Cable Pulling Loads: How to Estimate Friction, Lift and Bends

Estimate cable pulling loads one route segment at a time: add straight-run resistance, include the weight of unsupported rises, then calculate bend tension and sidewall loading. I begin at the reel and follow the pull direction. The tension leaving one segment becomes the starting tension for the next, so the order of straights and bends matters.

I compare the calculated force with the capacity of the cable, pulling attachment and supports. The examples use explicit assumptions; for an actual installation, put the current limits for its cable and equipment beside the calculated loads.

Key takeaways

• A level sliding calculation gives added friction; include incoming tension separately.
• Convert mass in kg to force in N once, using gravitational acceleration.
• Check sidewall loading at each bend with its local exit tension and applicable radius.
• Use a sliding, roller or conduit model that represents the actual contact.
• Assign suspended weight and other route loads to designed supports before making off the gland.

Divide the route into load cases

A route segment is a length of cable with a defined direction, support condition and calculation model. Mark the reel, pulling end, bends, changes in elevation and support points. I give each straight length its own identifier and record whether the cable slides on a surface, rolls on equipment or hangs freely.

Follow the force from the cable into its supporting structure. A guide can carry a lateral reaction while the pulling attachment carries axial tension. Assign each component the load it actually carries.

How do I calculate a level sliding segment?

For a level sliding run, multiply the supported cable weight by the applicable friction coefficient. Add that resistance to the incoming tension. The following model assumes one straight cable, steady motion and constant contact conditions:

Here $\mu$ is the dimensionless friction coefficient, $\lambda$ is cable mass per length in kg/m, $L$ is the sliding length in m, and $g$ is gravitational acceleration in m/s². The surface supports the cable weight. Bends, slopes, obstructions and other loads belong in separate parts of the route model.

Incoming tension is the axial load already present at a segment’s entrance. For this steady horizontal balance, the exit tension is the incoming tension plus the segment’s friction. This is the same segment-by-segment approach developed in Southwire’s Power Cable Installation Guide, using consistent SI units here.

For example, take a hypothetical cable with mass 0.8 kg/m, a level sliding length of 20 m, an assumed coefficient of 0.30 and $g=9.80665$ m/s². Assume steady motion and no other resistance in that segment:

I carry the 72.07 N exit tension into the next segment. The 47.07 N friction is only this segment’s addition to the incoming 25 N.

ToolMechanical loadsCable Pulling Force CalculatorEnter cable mass per length, level sliding length, the applicable friction coefficient and gravity. Add the resulting friction contribution to the known incoming tension before moving to the next route segment.Fpull = μλLgCable mass per lengthPulled cable lengthFriction coefficientGravitational accelerationOpen calculator ->

Test the uncertain contact input

With the same cable and length, assumed coefficients of 0.20 and 0.40 give 31.38 N and 62.76 N. Doubling the coefficient doubles friction in this model. These sensitivity inputs show why establishing the contact condition matters more than another decimal place.

I distinguish starting from moving contact, then identify the jacket, conduit or support surface and compatible lubricant. Rollers need their applicable model and equipment condition; a horizontal route does not make them sliding surfaces.

How much load does a vertical cable add?

For a freely hanging, unsupported cable length in static equilibrium:

The suspended weight is the gravitational force from the unsupported mass below the point being checked. Here $L_v$ is that length in metres. I mark intermediate supports before deciding which length loads each point.

Consider a separate 6 m hanging length of the same hypothetical 0.8 kg/m cable. It weighs 47.07 N. If its upper support also carries a 1.5 kg attachment, that attachment adds 14.71 N, giving 61.78 N in this static example. Count the attachment once. Count the horizontal and vertical lengths separately if the actual route contains both.

Mass and weight need distinct units. The NIST SI tables define mass in kilograms and force in newtons. If the cable data already give weight per length in N/m, multiply by length directly. Applying gravity again would overstate the force.

For lifting, include acceleration, support friction, attachments and travel direction through the applicable dynamic model. For a stopped run, identify the permanent supports that carry the weight after removing the pulling equipment.

What changes at a bend?

A bend changes both the pulling tension and the load pressing the cable against its guide. I calculate the local exit tension first, then use it in the applicable sidewall-load expression. Cable formation matters: a single cable and several individual cables sharing a conduit have different contact arrangements.

For a single cable with sliding contact, a commonly used preliminary bend approximation is:

Here $\mu_b$ is the bend’s dynamic-friction coefficient, $\theta$ is the contact angle in radians, and $e$ is the exponential constant. This is the single-cable case of Southwire’s approximation E-15, with its weight-correction factor equal to one. A 90° turn is $\pi/2$ radians. Entering 90 as a radian value would give an entirely different result.

The approximation represents sliding contact, not every rotating sheave or stiff-cable bend. Southwire treats freely turning sheaves and cable bending resistance separately. For the actual route, use the cable formation, bend geometry and equipment model that apply; retain any additional bending or starting resistance needed by that model.

Keep sidewall loading in N/m

For one single-conductor cable, or multiple conductors within one common jacket, the simple sidewall index is:

Use the local tension leaving the bend in N and the applicable bend radius in m to obtain N/m. Southwire gives this form as E-20 and different expressions for other cable formations. Keep the model’s radius convention with the drawing; a sheave outside diameter is not automatically the required radius.

