Cable pulling · Tension · Sidewall pressure

Cable pulling tension, sidewall pressure & jam ratio

The formulas every heavy pull runs on — maximum conductor tension, straight runs, bends, sidewall bearing pressure, weight correction, and jam ratio — worked through on a real-world pull.

The pulling equations

What you're finding	Formula	Terms
Maximum tension on the conductors (pulling eye on the conductors)	T_max = S × cmil × n (n ≤ 3); T_max = S × cmil × n × 0.8 (n > 3)	S = allowable conductor stress: 0.008 lb/cmil copper; cmil = area of one conductor; n = number of conductors
Tension in a straight horizontal run	T_out = T_in + L × w × f × Wc	L = length (ft); w = total cable weight (lb/ft); f = coefficient of friction; Wc = weight correction factor
Tension out of a bend (horizontal approximation)	T_out = T_in × e^(Wc × f × θ)	θ = bend angle in radians (90° = 1.571)
Sidewall bearing pressure — one cable	SWBP = T_out / R	R = bend radius (ft); result in lb/ft
Sidewall pressure — 3 cables, cradled	SWBP = (3Wc − 2) × T_out / (3R)	The bottom-center cable takes the most pressure
Sidewall pressure — 3 cables, triangular	SWBP = Wc × T_out / (2R)
Weight correction factor — 3 cables, cradled	Wc = 1 + (4/3) × (d / (D − d))²	d = cable OD; D = conduit ID
Weight correction factor — 3 cables, triangular	Wc = 1 / √(1 − (d / (D − d))²)
Jam ratio (3 single cables)	Jam ratio = D / d	Some guides use 1.05 × D / d to allow for conduit ovality in bends

Formulas are the industry-standard pulling equations as published in manufacturer guides (e.g. Southwire). Always use the cable manufacturer's own limits — maximum tension, sidewall pressure, and minimum bend radius — and the rating of your pulling eye, grip, rope, and puller. For critical or medium-voltage pulls, run the manufacturer's pull calculator or have the pull engineered.

Worked example — 3 × 500 kcmil copper single conductors in conduit

Step	Calculation	Result
Max tension on the conductors (pulling eye)	0.008 × 500,000 × 3	12,000 lb
Cable data (illustrative — use your data sheet)	OD d = 1.0 in; weight 1.6 lb/ft each → w = 4.8 lb/ft	—
Conduit	ID D = 4.0 in	—
Jam ratio	4.0 / 1.0 = 4.0	Outside 2.8–3.2 → no jam; cradled
Weight correction factor (cradled)	1 + 4/3 × (1.0 / 3.0)²	Wc = 1.148
Coefficient of friction (illustrative, lubricated)	f = 0.35	—
Section 1 — 200 ft straight	0 + 200 × 4.8 × 0.35 × 1.148	386 lb
Section 2 — 90° sweep, 3 ft radius	386 × e^(1.148 × 0.35 × 1.571) = 386 × 1.88	725 lb
Sidewall pressure in the sweep (cradled)	(3 × 1.148 − 2) × 725 / (3 × 3)	≈ 116 lb/ft
Section 3 — 100 ft straight	725 + 100 × 4.8 × 0.35 × 1.148	918 lb at the puller
Checks	918 lb ≪ 12,000 lb conductor limit; compare 116 lb/ft with the manufacturer's sidewall limit; check eye/rope/puller ratings	OK to proceed if all pass

The 0.008 lb/cmil stress is from Southwire's published maximum-pulling-tension guidance; cable OD, weight, and friction coefficient here are illustrative. Use your cable's data sheet and the lubricant maker's friction values.

Reading the jam ratio and configuration

D / d	How three cables sit	What it means
Below about 2.8	Triangular (cables nest in a triangle)	No jamming path; check clearance and fill
About 2.8 to 3.2	Can go either way	Jam zone — the center cable can wedge between the outer two in a bend. Avoid this range
Above about 3.2	Cradled (side by side in the bottom)	No jamming; use the cradled weight correction factor

Jam risk only exists with three (or more) single, non-plexed conductors. Triplexed cable doesn't jam. Formulas are the industry-standard pulling equations as published in manufacturer guides (e.g. Southwire). Always use the cable manufacturer's own limits — maximum tension, sidewall pressure, and minimum bend radius — and the rating of your pulling eye, grip, rope, and puller. For critical or medium-voltage pulls, run the manufacturer's pull calculator or have the pull engineered.

