Why 360 Degrees max in conduit run?

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It question came up during a small debate over NEC rules etc.. I never thought about this one, just always accepted it. Where did they get the 360 degrees for max in a conduit run? Was it ever more than that in the early years? I have seen people 'cheat' on conduit runs when it was 'convenient' seen as high as 400 degrees in the runs.

So is there a 'history' for the '360' max in a conduit? Not just because it represents a circle. lol
 
To minimize damage, yes. The question is what's so special about 360 degrees? They could have just as easily said 270, or 400, or even 327. I'm going to guess that number is completely arbitrary and selected only because most people know that a full circle is 360 (or 2pi radians or 32 compass points, etc :D ) so it's easy to visualize.
 
Maybe it's akin to a rope around a drum or capstan, and the friction rises exponentially with wraps.
 
The capstan equation is exactly the reason for this limit.

The specific value of 360 degrees is an arbitrary round number.

But the key is that pulling forces around turns are exponential with the amount of bend. Each additional radian of bend (about 57 degrees) multiplies the pulling forces by a factor of e (about 2.8). (Or each additional 90° multiplies the pulling forces by a factor of 4.8)
 
The people who wrote previous editions of the code actually had some experience. Anyone who ever pulled wire through four 90s knows that it would be next to impossible with very little more.
 
CoolWill said:
The people who wrote previous editions of the code actually had some experience. Anyone who ever pulled wire through four 90s knows that it would be next to impossible with very little more.
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Is somewhat of situational thing though. I've pulled small THHN/THWN stranded conductors through more than 360 degrees of bend rather easily a time or two, especially in metal raceways that usually have less drag than PVC has.

Also had instances with larger conductors in underground PVC where adding a third 90 degree bend made pulling by hand pretty much impossible where the same conductor/same raceway combination often can be done by hand pulling when there is no more than two 90 degree bends.
 
CoolWill said:
The people who wrote previous editions of the code actually had some experience. Anyone who ever pulled wire through four 90s knows that it would be next to impossible with very little more.
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Code doesn't prohibit pulling wire through more than four 90s. It requires a pull point so that the wire _could_ be pulled into the conduit system with no more than four 90s. Lots of people report installing wire going right through the code required pull point without stopping.

My guess is that the 360 degree limit has history with older rubber and fabric covered wires rather than modern high lubricity insulation.

The same physics still applies; but since the friction of modern wires is lower, more and more people will experience the 360 degree limit being very conservative.

Something else to keep in mind: it's probably lots easier to pull the wire in when it is new and the conduit is fresh. Those intermediate pull points are probably a big help down the road when the conduit needs to be re-used.

In a similar vein, bends are a tension multiplier. This means that if you have a bend right near the beginning of a pull (say the initial 90 of an underground run), if you _push_ the wire into that bend you can 'unload' it and that bend won't add to the total pulling tension. But down the road if you need to pull the wire out you can't 'unload' it, and it will multiply the pulling tension.

Jonathan
 
With stranded conductors you can easily pull through more than 360° worth of bends. With a tugger we would typcially pull through 6+ 90's with no problems.
 
kwired said:
Also had instances with larger conductors in underground PVC where adding a third 90 degree bend made pulling by hand pretty much impossible where the same conductor/same raceway combination often can be done by hand pulling when there is no more than two 90 degree bends.
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That is the reason that the code allows the use of rigid metal 90s in place of PVC for underground PVC conduit. Rigid 90s have far less drag than PVC ones do and you cannot burn through the inside curve of a Rigid Metal 90 like you can with PVC. There is one added challenge though. All parts of the unbonded Rigid Metal 90 must be 18 inches below ground. That means that the trench must be 18 inches plus the height of the throw of that size of 90 deep from the ground level to the bottom of the trench.
infinity said:
With a Tugger we would typically pull through 6+ 90's with no problems.
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If there are PVC 90s in the run you are very likely to burn through the inside curve of one or more of the 90s It that happens you have to expose the PVC 90, free the now trapped conductors, replace the 90, and repull the wire without cutting through the PVC again.
 
winnie said:
Something else to keep in mind: it's probably lots easier to pull the wire in when it is new and the conduit is fresh. Those intermediate pull points are probably a big help down the road when the conduit needs to be re-used.
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We had to redo the feeders for the escalators in a very large building. The existing feeders were the old AA-1350 aluminum and had been in the conduit for over 50 years. The aluminum wire and hardened into place and all of the attempts to pull it out failed. The project manager got all ticked off and called us a bunch of wimps because his estimate did not include any rebuilding of the conduit. The foreman got very hot because of the project managers insulting comments and sent 2 of us back to the shop to get every wire pulling machine they had. He warned the construction superintendent that it was futile and that the pulling machines would get destroyed. The Super hated that project manager because he always blamed the electricians when anything didn't work. So the Super said "Burn them up. Lets see him lie his way out of this one." We burned up all 5 wire pulling machines before the shop new what was happening. When we asked for new pullers so we could destroy them to they went ballistic. The owner told the project manager to get work clothes on and go over and show us how he could get it done. Not wanting anyone badly hurt I called a safety stop on the pull. The owner the super and a Union business agent got together and they all agreed that there was a lot of risk of the pulling machine or the wire pulleys breaking loose and cutting someone in half. The business agent said he was going to move me to another company to prevent retaliation but they owner convinced him that was unnecessary because he would fire anyone who tried to jam me up. I thanked him and shook his hand. In the end we had to cut several of the 90s out of the runs and install new ones using Erickson couplings. We didn't use any more tuggers because of the hazard of the machine breaking free of its drop in anchors. I went to an arborist supply and bought 2 of the 3 sheave pulleys and 150 feet of bull rope, which is what the arborist call the 1 inch nylon static kernmantle rope used to lower entire sections of trees down to the ground, when you have to "top" the tree down to a length were you had enough room to fell it safely. The 3 sheave pulley that was moving cut the required force to 1/8th of the actual force needed so we were able to save several of the 90s that were in places that would have been grim to rebuild.
 
hornetd said:
If there are PVC 90s in the run you are very likely to burn through the inside curve of one or more of the 90s
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Is this really a thing? I've heard about this forever but I have never seen it happen when you use the correct size and type of rope.
 
infinity said:
Is this really a thing? I've heard about this forever but I have never seen it happen when you use the correct size and type of rope.
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Consider that the size and type of rope does not change the amount of force that needs to be applied to get the wire to go through the conduit and around each bend. In long runs the amount of force needed can cause the wires to cut through the schedule 40 PVC elbow as all of that pulling force is imposed on the PVC that the wire has to pass over. The bends closest to the place were the wire entered the conduit have to hold up against the longer length of the wire that will grind over it to go to the end of the run. If it is a high tension pull and especially if it is done too quickly the heat of the friction can rise to the point were the PVC's yield temperature is exceeded. It doesn't have to get hot enough to melt the PVC. It only has to heat it enough so that it begins to soften. As more wire passes over that small portion of the elbow it gets hot enough for the wire to erode the PVC until it is cut through.

When Rigid metal conduit elbows are used the steel conducts the heat away from the friction point more quickly thus holding the temperature down and it is a lot harder than PVC so its abrasion resistance is far better than that of PVC.
 
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