The best way to reduce sidewall loading is to increase the bend radius at every curved point in your pull, choose the correct pulling direction, and use lubricant to lower friction. Sidewall loading (also called sidewall bearing pressure) is the crushing force a cable experiences as it’s pulled around a bend in conduit, and it’s the single most common cause of cable damage during installation. The good news: every factor in the equation is something you can control with planning.
How Sidewall Pressure Works
Sidewall pressure follows a simple relationship: it equals the pulling tension coming out of a bend divided by the radius of that bend. A tighter bend or higher tension means more force pressing the cable against the conduit wall. This force can crush insulation, deform shielding, and permanently damage conductors. When metal-taped cables are bent too sharply under tension, the helical tapes can separate, buckle, and cut into the insulation beneath them.
Because the formula is a ratio, you have two direct levers to pull: make the bend radius larger, or reduce the tension reaching that bend. Every strategy below targets one or both of those variables.
Use the Largest Possible Bend Radius
This is the single most effective change you can make. Doubling the bend radius cuts sidewall pressure in half, all else being equal. Southwire’s engineering guidance states that conduit bends, sheaves, and any curved surface a cable passes over under tension should use radii larger than the cable’s minimum bending radius specifically because of sidewall bearing pressure limits.
In practice, this means specifying long-sweep conduit elbows instead of standard 90-degree bends wherever the route allows. If you’re designing a new run, plan the route to avoid tight turns entirely. Where bends are unavoidable, factory-made sweeps with a known radius are preferable to field bends, which can end up tighter than intended. Even a modest increase in radius pays dividends. For example, going from a 24-inch radius to a 36-inch radius on the same pull reduces sidewall pressure by a third.
Pull From the Right Direction
Pulling direction has a surprisingly large effect on peak tension, and therefore on sidewall pressure. The general rule: feed cable into the end of the conduit run that has the most bends, or into the uphill side of the route. This keeps cumulative tension lower through the sharpest curves.
The reason is that tension builds as cable passes through each bend and each straight section of conduit. If you pull so that the cable hits the tightest bends early (near the feed point, where tension is still low), those bends multiply a smaller number. Pull from the wrong end, and the cable arrives at those same bends already carrying high tension from the entire run behind it.
Real calculations show the difference is significant. ELEK Software modeled a conduit route and found that pulling from the optimal direction required 20% less force than pulling from the opposite end. In a lubricated conduit, the peak tension dropped from roughly 2,800 N to about 2,200 N just by reversing the pull direction. That’s a meaningful reduction in sidewall pressure at every bend along the route, with zero additional hardware or cost.
Reduce Pulling Tension With Lubricant
Cable pulling lubricant reduces friction between the cable jacket and the conduit wall, which directly lowers the tension that builds over the length of the run. Lower tension at each bend means lower sidewall pressure at each bend. In the same ELEK model mentioned above, lubricating the conduit cut the peak pulling tension by roughly 65% compared to a dry pull. That single change transformed a borderline pull into a comfortable one.
Apply lubricant generously at the feed point and at any accessible intermediate point. The lubricant needs to coat the cable and the inside of the conduit continuously, not just at the entrance. For long runs, a lubricant pump or drip system at the feed end helps maintain coverage. The type of lubricant matters too: it should be compatible with both the conduit material and the cable jacket. Using the wrong product can swell or degrade certain jacket compounds over time.
Use Larger Sheaves and Rollers
Anywhere the cable passes over a sheave, roller, or guide during installation, the diameter of that device acts as the bend radius in the sidewall pressure equation. A larger sheave spreads the load over a longer arc of cable, reducing the peak force at any single point.
The impact on cable life is dramatic. Mazzella Companies published fatigue data showing that increasing sheave diameter from 12 inches to 22.5 inches for a 3/4-inch wire rope extended fatigue life by nearly five times. While that example applies to wire rope rather than power or fiber cable, the underlying physics is identical: a gentler bend means less stress per unit length of cable. For cable installation, oversized sheaves at pull points and direction changes are one of the easiest upgrades to specify.
Limit the Number of Bends Per Pull
Every bend in a conduit run multiplies the incoming tension by a factor that depends on the bend angle and the friction coefficient. Three 90-degree bends in sequence don’t just add pressure; each one amplifies the tension for the next. Industry practice typically limits a single pull to no more than 360 degrees of total bending (the equivalent of four 90-degree turns), but sidewall pressure often becomes the limiting factor well before that point.
If your route requires more bending than a single pull can safely handle, break the run into segments with pull boxes or junction boxes at intermediate points. A pull box lets you de-tension the cable and start the next segment fresh, resetting the cumulative tension to near zero. This is especially important on long runs with elevation changes, where gravity adds to the tension on uphill segments.
Monitor Tension in Real Time
Pre-calculation tells you what the tension should be. Real-time monitoring tells you what it actually is. Tension can spike unexpectedly due to a conduit joint that wasn’t fully seated, a cable snag, or lubricant that dried out partway through the run. A dynamometer or load cell on the pulling line gives the crew a live reading so they can stop before sidewall pressure exceeds the cable’s rated limit.
For critical installations, continuous tension monitoring has become standard practice. The principle is the same one used in bridge construction, where engineers install sensors on cable strands to track tension fluctuations over months of work. In cable pulling, the monitoring window is shorter but the stakes are similar: once sidewall pressure crushes an insulation layer or deforms a conductor, the damage is hidden inside the conduit and may not show up until the cable fails in service.
Know Your Cable’s Sidewall Pressure Limit
Every cable type has a maximum allowable sidewall pressure, typically expressed in pounds per foot. Exceeding it risks crushing the insulation or deforming internal components. For power cables, this limit varies by construction: a solid dielectric cable tolerates different forces than a paper-insulated lead-covered cable. Fiber optic cables have their own crush load criteria defined by industry standards from ICEA, and manufacturers like Corning publish specific recommendations for each cable family.
Before any pull, calculate the expected sidewall pressure at every bend using the formula (tension out of bend divided by bend radius) and compare it to the cable manufacturer’s limit. If the numbers are close, that’s the signal to increase a bend radius, add lubricant, change pull direction, or break the run into segments. Running the calculation after the cable is already stuck in the conduit leaves you with no good options.
Sidewall Loading in Silos and Bins
Outside of cable installation, sidewall loading also refers to the lateral pressure that stored bulk materials exert on the walls of silos, bins, and hoppers. The physics differ, but the reduction strategies share a common theme: control the geometry.
Engineers use internal inserts, such as cones, inverted cones, and convergent rings, placed at calculated positions inside the silo to redirect material flow and redistribute wall pressure. Research has shown that the shape, size, and placement of these inserts significantly affect both the flow pattern and the pressures on the silo wall. For cylindrical metal silos, one proven configuration places two internal convergent rings at heights proportional to the silo diameter, with specific angles and lengths tuned to the stored material. These inserts were first proposed by Johanson and Kleysteuber and have since been validated in both laboratory models and full-scale installations. Beyond inserts, modifying the hopper geometry itself, using a mass-flow design instead of a funnel-flow design, produces more uniform wall loading and reduces the peak lateral pressures that cause structural problems.