Field practices that keep tension down

Practice	Why it works
Feed from the end nearest the bends	Bends multiply the tension coming into them — keep that tension low
Lubricate continuously, not just at the start	Friction coefficient drives every straight section and is an exponent in every bend
Use a pulling eye on the conductors for heavy pulls	A basket grip loads the jacket and insulation, which limits allowable tension
Use large-radius sweeps	Sidewall pressure is tension divided by radius
Watch the dynamometer and log it	A tension record proves the cable wasn't overstressed
Don't count shields, armor, or the small EGC as tension members	Only the phase and neutral conductors carry the pull

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Planning a pull, in plain language

Sources. The equations on this page are the standard pulling equations published in manufacturer installation guides — see Southwire’s Maximum Pulling Tension bulletin and Power Cable Installation Guide. We don’t reproduce manufacturer tables; use your cable manufacturer’s limits and data sheet.

Three limits, check all of them. The conductors can only take so much tension (0.008 lb/cmil for copper on a pulling eye). The cable insulation can only take so much crushing in a bend (sidewall bearing pressure). And the hardware — eye, grip, swivel, rope, puller, and the structure you anchor to — each has its own rating. On big single-conductor pulls, sidewall pressure usually runs out first.

Bends are exponential. Straight sections add tension linearly with length, weight, and friction. Bends multiply it by e^(Wc·f·θ), so a 90° bend near the end of a long pull costs far more than the same bend near the start. That’s why you feed from the end nearest the bends, and why lubricant matters so much.

Worked example. Three 500 kcmil copper conductors in a 4 in conduit: the conductor limit is 12,000 lb. A 200 ft straight run builds 386 lb, a 90° sweep multiplies that to 725 lb with about 116 lb/ft of sidewall pressure on a 3 ft radius, and another 100 ft brings it to 918 lb at the puller — well within every limit, provided the sidewall figure is under the manufacturer’s limit.

Related: wire gauge conversion & conductor weights, the conduit fill chart, NEC Chapter 9 dimensions, and the conduit fill calculator. Record each pull — crew hours, notes, and tension readings — on the Field PM daily report.

FAQ

What is the maximum pulling tension for copper conductors?+

Using a pulling eye attached to the conductors, the common manufacturer limit is 0.008 lb per circular mil of copper, times the number of conductors (with a 0.8 factor when pulling more than three). Three 500 kcmil copper conductors work out to 0.008 × 500,000 × 3 = 12,000 lb. The eye, grip, rope, and puller can have lower limits, and a basket grip is limited further by the jacket.

How do I calculate tension through a bend?+

Multiply the tension going into the bend by e raised to (weight correction factor × friction coefficient × bend angle in radians). A 90° bend is 1.571 radians. With Wc = 1.148 and f = 0.35, a 90° bend multiplies tension by about 1.88.

What is sidewall bearing pressure?+

The crushing force per foot of bend radius that a cable sees as it is pulled around a bend — tension out of the bend divided by the bend radius for a single cable, with adjustments for three cables cradled or triangular. It often limits large single-conductor pulls before tension does. Compare the result to the manufacturer's sidewall pressure limit.

What is the jam ratio?+

The conduit inside diameter divided by the cable outside diameter, for three single conductors. When it falls in roughly the 2.8 to 3.2 range, the center cable can wedge between the other two in a bend and jam the pull. Some guides multiply the conduit ID by 1.05 first to allow for bend ovality.

Where do I get the cable weight and OD?+

From the cable manufacturer's data sheet for the exact product (insulation type, voltage rating, and construction change both). Our wire gauge page gives bare-conductor weights computed from density, but insulated cable is heavier.

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