How is pulling tension through a bend calculated?

Bends multiply tension by the capstan formula: outgoing tension equals incoming tension times e raised to the friction coefficient times the bend angle in radians. A 90 degree bend with a friction of 0.3 multiplies tension by about 1.6.

What is sidewall bearing pressure?

Sidewall bearing pressure is the force the cable presses against the inside of a conduit bend, equal to the tension at the bend divided by the bend radius in feet. Excessive pressure crushes the insulation, so NEMA WC 74 sets limits, commonly around 500 lb/ft for single conductors.

What friction coefficients should I use?

With cable lubricant, typical values are about 0.25 for HDPE duct, 0.30 for PVC, 0.35 for rigid steel and 0.40 for EMT. Dry pulls are far higher. The tool bundles lubricated defaults and lets you override with a measured value.

How do I handle multiple bends?

Calculate one section at a time. Take the tension after one bend and enter it as the incoming tension for the next straight run and bend. Chaining sections this way builds the full tension profile of a complex pull.

What is the maximum allowable pulling tension?

For copper conductors a common rule is 0.008 lb per circular mil of total conductor area, capped by the pulling-eye or basket-grip rating. Compare the calculated final tension against that limit and stop the pull or add a feed point if it is exceeded.

Wire Pulling Tension & Sidewall Pressure Calculator

Pulling large feeder or medium-voltage cable through conduit can build dangerous tension, especially at bends. Exceed the cable’s limit and you stretch conductors or strip the jacket; exceed the sidewall bearing pressure and you crush insulation against the bend. This calculator estimates tension through a straight run and a bend and checks sidewall pressure against NEMA WC 74 limits.

How it works

A straight horizontal run adds tension from friction against the conductor weight:

T_straight = T_in + w × L × μ

A bend multiplies tension by the capstan (belt-friction) equation, the dominant effect in any pull:

T_out = T_straight × e^(μ × θ)

where θ is the bend angle in radians and μ the friction coefficient. The sidewall bearing pressure for a single cable at the bend is the tension divided by the bend radius:

SWBP = T_out / R

Worked example

Pulling cable weighing 1.5 lb/ft through 200 ft of lubricated PVC (μ = 0.3) into a 90 degree bend of 2 ft radius, starting from zero tension:

T_straight = 0 + 1.5 × 200 × 0.3 = 90 lb
θ = 90° = 1.571 rad
T_out = 90 × e^(0.3 × 1.571) = 90 × 1.602 ≈ 144 lb
SWBP = 144 / 2 = 72 lb/ft

That 72 lb/ft is well within a 500 lb/ft single-conductor limit, so the pull is safe on both counts.

Tips and notes

Always pull with the correct lubricant; a dry pull can double the friction coefficient and the resulting tension. Increase the bend radius to lower sidewall pressure, since SWBP is inversely proportional to radius. For multi-bend routes, feed the output tension of one section into the next, and compare the final tension against the cable’s maximum allowable pulling tension, roughly 0.008 lb per circular mil of copper, capped by the pulling-eye rating.