How to calculate cable pulling tension

Pulling cable through conduit fails one of two ways: the tension exceeds what the conductors (or the pulling eye) can take, or the sidewall pressure at a bend crushes the cable. Both come from the same short calculation — and the bends are where it gets dangerous fast.

Straight friction, then the capstan bends

straight: T = μ·w·L · bend: T_out = T_in · e^(μθ) · sidewall: SWP = T / R

A straight run just adds friction — coefficient μ times cable weight per length w times length L. Bends are different: they multiply tension by e raised to (μ × bend angle in radians) — the capstan equation, the same physics as a rope around a bollard. The conduit pull tension calculator chains the straight runs and bends and reports both the final tension and the sidewall pressure.

Worked example — 100 ft, two 90° bends

0.5 lb/ft cable, μ = 0.35, 100 ft straight then 180° of bends (two 90s) at a 6 in radius, starting at 20 lbf:

straight: 20 + 0.35·0.5·100 = 37.5 lbf · bends: ×e^(0.35·π) = ×3.0 → 113 lbf · SWP = 113 lbf / 0.5 ft = 225 lb/ft

The final 113 lbf is a comfortable 23% of a 500 lbf pull limit — but the sidewall pressure is 225 lb/ft, 75% of a 300 lb/ft cable limit. Sidewall pressure governs, not tension. Tighten that bend to a 3 in radius and SWP doubles to 450 lb/ft — over the limit, even though the winch never strains.

Where it connects

This is the pulling side of conduit work; how many conductors legally fit is the conduit fill calculator, and laying out the bends themselves is the conduit bend offset calculator. The tension and sidewall limits themselves come from the cable manufacturer — this screen runs the route, you supply the ratings.

Common mistakes

• Sizing only on tension. Sidewall pressure at a tight bend often fails the cable first — check SWP = T/R at every bend, not just the winch readout.
• Pulling from the wrong end. Bends multiply the tension entering them; arrange the pull so the bends see the lowest tension (usually pull toward the straightest end).
• Ignoring lubricant. μ drops from ~0.5 dry to ~0.2 lubricated — and since bends use e^(μθ), halving μ roughly squares-roots the bend multiplier. Lube is the cheapest tension reduction there is.
• Adding bend tension instead of multiplying. Bends are exponential (e^(μθ)), not additive — two 90° bends are ×3.0 at μ=0.35, not "+friction twice."

Frequently asked questions

How do you calculate cable pulling tension?

Two effects add up. Straight runs add friction: T = μ·w·L (friction coefficient × cable weight per length × length). Bends multiply tension by the capstan equation: T_out = T_in·e^(μθ), with θ in radians. A 100 ft pull of 0.5 lb/ft cable at μ = 0.35, then two 90° bends, runs 20 → 37.5 lbf out of the straight, then ×3.0 through the bends = about 113 lbf.

Why do bends increase pulling tension so much?

Because the effect is exponential, not additive — the same capstan/belt-friction equation that lets a small force hold a big load around a bollard. Tension out of a bend is the tension going in times e^(μθ). At μ = 0.35, every 90° bend multiplies tension by about 1.73, and 180° of bends by 3.0. Tension entering a bend matters most, which is why you pull from the end that puts the bends last.

What is sidewall pressure and why does it often govern?

Sidewall pressure (SWP) is the bearing force the cable presses into a bend, SWP = T / R (tension ÷ bend radius). It is frequently the real limit, not total tension: in the worked example the 113 lbf final tension is only 23% of a 500 lbf pull limit, but the SWP at the 6 in radius is 225 lb/ft — 75% of a 300 lb/ft cable limit. A tighter radius spikes SWP and can crush the cable long before the winch reaches its tension rating.

How do you reduce cable pulling tension?

Pull from the heavy/bend-heavy end so bends see lower entering tension; use cable lubricant to drop μ (often from ~0.5 dry to ~0.2); use larger sweep radii to cut sidewall pressure; add pull boxes to break a long bendy run into shorter segments; and feed in with as little starting tension as the setup allows.