NEC Bend rule for secondary structure?

I had a job once in a high rise rewiring some new chillers. It was 2" pipe in the slab and can't remember what size the conductors were but they were THW and aluminum we couldn't pull them out, probably a 100 foot run. Based on the exposed elbows we could see they had more than 360 degrees even if they went like a crow flies and based on the wire size the pipe was overfilled. We ended up pulling them out with a comealong and had to constantly re-rig. The new wire was copper and smaller so they went in ok pulling by hand.
 
Code permits exactly 360 degrees and no more.

The friction equation used to calculate pull tension around bends is called the capstan equation, and in this equation the radius of curve is ignored. This equation is only approximate.

The reality is that a very broad sweep is different than a tight radius bend. But this doesn't matter from a code perspective.
 
You could install a pull box and bury it but you would probably pull through it anyway so it's usefulness is non-existent. I wouldn't do a thing until I failed the trench inspection.
 
JimFromAustin said:
Inspector is essentially counting a change in direction as 45 degrees. Even for long sweeping bends.
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I'm reading this to mean the inspector has already given thumbs down

JimFromAustin said:
Is there a method to calculate the radius (or some other parameter) of the bend and show that the bend may actually be only 15 degrees, not 45 degrees?
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As I described above, there is no such method in code.

One slim possibility is that if you can get a pull tension calculation that takes into account shallow sweeps, that the jurisdiction might grant special permission to exceed the code limit. This is probably more expensive than adding a pull point, however.
 
I'd expect a sweep to introduce less friction than a tight bend of the same angle.

I don't have direct experience with this, but other members with field experience of large pulls can probably chime in
 
winnie said:
I'd expect a sweep to introduce less friction than a tight bend of the same angle.
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In this case where the conduit kind of flows freely back and forth the natural bends won't change the pulling very much when compared to a straight run. When straight pulling the wire is going to lay on the bottom of the raceway, the gradual changes in bend will cause it to contact the sides of the raceway but with much less friction than it would going around a factory 90° elbow. I'm guessing that if you did some fancy calculation it might say something different which isn't happening in the real world. Trying pulling through a plumbing elbow which is 90° versus a long 90° sweep and let me know which was easier. :)
 
winnie said:
I'd expect a sweep to introduce less friction than a tight bend of the same angle.

I don't have direct experience with this, but other members with field experience of large pulls can probably chime in
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I always was taught that the purpose of sweep or large radius bends was to reduce friction- both pulling wire and carrying liquids.

-Hal
 
hbiss said:
I always was taught that the purpose of sweep or large radius bends was to reduce friction- both pulling wire
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Me too. For big data and telephone runs with 4" EMT we always installed extra long sweeps. Depending on the size in the spec a single sweep elbow might utilize an entire 10' length of EMT.
 
I would not add any more factory bends, and I would likely re-do the bends in and out of the trench.
What is the radius of the 2" 90's out of the trench? If they are standard PVC 90's I'd remove them and install 24" radius 90's.
 
infinity said:
In this case where the conduit kind of flows freely back and forth the natural bends won't change the pulling very much when compared to a straight run. When straight pulling the wire is going to lay on the bottom of the raceway, the gradual changes in bend will cause it to contact the sides of the raceway but with much less friction than it would going around a factory 90° elbow. I'm guessing that if you did some fancy calculation it might say something different which isn't happening in the real world. Trying pulling through a plumbing elbow which is 90° versus a long 90° sweep and let me know which was easier. :)
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I think we agree, if I wasn't clear :)
 
hbiss said:
I always was taught that the purpose of sweep or large radius bends was to reduce friction- both pulling wire and carrying liquids.

-Hal
Click to expand...

My understanding of the 'standard approximation' is that the sweep or large radius bend does not reduce total pulling friction but does reduce sidewall pressure.

And to repeat, I know this is an approximate equation and I expect that in the real world a sweep or large radius bend will reduce friction.
 
infinity said:
You could install a pull box and bury it but you would probably pull through it anyway so it's usefulness is non-existent. I wouldn't do a thing until I failed the trench inspection.
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yes, failed. have one too many 45 according to inspector. I think I see a way to solve this problem. This install is capable of 100Amps. The reason I'd ever pull was to install bigger capacity. Since conduit not capable of increased capacity, a new trench would be required anyway.

If you look at this photo, the entrance point is a 90, followed by "45" to maneuver around some dirt and foundation. Then another 45. if I eliminate that 45 near the entrance, and just make a beeline from the 90 to the 45 at the bottom, that should eliminate one of the 45's and it should be good. Not looking forward to the digging in 100 degree heat!

Also shown below are some ariels of the run.

QUESTION: What is the trick to cutting this conduit to make these changes w/o cutting the wire?

problem.jpg
IMG_5092.JPEG

IMG_5093.JPEG
 
For a trench like that I put in a quazite box pull box right in the middle of the run. Set the box with rebar to be flush with your new finished grade (I go in the sidewalk so its easy to find). I use a two 36" radius 45° bends to turn up into the bottom of the pull box to get about 3" into the bottom box. I glue on a 'end bell' for ease of pulling. Fill from the bottom of trench to bottom 2" of box with pea gravel or drain rock I am sure your electrician can assist with this.
1751213703417.png
 
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