conduit bend radius

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mikeybmdb32

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Just to set the stage, I'm an engineer and not a contractor/installer:

My question is ... how do you measure the radius of a 90 degree conduit bend? Do you measure it the same as you would the radius of a circle or do you sort of measure the circumference of the bend?

I ask because I'm working with a spec that calls for 18" radius on 90 degree bends and the contractor has installed bends that either have a radius of 8" (if I measure radius the same as on a circle) or 18" (if I measure the circumference of the bend). The contractor insists that trades measure the circumference and I'm extremely skeptical.
 
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Well, you just stepped in the doo-doo pile.

Field bends are made with tools. Factory bends are made with tools. The NEC seems to refer to two measurements: to the inside of the bend, and to the centerline of the bend- but nobody actually measures them. The bend is determined by the bending shoe, and we assume that it is correct- and as tight as we can go!
 
You measure the radius of the bend the same way as you measure the radius of a circle.

If he used standard bending shoes he will not get the radius you specified unless he has done segmented bending.
 
tkb said:
You measure the radius of the bend the same way as you measure the radius of a circle.

If he used standard bending shoes he will not get the radius you specified unless he has done segmented bending.
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I agree with Tim, the EC has not followed the contract documents and should be made to correct the installation.

Roger
 
The industry standard, for factory bent elbows, seems to be to the inside of the bend. I have seen some radii measured to the center or to the outside, but these have usually for matching/paralleling elbows with different conduit sizes. Never have I heard of the circumference being referred to as the radius.
 
From 358.24

"The radius of the curve of any field bend to the centerline of the tubing shall not be less than shown in Table 2, Chapter 9......."

344.24 also addresses the centerline being the point of the measured radius.

So, unless otherwise noted, the radius of conduit should be considered to the centerline, not the inside of the bend, nor the outside for that matter.
 
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Just to set the stage, I'm an installer JIW, not an engineer, and I passed geometry in grade 10.

We just did twenty 36" radius bends on 4" RMC. If we measured that thirty-six inches along the circumference, we'd be looking for work right now!
 
mikeybmdb32 said:
Just to set the stage, I'm an engineer and not a contractor/installer:

My question is ... how do you measure the radius of a 90 degree conduit bend? Do you measure it the same as you would the radius of a circle or do you sort of measure the circumference of the bend?

I ask because I'm working with a spec that calls for 18" radius on 90 degree bends and the contractor has installed bends that either have a radius of 8" (if I measure radius the same as on a circle) or 18" (if I measure the circumference of the bend). The contractor insists that trades measure the circumference and I'm extremely skeptical.
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not to put too fine a point on it, the contractor is trying to get out of fixing them. it's a BS answer.
i'm making a bold assumption and saying it's 1" conduit, and he bent it with a normal bending shoe
'cause he doesn't know how to bend them, and it's not a common thing to find sweeps that size with 18" radius.

now, any home desperate sells copies of ugly's. there are several pages devoted to concentric bending....
if you want to be really mean, get him a copy for the few dollars it costs, and give it to him at the same
time as you give him the correction notice... and make sure the project manager knows that the work
is not in compliance with contract drawings, as work not done correctly shouldn't be paid for.

even if you don't know segment bending, ya can throw a no dog on a stick of pipe, and bend it segmentally
in a one shot bender... you just need a digital level, and the willingness to learn how.... and the formula
for developed length....
 
If it was in the specs the contractor needs to provide the long radius bends. What is the purpose of the requirement? Is there a technical reason that the standard radius bends cannot be used? If not, maybe the answer would be a credit to the owner for the work that was not done. The cost to make segment bend would be at least 5 times that to make a standard one shot bend. Not sure what a factory 90 with an 18" radius would cost. It might be cheaper than making your own but it would require more couplings and cutting and threading.
 
that either have a radius of 8" (if I measure radius the same as on a circle) or 18" (if I measure the circumference of the bend).
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You do realize that radius and circumference are two entirely different things, don't you? I don't understand what you are measuring when you talk about the circumference of a bend.