Sidewall loading is force per unit length acting along the bend contact. Although often called sidewall pressure, this index is not pressure in pascals. NIST defines a pascal as N/m². Calculating local contact stress would also need the contact area and load distribution.

For an independent example with 400 N leaving a bend, a 0.50 m radius gives 800 N/m. At the same tension, 0.80 m gives 500 N/m. A larger radius reduces this index, but the chosen geometry must also meet the cable’s minimum bend radius and fit the route.

Carry tension through a straight–bend–straight route

This connected example uses the same hypothetical cable and gravity: one cable sliding steadily through a horizontal 20 m straight, a 90° bend and a 10 m straight. It contains no vertical segment.

Assume 25 N at the first straight’s entrance, 0.30 friction on both straights, and 0.20 at the bend. The bend radius for the sidewall model is 0.50 m. Apply the simple bend approximation above; the example omits starting transients, cable stiffness and any separate bend-weight correction beyond that approximation.

Use 122.21 N at the final exit and 98.67 N at the bend exit. Substituting the final value into that earlier bend check mixes route positions. I keep full precision between steps and round the displayed results.

From my analysis of this example, increasing the bend radius from 0.50 m to 0.80 m reduces its sidewall index from 197.35 to 123.34 N/m, a 37.5% reduction at the same tension. The exponential approximation gives the same tension because radius is absent from that expression. Real route design also needs the cable’s bend-radius and stiffness checks.

Changing the pull direction deserves a new calculation. Straights and bends then occur in a different order, and the tension entering each bend can change. For routes with elevation changes, the direction of the weight contribution changes too. Compare the recalculated local loads and equipment access, rather than merely reversing the arrows on the completed sheet.

Which limits control cable pulling loads?

Compare each calculated load with the limit for that cable, attachment, location and duty. A low tension at the final exit does not settle sidewall loading at a tight bend. Similarly, a generous cable tension limit does not increase a pulling eye’s or grip’s rated capacity.

Southwire’s guide separates pulling-device limits, cable tension, sidewall loading and route geometry. Use the current cable and attachment instructions for allowable values, and compare them with the calculated demand at each location.

The responsible installation engineer also resolves reel drag, starting effects, changes during the pull and obstruction or jamming risks. Include load monitoring and stopping criteria in the installation plan.

Where do cleats and the cable gland fit?

I assign the route load to the hardware designed to carry it. IEC 61914:2021 covers cable cleats and intermediate restraints, including declared duties and the specified mounting surface. IEC 62444:2010 covers complete cable glands. These are distinct component functions with their own installation conditions.

A gland’s cable-retention result does not make it a pulling attachment or an unrestricted vertical support. Establish the unsupported length, attached loads and support reactions, then match the hardware to that duty. The cleat and gland guide explains their relationship at an enclosure entry.

At the last bend, I check the required straight approach, enclosure projection and support location. Keep the cable from pressing sideways against the gland mouth. The bend-radius selection guide covers that geometry, while the strain-relief guide addresses service loads after installation.

Keep a usable route calculation sheet

When an input changes, I recalculate the affected downstream segments. Keep an unknown load visible rather than burying it in a general factor.

Conclusion

I calculate cable pulling loads by following the force through the route. A horizontal segment adds friction, an unsupported rise adds weight, and a bend changes tension and creates a local sidewall load. Carrying the exit tension forward keeps those contributions connected without confusing their units or locations.

Before selecting the pulling arrangement, compare the local results with the cable, attachment, bend and support limits. At the enclosure, provide the intended cable support and approach geometry so the gland performs its specified entry and retention functions.

FAQs about preliminary cable-pulling loads

Does the horizontal calculator give the total pulling tension?

It gives the added friction of a level sliding length. Add the incoming tension, then calculate any further straights, bends or vertical portions with their applicable models. I use the segment result as one line on the route sheet, not as the final value for every route.

Can I enter cable weight in kilograms as a force?

Kilograms describe mass. Multiply mass by gravitational acceleration to obtain force in newtons. If a datasheet already gives weight per length in N/m, multiply by length without applying gravity again. Label kg/m and N/m distinctly throughout the calculation.

Why is sidewall pressure expressed in N/m?

The simple single-cable bend index divides the local exit tension by the applicable bend radius. Its unit is force per length, even though it is commonly called sidewall pressure. A contact-stress calculation in N/m² would also need contact-area and load-distribution information.

Can a cable gland carry the weight of a vertical cable run?

Use it for that duty only when the exact gland, cable and mounting arrangement are specified for the resulting load. I first identify the unsupported length and other supports, then select the hardware for that load path. Cable retention alone does not make the gland a pulling attachment.

Will a larger bend always reduce the final pulling tension?

A larger radius lowers the simple sidewall index at the same tension. In the exponential bend approximation, tension depends on incoming tension, friction and angle, so changing radius alone leaves that estimate unchanged. Check the actual cable stiffness, bend geometry and equipment model as well.

Sources and technical references

• Southwire — Power Cable Installation Guide (2005), pulling tension and sidewall models, accessed 2026-10-28.
• NIST — Guide to the SI, Chapter 4: mass, force and pressure units, accessed 2026-10-28.
• IEC 61914:2021 — Cable cleats for electrical installations, accessed 2026-10-28.
• IEC 62444:2010 — Complete cable-gland construction and performance scope, accessed 2026-10-28.

Products

• Cable Glands