NFPA 70) Chapter 9, Table 2. For hand benders, those minimums are built into the bender shoe. For segment bending with a hickey, you choose the radius, so you need to verify compliance yourself.

Concentric Bend Spacing Chart

The chart below gives the center-to-center spacing for same-size conduit with a standard 1/2" gap between faces. These values come from the actual outside diameters specified in manufacturer data and industry references.

Concentric bend spacing chart: center-to-center distance by conduit type and trade size (1/2" gap)

Trade Size	EMT OD	EMT C-C	IMC OD	IMC C-C	Rigid OD	Rigid C-C
1/2"	0.706"	1.206"	0.815"	1.315"	0.840"	1.340"
3/4"	0.922"	1.422"	1.029"	1.529"	1.050"	1.550"
1"	1.163"	1.663"	1.290"	1.790"	1.315"	1.815"
1-1/4"	1.510"	2.010"	1.638"	2.138"	1.660"	2.160"
1-1/2"	1.740"	2.240"	1.883"	2.383"	1.900"	2.400"
2"	2.197"	2.697"	2.360"	2.860"	2.375"	2.875"

To adjust for a different gap, just swap the 0.5" in the formula. Some contractors prefer 3/8" for tight racks or 3/4" where couplings need clearance. The Parallel Offset Calculator on conduitbending.com handles custom gap values automatically.

How Segment Bending Works

You can't make a concentric bend with a standard hand bender in one shot. The bender shoe produces a fixed radius (the NEC minimum), and that radius is too tight for concentric work on outer conduits. Instead, you use segment bending: breaking one large-radius bend into many small, evenly spaced bends.

A hickey bender is the traditional tool for this. It bends a few degrees at a time, and you control the radius by adjusting the spacing between bends. Some electricians use a hand bender for segment work on smaller conduit, applying partial pressure to create shallow bends at each mark.

The Segment Bend Formulas

Three numbers drive every segment bend:

1. Developed Length (for a 90-degree bend):

DL = R x 1.5708

This is the arc distance along the conduit centerline through the bend. The 1.5708 constant is pi / 2. For any other angle, use: DL = R x (angle / 57.2958).

2. Number of shots:

Choose how many individual bends make up the 90. More shots = smoother curve, but more time. Common choices:

• 18 shots at 5 degrees each = 90 degrees
• 15 shots at 6 degrees each = 90 degrees
• 9 shots at 10 degrees each = 90 degrees (rougher curve, faster)

3. Mark Spacing:

Spacing = DL / Number of shots

This is the distance between marks on the conduit. Each mark gets one bend of the chosen degree amount.

Worked Example: Concentric 90 with Three 3/4" EMT

You're running three sticks of 3/4" EMT in a parallel rack and need a concentric 90-degree bend. You want 1/2" between conduit faces and plan to use 18 shots at 5 degrees each.

Step 1: Determine Center-to-Center Spacing

• OD of 3/4" EMT = 0.922"
• Gap = 0.5"
• C-C = 0.922 + 0.5 = 1.422"

Step 2: Choose the Innermost Radius

• NEC minimum for 3/4" EMT (one-shot bender) = 4"
• For segment bending, you want a larger radius for a cleaner look. Pick 8".

Step 3: Calculate Each Conduit's Radius

• Conduit 1 (inner): R = 8.000"
• Conduit 2 (middle): R = 8.000 + 1.422 = 9.422"
• Conduit 3 (outer): R = 8.000 + (2 x 1.422) = 10.844"

Step 4: Calculate Developed Length for Each

• Conduit 1: DL = 8.000 x 1.5708 = 12.566"
• Conduit 2: DL = 9.422 x 1.5708 = 14.800"
• Conduit 3: DL = 10.844 x 1.5708 = 17.034"

Step 5: Calculate Mark Spacing (18 shots)

• Conduit 1: 12.566 / 18 = 0.698" (roughly 11/16")
• Conduit 2: 14.800 / 18 = 0.822" (roughly 13/16")
• Conduit 3: 17.034 / 18 = 0.946" (roughly 15/16")

Step 6: Mark and Bend

Starting from a common reference point, lay out the marks on each conduit at the calculated spacing. Bend 5 degrees at each mark. Use a torpedo level or protractor to check progress every 3 to 4 shots. After all 18 bends, each conduit should show a 90-degree turn, and the spacing between them should remain uniform throughout the curve.