The circumference of an 8 inch radius circle will be about 201 inches. So a 90 would have a running length of 50.25 inches measured along the arc of the circumference.

I can't picture what you are asking, especially with the two measurements being so far off.

Any chance of a sketch?
 
even if you don't know segment bending, ya can throw a no dog on a stick of pipe, and bend it segmentally
in a one shot bender... you just need a digital level, and the willingness to learn how.... and the formula
for developed length....
Click to expand...

Not perfectly unless you know what you are doing.

I own 2 Chicago benders, the best for making large sweeps.

Consider 15 bends of 3 degrees each, spaced about three inches apart.

If you put a no-dog on the tail of the pipe and bend it 15 times, more often than not the pipe will be out of plane. This is due to a couple of things. The shoe of the bender pulls to one side just a bit. In order not to, you wouldn't be able to get the pipe in and out. Also, pipe can be softer on one side than the other.

Consider that any error in measurement will be 15 times greater doing a sweep with 15 bends.

There is an art to doing perfect segmented bends, the kind where you can lay 3 pieces of pipe you just bent on top of each other and they will not only all match perfectly, but will all be in perfect plane.

Along with the Chicago bender, I use a no-dog, a regular level, could be magnetic, it doesn't matter, a folding ruler, a tape measure and an incline level. Pencils are optional. I can do it without putting a single mark on the pipes. Think about it, if they have to be all perfect it's probably because people are going to be looking at it them....
 
Someone with all the experience on the pulling calculator, for practical application what is the difference in pulling tension if we measure, inside, centerline, or outside?

As far as the OP, even an apprentice should be able to see the difference between an 8" and and 18" radius. From here.

I take it he measured out 18" on a straight piece of conduit and made sure his bend fit inside ?? the lines.
 
I'm guessing the purpose was to relieve stress on the cable/wires during the pull. It is 1 1/4" PVC conduit. Thanks to all for your answers!
 
K8MHZ said:
The circumference of an 8 inch radius circle will be about 201 inches. So a 90 would have a running length of 50.25 inches measured along the arc of the circumference.
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We must have a typo here!
The circumference is about 50.25 inches.

8 + 8 * 3.14 = 50.25
 
mikeybmdb32 said:
I'm guessing the purpose was to relieve stress on the cable/wires during the pull. It is 1 1/4" PVC conduit. Thanks to all for your answers!
Click to expand...
The radius of the bends does not enter into the standard pulling tension or force calculations. The radius is used in calculating the sidewall pressure that is put on the insulation as the conductors are pulled around a bend.
 
I might be reading this wrong, but isn't 18" radius for 90 degree bends a bit generalized? That might be fine for smaller conduit, albeit a little long for smaller conduits; however, for 6" conduit it wouldn't even meet code if it was only 18" radius. I think the spec is faulty.
 
mikeybmdb32 said:
I'm guessing the purpose was to relieve stress on the cable/wires during the pull. It is 1 1/4" PVC conduit. Thanks to all for your answers!
Click to expand...

ah. thank you. i believe you can get 24" radius in 1 1/2" pvc... not positive if smaller is available in factory bends....

but bending pvc isn't a difficult chore... all you need is something 3' in diameter to wrap it around......
and a torch to heat it.... or a blanket.

if it was something that had to be spot on, i'd take my jigsaw, put the circle cutter on it, set it for 18"
and get three sheets of plywood.... and cut two of the sheets the right radius, and the third for a backing
piece.... make the first cut at the outside radius, the second cut the inside radius, and the screw both of
them to the third sheet, so you have a groove the right width of the conduit, and the right radius.....
and 1 1/2" deep....

heat up one pipe, toss it in the groove, and heat up the second one... when the second one is toasty,
the first one will be cool enough to hold it's set... remove it and replace with hot floppy one.... they
will come out looking like factory products.
 
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