Conduit Back-to-Back Bend Calculator: Mark Spacing & Shrink

Result

Formulas

\(T_1 = \frac{R_1}{\sin(\theta_1)}\)
\(T_2 = \frac{R_2}{\sin(\theta_2)}\)
\(S_{\mathrm{mark}} = T_1 + T_2\)
\(M_2 = M_1 + S_{\mathrm{mark}}\)
\(S_{\mathrm{shrink}} = R_1\tan\left(\frac{\theta_1}{2}\right) + R_2\tan\left(\frac{\theta_2}{2}\right)\)

A conduit back-to-back bend layout establishes two bend marks when each bend has its own rise and angle. The calculation produces the Second mark location from the conduit end, along with the Bend separation between marks, individual bend travel, combined rise, and estimated shrink.

Use the layout when the conduit must change elevation twice in close succession and the two bend events must remain separately marked. Typical applications include offsets around obstructions, routed raceways entering equipment at different elevations, and field layouts where two bends cannot be treated as a single standard offset.

The primary field number is usually Second mark. After locating the First mark from conduit end, measure the calculated bend separation along the conduit to place the second mark.

Bend Travel and Mark Separation

Each bend is calculated independently from its rise and bend angle.

For each bend:

text{Bend travel} = frac{text{Bend rise}}{sin(text{Bend angle})}

The calculator uses:

First bend rise (in) to calculate First travel
Second bend rise (in) to calculate Second travel
First bend angle (deg) and Second bend angle (deg) as the angle for each independent bend
First mark from conduit end (in) as the fixed reference point for the first bend mark

The two travel values are added to determine the distance between the marks:

\(\displaystyle \text{Bend separation} = \text{First travel} + \text{Second travel}\)

The second mark is then located from the conduit end:

\(\displaystyle \text{Second mark} = \text{First mark} + \text{Bend separation}\)

The calculator also reports:

\(\displaystyle \text{Combined rise} = \text{First bend rise} + \text{Second bend rise}\)

This is the total entered vertical change associated with both bend events. It is not a substitute for verifying the final installed conduit elevation, direction of travel, or bender orientation.

Calculator Inputs

Input	Electrical layout use
First bend rise (in)	Rise resolved by the first bend
Second bend rise (in)	Rise resolved by the second bend
First bend angle (deg)	Angle used to determine the first bend travel
Second bend angle (deg)	Angle used to determine the second bend travel
First mark from conduit end (in)	Reference distance from the conduit end to the first bend mark

A smaller bend angle requires more conduit travel for the same rise. For example, a 30-degree bend requires more travel than a 45-degree bend to produce an equal rise because the conduit changes direction more gradually.

The screen is intended for back-to-back geometry where separate marks must remain visible. It does not convert the two bends into a single offset multiplier or replace bender-specific layout instructions.

Calculation Example

Enter the following values:

Input	Value
First bend rise	4 in
Second bend rise	4 in
First bend angle	30 deg
Second bend angle	30 deg
First mark from conduit end	24 in

For a 30-degree bend:

\(\displaystyle \sin(30^\circ) = 0.5\)

First bend travel:

\(\displaystyle \frac{4\text{ in}}{0.5} = 8\text{ in}\)

Second bend travel:

\(\displaystyle \frac{4\text{ in}}{0.5} = 8\text{ in}\)

Mark separation:

\(\displaystyle 8\text{ in} + 8\text{ in} = 16\text{ in}\)

Second mark:

\(\displaystyle 24\text{ in} + 16\text{ in} = 40\text{ in}\)

The calculated results are:

Result	Value
First mark	24 in
Second mark	40 in
First travel	8 in
Second travel	8 in
Bend separation	16 in
Combined rise	8 in
Total estimated shrink	2.1436 in

Measure 24 inches from the conduit end for the first mark, then measure 16 inches farther along the conduit for the second mark. The estimated shrink identifies the calculated loss in developed length associated with the entered bends; confirm how that length affects the actual stub, coupling, box-entry, and finished raceway dimensions.

Field Verification

Back-to-back bend marks are layout dimensions, not a complete installation determination. Verify the following before bending conduit:

Conduit rotation and the intended plane of each bend
Bend direction and whether the second bend is made toward or away from the first bend
Required straight length between bend tangencies
Bender shoe geometry, centerline reference, arrow/star reference marks, and take-up conventions
Springback for the conduit material and bending method
Available clearance around equipment, structural members, other raceways, and conductors
Final conduit length at boxes, cabinets, pull points, and couplings

The Total estimated shrink is calculated from the entered bend geometry. Actual installed shrink can vary with bender geometry, conduit type, bend reference location, and field technique. Confirm the completed raceway against the required connection points before cutting conduit or committing final bends.