conduit bends

[sparkmantoo]

we were discussing the bends that can be made in a conduit run between junction boxes or whatever. we were discussing the maximum degree you can make in a SINGLE bend. someone mentioned a "shepards hook" bend. it looks kind of like a question mark, and it application is used for a run say up the wall and then have to ninety through it. in order to do this you must overbend the ninety and then flip the pipe around and pull the back side up until it come out ninety degrees.
i seem to recall that the code says no more than 4 quarter bends in any run between j-boxs, conduit bodies, and also shall not be less than the inside radius designated by table 344.26 i think. i don't have the book here. my interpritation is the quarter bends are just that quarter bends (4x90=360) and to make more than a 90 degree bend (single bend remember) is exceedind that rule and also the inside radius is smaller because you have a tighter ninety. any thoughts on this people i would greatly appreciate some feedback.

 

[don_resqcapt19]

Re: conduit bends

The code does not limit the maximum numer of degrees in a single bend, just the total number between pull points. Look at the code words:

There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, for example, conduit bodies and boxes.

The number of degrees in the bend does not change the radius of the bend. A 30? or a 135? bend made on the same bender will have the same radius.
Don

 

[sjaniga]

Re: conduit bends

I know what you are trying to say, I think the code leaves too much open on this, to be on the safe side we try very hard not to make a single bend more than ninety total degrees. As for changing the inside dia by making a larger readius bend would depend on if you kinked the pipe, I can make a 180 on an electric bender without kinking.

 

[don_resqcapt19]

Re: conduit bends

How does making a single bend greater than 90? change anything? Why would two 180? bends be a violation and 4 90? bends not a violation?
Don

 

[bennie]

Re: conduit bends

Making a bend over 90? may not be a code violation, but it is definitely a quality of workmanship issue, except for very short runs.

On Federal Spec projects, a 90% mandrel is required to pull through each conduit, prior to pulling wire or cable.

The overbent conduit can make the co-efficient of friction exceed the maximum tension for a cable pull.

[ July 20, 2003, 08:21 PM: Message edited by: bennie ]

 

[peter]

Re: conduit bends

Sparkman,
I think what you are describing is going up the wall, 45? away from the wall and then doing a 135? to bring the conduit horizontal and straight thru the wall. You could just use a regular 90? and cut a slot in the first part of the wall. Or you could just mount a junction box on the wall and penetrate the wall from the back. Or use a condulet.
But early in my career, I was faced with two outlet boxes at the same level in the same stud space but on opposite walls and I had to connect them with 1.2" conduit. Nothing worked until I did a Greek Omega thing. It went up, then 30? away and then 270? and then 30? to plunge back into the top of the other box. The foreman didn't like it. Called it a Shepard's Hook and couldn't get anything else to work. So he used flex.
~Peter

 

[sparkmantoo]

Re: conduit bends

my concern was when you overbend the conduit you do shorten that radius on the inside of the bend therefore violating the table i referred to. or am i wrong.

 

[pierre]

Re: conduit bends

I don't believe this violates the code. If the run is long enough, with other bends though, I would not want to be the one responsible for the pull. Even with soap who would be responsible for the integrity of the conductors. If there could be a violation, it would be 110.7.

Pierre

[ July 21, 2003, 07:21 AM: Message edited by: pierre ]

 

[don_resqcapt19]

Re: conduit bends

sparkmantoo,
How can the radius change if you are using only one bender??? The radius is fixed by the physical size of the bender. All bends from 1? to 360? will have the same radius if made on the same bender.
Don

 

[peter]

Re: conduit bends

Don,
"All bends from 1? to 360? will have the same radius if made on the same bender."
Not so. I use my [for instance] 3/4" bender to make large radius bends such as 12" radius and larger. I BELIEVE that this helps with the pulling process. Also this solves certain clearance problems.
The technique is to bend the conduit maybe 10?, advance the bender a couple of inches, bend another 10? and so on...
With all due humble respect,
~Peter

 

[bennie]

Re: conduit bends

One shot benders will make the same bends.

Segment benders, and bending, will bend any radius desired.

 

[don_resqcapt19]

Re: conduit bends

Peter,

I BELIEVE that this helps with the pulling process.

I used to think this too, until trying to pull wire in a pipe run with concentric bends, some of which took more than 10' of conduit for a 90? bend and found it pulls harder than when normal bends are used.
don

 

[stevenb]

Re: conduit bends

You would bend it if you had to pull it.


As an apprentice, I was always reminded of the above words

 

[peter]

Re: conduit bends

Don,
You 90? bend using 10' of conduit seems to be a 12.73' radius. I am puzzled as to why it would be harder to pull. This seems to be counter-intuitive.
For example, if you used a plumber's elbow, [perhaps 1/2" radius] the pull would be obviously impossible. A straight pull would be much easier. Somehow a large radius bend would be closer to a straight pull than a smaller radius. Think of curves on freeways.
Also wire, especially solid wire, is stiff and during the pulling process, the wire must be bent to the radius of the conduit and the smaller the radius, the more it must be bent. Thus more force should be required.
Do the power companies and the communication guys know something we don't? The power companies specify large sweeps and the commies specify 3/4" tubing for their little cables - perhaps realizing that 3/4" conduit uses a larger radius than 1/2" and is thus easier for them to pull. (I know that there is minimun radius for fiber-optic cable but they require this for phone cable too.)
Has any scientific research been done on this since the 1930's? It seems like a good subject for a doctoral disertation for somebody.
~Peter

 

[jrdsg]

Re: conduit bends

is there room for a little whining from the low-voltage side of things?

quite often our contracts aren't let until division 16 has already installed whatever conduit the electrical designer deigned to leave us. save a little pity for the security & comms guys that have to pull 24 ga. wire through all those 90's.

buried conduit is the worst, since it is often full of water and sometimes even sand and gravel by the time we get to the site.

 

[ryan_618]

Re: conduit bends

Originally posted by jrdsg:
is there room for a little whining from the low-voltage side of things?

Um....no.

 

[don_resqcapt19]

Re: conduit bends

Peter,
Yes, an extremely short radius would make it impossible to pull. I would never install 1/2" rigid using the 1/2" shoe on a mechanical bender as the radius makes for a very difficult pull. It has just been my experience that very long radius bends seem to be harder to pull than standard radius bends. I have never used a pulling force meter to prove this, it just feels that way when I'm pulling the wire. The runs that I am talking about are 3/4" conduit with 4 #12s and 5 #14s. And, yes, the pulling force calculation says it is easier to pull through the large radius bends, but it doesn't feel that way to me.

The radius of the bend does make a big difference in the crushing force (sidewall pressure) that is applied to the conductors as they are pulled in and often large radius bends are specified to limit this pressure. If you do pulling calculations for larger cable runs the sidewall pressure rather than the actual pulling force is often the limiting factor.
Don

 

[bennie]

Re: conduit bends

If I remember correctly, the co-efficent of friction created by a 90, has an increase above and below a certain radius. I don't know what the figures are. I also do not know the ideal radius either.

 

[sparkmantoo]

Re: conduit bends

i thank each and everyone of you for your input.