Cable Pulling Tension & Sidewall Pressure Calculator (AEIC CG5)

Overview

Cable pulling tension and sidewall pressure (SWBP) are not lookup numbers. They depend on the order of the run: a 90° sweep at the start is harmless; the same sweep at the end can crush the cable. That is why this tool builds the duct run section by section, walks the tension along in one direction, then flips the route and checks both ends.

The method follows the AEIC CG5 (2nd edition) Underground extruded power cable pulling guide. The pressure equations also appear in IEEE 1185 and in manufacturer installation manuals, so the pressure side is well corroborated. The tension side is where methods diverge — that is covered below.

How the calculation works

Two passes. Walk tension along the whole route, section by section. Then derive the sidewall pressure at each bend from the tension leaving the bend. One tension number into each bend can give two completely different pressures depending on where it sits in the run.

The pressure at a bend

For one cable in a duct, sidewall pressure is the tension leaving the bend divided by the inside radius in feet. Three cables split into two formations, and which one applies depends on the jam ratio (duct I.D. ÷ cable O.D.):

• Cradled — the middle cable takes the load. (3Wc − 2) × T ÷ 3R
• Triangular — the bottom two share it. Wc × T ÷ 2R

Below jam ratio 2.4 the cables ride triangular. From about 2.6 upward the published guidance describes the formation as first uncertain and then increasingly cradled. The tool assumes cradled across that whole range because cradled is the harder case, and assuming it is what the guide recommends when the formation is not clear. Triplexed assemblies hold a triangle throughout a pull, so jamming is unlikely for them whatever the calculated ratio.

Formation and jamming are separate questions. A jam ratio between 2.8 and 3.0 is the band where one cable can slip between the other two and wedge in a bend, and the tool flags that independently of which formula it used.

Weight correction earns its keep twice

Three cables in a duct press harder on the wall than their combined weight suggests, because they also press on each other. The weight correction factor Wc captures that.

It shows up in two places. Once in the pressure formulas above. And again inside the tension calculation, where it multiplies the friction coefficient. That second use is the one people leave out, and it matters more than the first.

Friction is not one number

The friction coefficient drops in a tight bend. It sounds backwards, but the physics is straightforward: at low contact force the lubricant film is thick and you are shearing the compound itself. Squeeze it thin and boundary lubrication takes over, and the compound starts doing its actual job.

The threshold sits around 150 lb/ft. The tool tries the low-pressure value first at every bend. If the result lands at or above 150, it recalculates with the high-pressure value. If that recalculation drops the pressure back under 150 — a contradiction — it keeps the low-pressure result, because that is the conservative one.

Direction decides the pull

The same duct run gives different tensions depending on which end you feed from. Bends near the pulling end multiply tension that has already built up. Bends near the reel multiply almost nothing. Feeding into the end with the most bends, or into the uphill side, usually wins.

This calculator runs both directions every time and shows you the better one, with the other reported underneath. On a marginal run this single choice is often the difference between a pull that works and one that stops halfway.

Worked example: 3 × 500 kcmil into 5" galvanised rigid steel

Cable O.D. 1.60 in, 2.2 lb/ft each. Duct I.D. 5.07 in. XLPE outer covering. Route from the switchgear: 10 ft straight, 90° sweep, 75 ft straight, 90° sweep, 635 ft straight, 90° sweep, then 30 ft into the transformer.

Jam ratio = 3.17 → the cables cradle. Weight correction comes out at 1.283.

Both friction cases show up here. At section 4 the low-pressure coefficient gives 285 lb/ft, but recalculating with the high-pressure value drops it to 127 — under the threshold, so the tool keeps the conservative low-K result. At section 6 the high-pressure value still leaves 1 026 lb/ft, so that one stands.

A compression eye on 500 kcmil copper, with two of the three cables sharing the load, allows 11 000 lb. The pull finishes at 10 879 — 99 percent of the limit. Now feed from the transformer end instead: 12 506 lb and 1 193 lb/ft, 114 percent of the limit. The pull fails.

AEIC CG5 recommends keeping the calculated maximum at or below 80 percent of allowable as good design practice, with headroom for start-up and surging. This run needs a pull box in the middle of the 635 ft straight — or a larger sweep on the last bend — before anyone touches it.

Why some calculators report lower numbers

A lot of free tools use a lookup table of bend multipliers — typically 2.2 for a 90° bend at K = 0.5. That figure comes from raising e to the power of friction × angle, and it leaves the weight correction factor out of the exponent.

Put Wc back in, at 1.28, and the same multiplier becomes 2.74. Across three bends that compounds: roughly 8 000 lb by the old method against nearly 10 900 here. The published guide is explicit that the coefficient used in these equations already carries the weight correction factor, and its own worked examples confirm it.

If a tool gives you a comfortably lower answer for a multi-bend pull, check whether it is applying Wc to the bends or only to the straights.

What has to pass besides pressure

Tension at the pulling end has to stay under whichever is lowest: the eye / grip rating, or the conductor stress limit. Clearance has to be at least half an inch so the cable and pulling hardware physically fit. Jam ratio has to stay clear of the 2.8 to 3.0 band. And no bend can be tighter than the cable's own minimum radius, which is a multiple of its diameter set by the shield construction.

The tool flags all four alongside the pressure result.

Use this alongside cable sizing (to pick the cross-section in the first place) and voltage drop (to verify drop after the cross-section is fixed). The three together give you: size → drop → field feasibility.