Gradual bends in underground PVC

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Carultch said:
I defined "gradual bend" when I opened this discussion, as a bend that is made with the room temperature flexibility of the conduit. I.e., no factory elbows or sweeps are used, no 5 degree couplings, no bending equipment that applies heat to make it bendable, and no bending equipment of any kind.

Pick up a piece of 1" PVC conduit, and observe how it bends under its own weight. Now with a partner, see how much curvature the two of you can bend the conduit by human forces alone, and without intentionally heating it to make it bend more. That's what a "gradual bend" is.

If desired, we could consider a radius of curvature for this, such as 40 ft, to make a more objective definition.
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Thanks, now I know what gradual is. Regardless, it will compressed to some degree across this gradual bend. It is to what greater of compression would be allowed which is ambiguous. It is of my opinion that if one were to use common sense this would not be issue. Which is sometimes elusive.
 
A lot of hair splitting going on here when in real life, at least in my experience, inspectors are not breaking out calculators and transits to determine the exact degrees of deflection. :cool:


Going above ground for a moment and I will ask how many of you count the box offsets you may make?
 
Carultch said:
I defined "gradual bend" when I opened this discussion, as a bend that is made with the room temperature flexibility of the conduit. I.e., no factory elbows or sweeps are used, no 5 degree couplings, no bending equipment that applies heat to make it bendable, and no bending equipment of any kind.

Pick up a piece of 1" PVC conduit, and observe how it bends under its own weight. Now with a partner, see how much curvature the two of you can bend the conduit by human forces alone, and without intentionally heating it to make it bend more. That's what a "gradual bend" is.

If desired, we could consider a radius of curvature for this, such as 40 ft, to make a more objective definition.
Click to expand...
Thanks, now I know what gradual is. Regardless, it will compressed to some degree across this gradual bend. It is to what greater of compression would be allowed which is ambiguous. It is of my opinion that if one were to use common sense this would not be issue. Which is sometimes elusive.
 
1 inch schedule 40 and between 70 and 100 deg F one can probably pretty easily make a complete circle with only four 10 foot pieces without heating it up or using other mechanical means to bend it. Can possibly come pretty close to doing that with only one piece of 1/2 inch.
 
iwire said:
A lot of hair splitting going on here when in real life, at least in my experience, inspectors are not breaking out calculators and transits to determine the exact degrees of deflection. :cool:


Going above ground for a moment and I will ask how many of you count the box offsets you may make?
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We count offsets, or at least I used to. My guys were trained to but I haven't checked up on them with a protractor and calculator.
 
templdl said:
Thanks, now I know what gradual is. Regardless, it will compressed to some degree across this gradual bend. It is to what greater of compression would be allowed which is ambiguous. It is of my opinion that if one were to use common sense this would not be issue. Which is sometimes elusive.
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In many cases, the code doesn't "allow" common sense. In many of those cases, it is understandable because the code has to put its foot down and set a limit at some point. I have seen the number of bends rule broken as many times as just about any code, so I would guess that inspectors are not really aggressive on this code.
 
Strathead said:
In many cases, the code doesn't "allow" common sense. In many of those cases, it is understandable because the code has to put its foot down and set a limit at some point. I have seen the number of bends rule broken as many times as just about any code, so I would guess that inspectors are not really aggressive on this code.
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I thought this string of posts was in regard to "gradual bends" a term that is ambiguos which was the intent of my response. I realize that ast there are those who are not able to think and must really on simple black and white answers.
 
templdl said:
I thought this string of posts was in regard to "gradual bends" a term that is ambiguos which was the intent of my response. I realize that ast there are those who are not able to think and must really on simple black and white answers.
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And I wasn't saying that your common sense approach is not desirable, just that it is usually not the journeyman in the field who gets to decide to use common sense. regarding the term "gradual bends"... it still contains the word bends and that isn't ambiguous.
 
Strathead said:
And I wasn't saying that your common sense approach is not desirable, just that it is usually not the journeyman in the field who gets to decide to use common sense. regarding the term "gradual bends"... it still contains the word bends and that isn't ambiguous.
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It is when nothing is defined but generalizations are used that it somewhat troublesome when there are no real standards or references.
 
iwire said:
A lot of hair splitting going on here when in real life, at least in my experience, inspectors are not breaking out calculators and transits to determine the exact degrees of deflection. :cool:


Going above ground for a moment and I will ask how many of you count the box offsets you may make?
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Yes, it is getting to the point where thinking is being removed from the trade and all that needs to be done is to be able to read and memorize following the rules in order to qualify as an electrical.
 
iwire said:
....Going above ground for a moment and I will ask how many of you count the box offsets you may make?
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I don't. Nor do I shoot a laser to make sure that every bar joist is exactly in line with the one before it or after it. Nor do I check that hard to make sure that the sum total of every stick I run in any direction, be it north, south, east, or west is so perfectly flat, true, level, and plumb, and that it has zero degree deflection in it. I count my nineties and my big offsets, anything else is too much to worry about. Send me to electrician purgatory if you want, I've got some more work to get done before I check in there.
 
iwire said:
A lot of hair splitting going on here when in real life, at least in my experience, inspectors are not breaking out calculators and transits to determine the exact degrees of deflection. :cool:


Going above ground for a moment and I will ask how many of you count the box offsets you may make?
Click to expand...

