Cable Pull Estimator

01 · Friction

μ Typical values, lubricated cable

This has the biggest effect on the result. Picking a row fills in the top of its range.

02 · Cable

Weight per metrekg/m Min bend radiusmm Max safe pulling tension kg From the cable maker, or about 7 kg/mm² (70 N/mm²) × total copper area if pulling on the conductors.

03 · Rope and sock

Rope weightkg/m Rope capacitykg Sock capacitykg

04 · Advanced

Back tension

From the cable drum

Estimate Measured None

Rule of thumb: cable weight per metre × 25 is a common starting point. Use a measured value if you have one.

Factors and limits

Working load fraction 0.70 keeps a 30% margin on every component. Sheave allowance divisor Extra derating on sheave SWL. 1.1 allows a further 10%. Weight correction factor 1.0 for one cable. Use about 1.1–1.4 for three cables in one conduit. Sidewall pressure limit kg/m From the cable maker. Leave blank to show values without a check.

Checking pull…

Required pull: —
Winch setting: —
Bend radius: —
Total bends: —

05 · Route

Route legs

Enter how far the route turns at the end of each leg: 90 for a right-angle bend, 45 for a 45° bend.

Leg	Length m	Rise + / drop − m	Bend at end °	Rope only	Corner radius mm	Sheave SWL kg	Actions

Leg 1 starts at the cable drum. Leave the bend blank where a leg has no bend, and the rise blank on a level leg.

Reference

Method and standards

How tension builds up

Working from the drum, each leg adds friction and gravity, and each bend multiplies the tension.

Straight leg, slope φ

T₂ = T₁ + w·L·(K·μ·cos φ + sin φ)

Bend of θ radians

T₂ = T₁ · e^(K·μ·θ)

w is weight per metre, L is leg length, K is the weight correction factor. Tension never drops below zero.

Checks

Winch setting is the working load fraction × the weakest of the cable, rope, sock and derated sheaves.
Sheave SWL is divided by the resultant load factor 2·sin(θ/2), capped at no less than 1, then by the allowance divisor.
Sidewall pressure is tension leaving the bend ÷ bend radius, for a single cable.
Bends are totalled against 360° between pull points.
A bend with no sheave rating is left out of the winch limit, and a bend with no corner radius shows as incomplete.
The pull is also worked from the far end, as a cross-check on which direction is easier.

Standards and sources

AS/NZS 3000 (Wiring Rules) requires cables to be installed without damage and bent no tighter than the manufacturer's recommended radius. It does not set out a pulling tension calculation.
The straight and bend equations are the capstan method used in cable makers' installation guides and in IEEE 1185 (not an Australian Standard).
Take the maximum pulling tension, minimum bend radius and sidewall pressure limit from the cable maker's data sheet, and the rope, sock and sheave ratings from their makers.
This is an estimate for planning a pull. It does not replace the manufacturers' instructions or an engineer's check.