NEC Bend rule for secondary structure?

I would just bury junction box somewhere near the middle of the run and call it a day. You won't use it anyway.
 
If the wire is in already what’s the point of fixing it. For later use of the conduit?
 
LarryFine said:
My first choice would have been no elbows in the horizontal runs at all, only sweeping bends.
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Are you saying sweeps don't count as bends for code sections 3xx.24 not more than 360o between pull points? Code says bends not elbows.

326.26, 342.24(B), 344.24(B), 348.24(B), 350.24(B), 352.24(B), 353.24(B), 354.24(B), 355.24(B), 356.24(B), 358.24(B), 362.24(B) All have the same wording.
(B) Number in One Run.
The total degrees of bends in a conduit run shall not exceed 360 degrees between pull points.
 
Fred B said:
Are you saying sweeps don't count as bends for code sections 3xx.24 not more than 360o between pull points? Code says bends not elbows.
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Not at all. I'm saying that it would provide for the absolute minimum total bend angle.

A 45 adds its fixed amount of angle to the run, where a long sweep would absorb it.

A run of long sweeps would be tallied by the total turning per a compass while walking it.

I had an inspector who counted every offset as a pair of 45s, regardless of how shallow it was.
 
Speaking from actual pulling experience. A long gradual bend presents much more resistance than a tight "factory" made bend. I believe that it has to do with the amount of surface area contact that the wire and pipe have along the inside radius of the bend. I had a 100 foot pull for a pool. 2 90s and one long, 30 foot sweep with #12 THWN. It took us about an hour and 3/4 of a bottle of lube to get the conductors in. We set up test at the shop because i couldn't figure out why it was so hard. I recreated the pull exactly as it was on the site and then I eliminated the 30' sweep and installed a "factory" 90. The conduit length for the factory bend was a longer pull of course. The longer pull with the factory bend took all of about 10 minutes including hooking the conductors to the snake. Once again the pull with the long 30' sweep took just as long as it did at the job site. We all have to pay for our education.

I pass this information on when ever I can and recently spoke with a contractor that had about a 50' sweep. I had warned him about the difficulty he was about to run into. He followed the trench that was dug for him. Upon meeting up with him, he confirmed that the pull was an absolute struggle and use 2 bottles of lube. Never again will he not use the tight "factory" type bends.

If we look at the amount of contact surface area when we use tight bends versus a long gradual bend the difference can be staggering. Take a 1" sweep with an 8" radius, 8"x4=32" with the max allowed of 360*. That gives me just 32" of surface area resistance that we are pulling against. Now look at the long gradual sweep from my personal experience. 2 sweeps with 8" radius each. 8"x2=16". 16" + the long 30' inside radius sweep. That gives me a total of 376" of surface resistance that we are pulling against and the harder we pull, the more additional resistance we create as we pull the wire tighter against the wall of the conduit.
 
Thanks for both the anecdote and then doing a controlled test. That is real science. I wonder how well pulling tension calculators agree with your results

( Old joke: What's the difference between theory and practice?

In theory there is no difference.)

The approximation for friction is that it depends only on the materials involved and the normal force pushing the materials together, not the area of contact. The other approximation is that a bend introduces a tension multiplier that only depends on the bend angle. Clearly these approximations fail in your long sweep case, and fail in a way that goes against my intuition.
 
I'm not sure what you are saying Winnie. I didn't state that I used actual engineering math. I simply did a real world test and presented my results. My guessing at the resistance factor is purely conjecture. However the real world practical application has been proven time and time again in my 40 year career as an electrical contractor.
 
eagleelec said:
I'm not sure what you are saying Winnie. I didn't state that I used actual engineering math. I simply did a real world test and presented my results. My guessing at the resistance factor is purely conjecture. However the real world practical application has been proven time and time again in my 40 year career as an electrical contractor.
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I'm saying that your real world experience combined with doing a controlled test adds useful knowledge to the world because it shows the limits of theory.

I described a couple of approximations used in the theory and saying that your real world measurements show the approximations to be wrong, and wrong in the not conservative direction. This makes your data especially useful.
 
The few times I've played around with pull tension calculators the tension always increased when the radius of the bend decreased. This is why we don't pull wires through bends like plumbing elbows.
 
Just glue the conduit together as you pull the wire in, so the pull tension is minimal.
I herd that on authority from the sales guy at home-depot electrical isle and read that in DIY chat room.
 
It is certainly interesting to hear someone say the larger radius the larger the tension. A Winnie noted, frictional force does not depend on surface area , at least theoretically. There could be other factors at play. One factor that is often ignored is the force required to bend an unbend the conductors. This would be more prevalent with larger conductors.
 
electrofelon said:
It is certainly interesting to hear someone say the larger radius the larger the tension. A Winnie noted, frictional force does not depend on surface area , at least theoretically. There could be other factors at play. One factor that is often ignored is the force required to bend an unbend the conductors. This would be more prevalent with larger conductors.
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It would be fun to see a curve of pulling tension vs bend radius, everything else held constant.
 
You would never be able to pipe a gas station if you couldn't get away with flexing pipe in trenches without counting it against the 360 rule. I've never counted it, and never had an inspector have a problem with it, including state inspectors. The opposing bends by the trash cans is a bit much though. That trench should have been cut a bit straighter.
 
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