Summary: concentric 90 with three 3/4" EMT (8" inner radius, 18 shots at 5 degrees)

Conduit	Radius	Developed Length	Mark Spacing
#1 (inner)	8.000"	12.566"	0.698" (11/16")
#2 (middle)	9.422"	14.800"	0.822" (13/16")
#3 (outer)	10.844"	17.034"	0.946" (15/16")

Concentric Offset Bends

Concentric 90s get the most attention, but concentric offsets come up just as often. When a rack of parallel conduit needs to jog around a beam, a panel, or a piece of equipment, each pipe needs a different mark placement to keep the spacing uniform through the offset.

The key formula for concentric offsets:

Mark adjustment = C-C x tan(bend angle / 2)

This gives you the distance to shift each successive conduit's first mark relative to the conduit before it. The bend angle is the angle of each offset bend (typically 30, 45, or 60 degrees).

Offset Example: Two 1" EMT, 30-Degree Offset

• C-C for 1" EMT with 1/2" gap = 1.163 + 0.5 = 1.663"
• Bend angle = 30 degrees
• tan(15 degrees) = 0.2679
• Mark adjustment = 1.663 x 0.2679 = 0.446" (roughly 7/16")

So you'd start the second conduit's first mark 7/16" farther from the end than the first conduit's mark. Both conduits use the same offset height and the same multiplier from the bending formulas chart. The only difference is where the first mark lands. For three or more conduits, keep adding 7/16" for each successive pipe.

The Offset Bend Calculator can handle these adjustments when you enter parallel conduit counts.

Concentric 90-Degree Stubs (Hand Bender Method)

If you're bending 90-degree stubs with a hand bender rather than doing full segment bends, there's a simpler approach for parallel conduit. Since each pipe uses the same bender (same radius, same deduct), the only variable is where you place your mark.

The rule: add one C-C spacing to each successive stub-up mark.

Say you need three 3/4" EMT stubs at 24 inches, spaced 1/2" apart:

• Deduct for 3/4" EMT = 6"
• C-C = 1.422"
• Conduit 1: mark at 24 - 6 = 18" from end
• Conduit 2: mark at 18 + 1.422 = 19.422" (19-7/16") from end
• Conduit 3: mark at 18 + 2.844 = 20.844" (20-13/16") from end

The result: the innermost conduit stubs up at 24", and each outer conduit stubs up progressively higher to account for the wider arc of the hand bender at the same radius. The spacing stays uniform from the straight run through the bend and into the vertical stub.

This is the method most electricians use day-to-day. It's fast and requires zero segment bending. The Conduit Bending Guide covers the deduct values for all standard EMT sizes if you need a refresher.

NEC Requirements for Concentric Bends

Two NEC rules directly affect concentric bending:

Minimum Bend Radius (Chapter 9, Table 2)

Every conduit bend must meet the minimum radius for its type and size. When segment bending, you control the radius, so you need to verify it. The minimum radii for EMT with one-shot benders are:

NEC Chapter 9, Table 2: minimum bend radius for EMT (one-shot and full-shoe benders)

EMT Size	Min. Radius (One-Shot)
1/2"	4"
3/4"	4"
1"	5"
1-1/4"	6"
1-1/2"	8"
2"	9"

For segment bending, your innermost radius should exceed these minimums. A good practice is to double the NEC minimum for your innermost pipe.

360-Degree Rule (NEC 358.26)

The total degrees of bends between pull points can't exceed 360. This applies to each individual conduit in your rack. A concentric 90 counts as 90 degrees toward the limit, just like any other 90. If the conduit run includes other bends (offsets, kicks), add them all up. Hitting 360 degrees means you need a pull box or conduit body before the next bend.

Common Mistakes with Concentric Bends

• Using the trade size instead of the actual OD: "3/4-inch" EMT is not 0.75 inches in outside diameter. It's 0.922 inches. Using the trade size for spacing calculations guarantees incorrect gaps. Always look up the actual OD.
• Picking too tight an inner radius: If your innermost radius barely clears the NEC minimum, the segment marks will be so close together that bending accuracy suffers. Start with at least 1.5x to 2x the NEC minimum for the inner pipe.
• Inconsistent bend angles: Every shot in a segment bend must be the same degree. If some shots are 4 degrees and others are 6 degrees, the curve won't be smooth. Use a protractor or the degree indicator on your bender.
• Not bending the inner conduit first: Always start with the innermost pipe. Verify it before moving outward. If the inner conduit is off, every outer pipe will compound the error.
• Forgetting that segment bending takes time: A rack of 5 or 6 parallel conduits with concentric 90s can take an entire day. Factor this into your labor estimate. As noted on the Mike Holt forums, some experienced electricians budget 2+ hours per concentric 90 on large racks.
• Skipping concentric bends on offsets: Electricians often remember concentric spacing for 90s but forget it on offsets. If your rack has uniform spacing on the straights and through the 90s, a non-concentric offset will ruin the look.

