Straights add, bends multiply
A straight length of raceway costs you the weight of the cable in it, multiplied by the length, multiplied by the coefficient of friction. That is an addition, and it is the same addition wherever in the run it happens. Fifty feet of straight pipe adds the same tension whether it is the first fifty feet or the last.
A bend does not work like that. A bend multiplies whatever tension has already built up behind it, by e raised to the power of the friction coefficient times the bend angle in radians. At a friction figure of 0.5 a ninety degree bend multiplies by about 2.19. Behind 100 pounds it costs you 119 pounds. Behind 600 pounds the identical bend costs you 715. Same pipe, same angle, same lubricant, six times the price.
That is why the running order matters, and it is the single most useful thing this page has to say. Take the run the calculator loads with: 80 feet of straight, a ninety on a two foot radius, 30 feet, a forty five, and a fifteen foot vertical rise, with cable at 1.9 pounds per foot and friction at 0.5. Pulled in that direction it comes out at 318 pounds. Reverse it — pull from the other end — and the footage, the degrees and the friction are all identical, and the answer is 139 pounds. Less than half, for changing nothing but which end the winch is on. The calculator runs both directions on every set of numbers for exactly that reason.
Sidewall pressure is a different number and it is the one that hurts the cable
Tension is what the rope and the grip feel. Sidewall pressure is what the cable jacket feels where it is pressed against the outside of a bend, and it is the tension leaving that bend divided by the bend radius. A 900 pound tension coming out of a two foot radius bend is 450 pounds per foot pressing the cable into the pipe wall. The same 900 pounds out of a five foot sweep is 180.
Two consequences follow. The first is that the worst bend is nearly always the last one before the pulling end, because that is where tension is highest, even when every bend in the run is the same ninety degrees on the same radius. The second is that radius is a lever you can actually pull: replacing a tight shop bend with a factory sweep at the far end of a run cuts the sidewall figure at the place it matters, and it also lowers every tension figure downstream of it because the bend is doing the same multiplication either way.
The friction number is the weak link
Everything above is arithmetic you can check. The coefficient of friction is not — it is a property of a specific jacket compound against a specific raceway interior with a specific lubricant between them, at whatever temperature the job is happening at, and it is the input this whole calculation is most sensitive to. Change it from 0.4 to 0.6 and every bend multiplier moves, compounding down the run.
So take it from the people who publish it. Lubricant manufacturers publish friction figures for their product against common jacket and raceway combinations, and cable manufacturers publish theirs. This page supplies a placeholder so the arithmetic has something to run on, and that placeholder is not a recommendation, a typical value or a safe assumption. If the answer is close to any limit you care about, the honest response is to treat the friction figure as a range and run it at both ends.
What this model leaves out
| Not modelled | Which direction it moves the real answer |
|---|---|
| Weight correction for several cables in one raceway | Up. A triangular or cradled configuration presses harder than one cable lying alone. |
| Starting from a dead stop mid-pull | Up. Static friction exceeds sliding friction, and every stop restarts at the higher figure. |
| Lubricant that has been wiped off before the far bends | Up. The friction figure applies where the lubricant actually is. |
| Couplings, joints, an out-of-round raceway, debris | Up. |
| Cable that is warm and flexible rather than cold and stiff | Down, usually, but not by an amount worth planning around. |
Every one of those pushes the same way except the last. A calculated figure is a floor, not a forecast, and the margin between it and any limit you are working to is what absorbs all of it.
The pages either side of this one
Before the pull there is the question of whether it fits at all, which is the conduit fill calculator for conductors by insulation type and trade size, or the low-voltage pathway calculator for cables given by outside diameter — that one also covers the jam ratio, where three identical cables wedge across the bore and stop dead regardless of what the fill arithmetic says. The bends themselves are marked out on the conduit bend and offset calculator. The raceway material take-off is on the conduit run take-off calculator, and what the pull will drink is on the pull lubricant calculator. If the cable is coming off a reel and you want to know whether there is enough on it, that is the cable reel capacity calculator.
The parts this page will not touch
A cable under pulling tension is a spring with several hundred pounds in it. When a rope, a swivel, a grip or an eye lets go, the stored energy comes out along the line of the pull, and everything standing in that line — people first — is in the path. The tension figures on this page exist so you know how much energy is in the system, not so you can decide it is acceptable. Rope, grip, swivel and anchor ratings come from their makers, and the people who set up a pull of any size are the ones qualified to judge it.
A raceway you are pulling into is very often a raceway that has something else in it, and the assumption that a circuit is dead is the assumption that kills people in this trade. Whether a conductor is energised, how that gets established, and who is permitted to work on or near it are matters for a licensed electrician working to the rules adopted where you are. Nothing on this page establishes any of it and nothing here is a procedure.
Vaults, manholes, pull pits and crawl spaces where cable pulls happen are confined spaces, and the hazard in them is atmospheric long before it is physical. People die in them without warning, and rescuers die going in after them. This is named here so it is not a surprise; it is not something a calculator page should give a procedure for.
This page does not supply limits and cannot judge yours. Fill percentages, support and strap spacing, minimum bend radii, tray loading rules, cover depths and every other installation limit come from the code your jurisdiction has adopted and from the manufacturer of the product you actually bought, and the two do not always agree. The arithmetic here runs on figures you type in. Whether the result is acceptable is a conversation with a licensed electrician and with the inspector who signs the job off.
Questions people ask
Why does the same run give a different tension depending on which end I pull from?
Because bends multiply rather than add. A bend applies its multiplier to whatever tension has already accumulated behind it, so a bend sitting near the feeding end multiplies a small number and the same bend near the pulling end multiplies a large one. Reverse the run and the bends swap positions relative to the accumulated straight-length tension, and the final figure moves even though the total footage and the total degrees are unchanged. The calculator runs both directions every time so the comparison is in front of you before the rope goes on.
What coefficient of friction should I use?
The one published for your jacket compound, your raceway material and your lubricant, by the people who make them. There is no general answer and this page deliberately does not offer one — the default in the field is a placeholder so the arithmetic runs, and the hint on the field says so. It is also the input the result is most sensitive to, more so than length, so if the answer lands anywhere near a limit you care about, run it at both ends of whatever range you were given rather than at a single number.
What is sidewall pressure and why is it separate from tension?
Tension is the pull along the cable, in pounds. Sidewall pressure is the force squeezing the cable against the outside of a bend, in pounds per foot of bend radius, and it is simply the tension leaving that bend divided by the radius. They are different because a small radius concentrates the same tension into a much higher pressure. It is normally the last bend before the pulling end that is worst, because tension is highest there, and increasing that radius is the most direct thing you can do about it.
Does the calculator tell me whether my pull is within the cable limits?
No, and it is built not to. Maximum pulling tension and maximum sidewall pressure are figures from the manufacturer of the cable you are actually pulling, and they depend on the conductor material and size, the construction and the type of grip. Enter yours and the page restates them beside the computed figures so you can see both together. It will not tell you the pull is acceptable, because that judgement belongs to a licensed electrician looking at the actual installation.
What does back tension at the feeding end mean and should I put a number in it?
It is whatever is already resisting the cable before it enters the first foot of raceway — reel drag, the payoff arrangement, a bend in the slack between reel and pipe. Zero assumes the cable arrives completely free, which never quite happens. Anything you enter there is multiplied by every bend downstream of it, so a modest reel drag at the feed can turn into a large number at the far end. That is the same compounding described above, working against you from the first foot.