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 examples

Example 1 — Moderate pull, single 90° bend:

Cable weighing 1.5 lb/ft through 200 ft of lubricated PVC (μ = 0.3) into a 90° 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

At 72 lb/ft, this is well within a 500 lb/ft single-conductor limit — a safe pull.

Example 2 — Heavier cable, same geometry:

The same run but with a heavier cable at 3.0 lb/ft:

T_straight = 0 + 3.0 × 200 × 0.3 = 180 lb
T_out = 180 × 1.602 ≈ 288 lb
SWBP = 288 / 2 = 144 lb/ft

Still within limits, but the tension at the pulling end has doubled — important for planning the pulling equipment and checking the pulling-eye rating.

Example 3 — Tight bend radius, higher SWBP:

Same 1.5 lb/ft cable, same 90° bend, but a smaller 1 ft bend radius:

T_straight = 90 lb (unchanged)
T_out = 90 × 1.602 ≈ 144 lb (unchanged — radius doesn't affect tension)
SWBP = 144 / 1 = 144 lb/ft

Notice that the tension at the exit of the bend is identical — the radius does not appear in the capstan equation. But the sidewall bearing pressure doubles when the radius halves. This is why tight bends damage insulation even when the tension seems acceptable.

Friction coefficients by conduit type

Always use a compatible cable pulling lubricant. A dry pull can increase μ by 50–100%, which more than doubles tension through a bend due to the exponential nature of the capstan formula.

Multi-bend routes

For a conduit run with several bends, calculate section by section:

1. Compute T_out for the first straight run + first bend.
2. Use that T_out as T_in for the next section.
3. Check SWBP at each bend separately — the highest-pressure bend is often not the last one.
4. Compare the final T_out against the cable’s maximum allowable pulling tension (for copper, typically 0.008 lb per circular mil, capped by the pulling-eye or basket-grip rating).

If the final tension or any SWBP exceeds the limit, options include: adding a midpoint pull or feed box, increasing the bend radius, switching to a lower-friction conduit type, or splitting the run into smaller sections.