Parallel offset calculator

When a rack of conduits all take the same offset, each pipe has to start its jog a little further along than the one beside it, or the diagonal sections crowd together. Enter the centre-to-centre spacing and the bend angle; the calculator gives the stagger to advance each successive conduit, and the offset mark spacing if you add a rise.

Worked example

Say four conduits run 3 inches apart and all offset at 30°. The stagger is the spacing times the shrink constant for 30°, 3 × 0.268 ≈ 13/16 inch. So if the first conduit starts its offset at some mark, the second starts about 13/16 inch further along, the third about 1 5/8 inches, the fourth about 2 7/16. Every conduit gets the identical 30° offset; only the starting point walks along by 13/16 inch each time, and the group stays 3 inches apart through the jog.

The formula

The stagger per conduit is the spacing times the offset shrink constant for the angle:

stagger = spacing × (1 ÷ sin θ − 1 ÷ tan θ)

That shrink constant is also tan(θ/2), which is what falls out of the geometry: to hold the same perpendicular spacing between two parallel runs once they tilt by θ, adjacent bend points have to sit tan(θ/2) of the spacing apart along the run. It is the same constant as offset shrink, so the number you already know for the angle does this job too.

Exact vs. the field rule

The stagger constant is the shrink figure for the angle, so the same rounding applies: the field rules are close on tight spacing and drift as the group gets wider. On a big rack it is worth taking the stagger off the exact constant, because the error compounds across every conduit in the run.

Questions

Why stagger the marks on parallel offsets?

So the runs stay parallel and evenly spaced through the jog. If every conduit bent at the same point, the diagonal sections would crowd closer together than the straight runs and the group would bunch. Advancing each successive pipe along its length by the right amount keeps the spacing constant all the way through.

How much do I advance each conduit?

By the centre-to-centre spacing times the shrink constant for the bend angle — the same 1/sin θ − 1/tan θ that gives offset shrink. At 30° with 3 inch spacing that is about 3/4 inch per pipe. Each conduit further along the group starts its offset that much later than the one before it.

Which conduit starts first?

The one on the inside of the jog — the side the run steps toward. Each conduit further out is advanced by one more stagger, so the group fans along a common line and the diagonals come out parallel. As long as you step them consistently in one direction it comes out right.

Is the offset itself the same on every conduit?

Yes. Every conduit gets the same rise, the same angle, and the same distance between its own two marks. Only the starting point moves. Work the offset once with the offset calculator, then apply the stagger to place where each run begins.

Related tools

• Conduit offset calculator — the offset each conduit in the group takes.
• Concentric bend calculator — parallel runs around a bend rather than an offset.
• Conduit shrink calculator — the same constant, doing its other job.