How Conduit Bending Handles Parallel Bends

Conduit Bending eliminates the manual math for concentric bends. The app's parallel bend feature calculates marks for every conduit in a rack simultaneously.

What it does:

• Automatic spacing: Enter the conduit type, size, number of parallel runs, and desired gap. The app calculates the C-C spacing and all mark positions.
• Multiple bend types: Parallel calculations work for 90s, offsets, saddles, and kicked 90s. Not just segment bends.
• Bender library: The app's bender library includes EMT, IMC, and Rigid from 1/2" to 4", with correct OD values and radii for every size.
• Fraction output: Results display in feet-inches-fractions, the format you actually mark on conduit. No decimal-to-fraction conversion needed on site.
• 16 bend types: Beyond parallel bends, it handles offsets, saddles, 90s, rolled bends, corner bends, and more.

For the three-pipe example above, you'd select 3/4" EMT, enter "3" for parallel conduits, set the gap to 1/2", and the app would output all the marks for each pipe. The entire calculation takes seconds instead of the 15+ minutes of manual math.

Quick Reference: Concentric Bend Formulas

Quick reference for all concentric bend formulas

Formula	Equation	When to Use
Center-to-Center	C-C = OD + Gap	Finding spacing for same-size conduit
Conduit Radius	R(n) = R(1) + (n-1) x C-C	Finding each pipe's bend radius
Developed Length (90°)	DL = R x 1.5708	Arc length for segment bending
Developed Length (any angle)	DL = R x (angle / 57.2958)	Arc length for non-90 bends
Mark Spacing	Spacing = DL / # of shots	Distance between segment bend marks
Offset Mark Adjustment	Adj = C-C x tan(angle/2)	Shifting marks for concentric offsets
Stub-Up Adjustment	Add C-C to each successive mark	Hand-bender parallel 90 stubs

Related Guides

• Single-conduit bending fundamentals: 90-Degree Bend Guide and Offset Bend Guide
• Multiplier and shrink values for offsets: Bending Formulas & Deduct Charts
• Conduit OD and wall thickness by type: EMT vs Rigid vs IMC
• All guides in one place: Conduit Bending Guide

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Frequently Asked Questions

What is a concentric bend in conduit?

A concentric bend is a large-radius bend made in multiple parallel conduits so they maintain equal spacing throughout the curve. Each conduit shares the same center point but uses a progressively larger radius as you move outward from the innermost pipe. The result is a clean, professional-looking rack of conduit that stays uniformly spaced before, during, and after the bend.

How do you calculate concentric bend spacing?

Calculate center-to-center spacing by adding the conduit outside diameter (OD) to your desired gap between conduit faces. For same-size conduit with a standard 1/2-inch gap: C-C = OD + 0.5". For 3/4" EMT (OD = 0.922"), the center-to-center spacing is 0.922 + 0.5 = 1.422", or roughly 1-7/16".

What is the concentric bend formula?

The concentric bend formula determines the radius for each conduit in a parallel run: R(n) = R(1) + (n - 1) x C-C, where R(1) is the centerline radius of the innermost conduit, n is the conduit number counting outward, and C-C is the center-to-center spacing. The developed length for a 90-degree bend is then DL = R x 1.5708.

What is the difference between concentric bends and segment bends?

Segment bending is the technique used to make concentric bends. A segment bend breaks one large-radius bend into many small, equally spaced bends using a hand bender or hickey. Concentric bending is the goal (parallel conduits with graduated radii), while segment bending is the method. You use segment bending on each individual conduit, adjusting the mark spacing for each pipe's unique radius.

How do you bend parallel conduit to maintain spacing?

For hand-bender 90s, add the center-to-center spacing to each successive conduit's stub-up mark. For segment bends, calculate a unique developed length and mark spacing for each conduit based on its radius. Always bend the innermost conduit first, verify the result, then work outward. The Conduit Bending app automates these calculations for parallel runs of any size.

What is the developed length formula for conduit bending?

The developed length formula for a 90-degree bend is DL = R x 1.5708 (R x pi / 2). For any angle: DL = R x (angle / 57.2958). The developed length is the total arc distance along the conduit centerline through the bend. Dividing DL by the number of segment shots gives you the spacing between marks.