I do, sort of. I don't put 4 full 90's in a conduit and a box offset as well.
 
templdl said:
Yes, it is getting to the point where thinking is being removed from the trade and all that needs to be done is to be able to read and memorize following the rules in order to qualify as an electrical.
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Blame that on lawsuits, technical details is what gets you into trouble when being sued. Inspectors? Kind of goes back to lawsuits to a certain extent though sometimes it is just abuse of authority.
 
Strathead said:
In many cases, the code doesn't "allow" common sense. In many of those cases, it is understandable because the code has to put its foot down and set a limit at some point. I have seen the number of bends rule broken as many times as just about any code, so I would guess that inspectors are not really aggressive on this code.
Click to expand...

I'm sure anyone can arrange 5 factory 90 degree bends end-to-end, and still pull a a bundle of wire that fills it the full 40% fill by hand without any struggle. The location within the run for where a bend is, also makes a big difference to the overall pulling tension.

It really is pulling tension and sidewall pressure, that govern whether or not a conduit run is possible for any given pull. And that govern whether the wire will still be decent for safe operation after the pull is complete. Still, the 360 degree rule still works as a general guideline to keep most conduit systems and the pulling tension within reasonable limits
 
Carultch said:
I'm sure anyone can arrange 5 factory 90 degree bends end-to-end, and still pull a a bundle of wire that fills it the full 40% fill by hand without any struggle. The location within the run for where a bend is, also makes a big difference to the overall pulling tension.

It really is pulling tension and sidewall pressure, that govern whether or not a conduit run is possible for any given pull. And that govern whether the wire will still be decent for safe operation after the pull is complete. Still, the 360 degree rule still works as a general guideline to keep most conduit systems and the pulling tension within reasonable limits
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Correct it is a combination of things that determines pulling tension. I have pulled 14 and 12 AWG with no struggle through 5+ 90 degree bends, and had cases with larger conductors and only 3 90 degree bends that needed mechanical assistance to make it past the third bend.
 
wwhitney said:
Thanks for the summary! Apparently it is called the capstan equation. Wikipedia has an entry, but I liked the derivation starting on page 3 of this PDF better.

Cheers, Wayne
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Thanks for finding that; I didn't know it was called the capstan equation, even though I knew the math applied to 'capstan rollers'. I learned about it in a book on bicycles, where the equation is used to calculate brake line friction.

My favorite use for the exponential way friction around bends depends on the input tension to the bend is in the 'torque multiplier', a mechanical component used in mechanical computers. This uses two capstans arranged to 'fight' each other. Small movements of an input change the _input_ tension on the string wrapping the capstans, which causes huge changes in the output tension. The device is essentially a fully mechanical 'class A' amplifier.

The wikipedia article does a good job of explaining the principle of the torque multiplier, but the meccano ones are much cooler:
http://www.meccano.us/differential_analyzers/robinson_da/torque_amp.html

-Jon
 
mgookin said:
I think you guys mean sweeps.
An elbow is an angle without a radius.
A sweep is a radius which results in an angle.
Agree? Disagree?
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So are you saying a pulling L or a condulet, for instance, would be an "elbow" and a 90 degree fitting would be a sweep? Never thought of it that way. Always thought "sweep" was just another term for an elbow.
 
arfrcchic said:
mgookin said:
I think you guys mean sweeps.
An elbow is an angle without a radius.
A sweep is a radius which results in an angle.
Agree? Disagree?
Click to expand...
So are you saying a pulling L or a condulet, for instance, would be an "elbow" and a 90 degree fitting would be a sweep? Never thought of it that way. Always thought "sweep" was just another term for an elbow.
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Well, it depends on who you ask because there is no definition for either.

Ask a plumber about anything that looks like a bent piece of pipe, and he'll likely say its a sweep.

To an electrician, however, and IMO, a short, factory-supplied piece of conduit with a standard radius bend is an elbow. Standard being anything close, but not smalller than the minimum permitted radius bend. A sweep meets the same criteria with the exception that the bend radius is substantially greater than the minimum.

A condulet, which an LB or LL or LR is, is not a part of this discussion IMO.
 
Many plumbing elbows still have a radius to the bend, it just happens to be too short to be practical to use for pulling wire and cable through it - though I have seen it done many times:blink:
 
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