Fordean
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
any good videos out there showing measurements, And Hydraulic benders
2.5 inch EMT 3 inch offset
- Thread starter Fordean
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If you're doing this offset with a hydraulic bender, which, and what is the shoe's bend radius?
Each bender will have minimum adjacent straight section requirements, e.g. distance to hook-end or roller(s).
You're probably looking at no more than 10? bends.
Carultch
Senior Member
- Location
- Massachusetts
I usually go here, to get the NEC requisite bend radii. Your bend radius might be bigger, depending on the tool you use.
http://www.lanshack.com/DesigningConduitRuns.aspx
A 2.5" conduit requires a bend radius of 10.5". A 2.5" conduit is approximately 3" in OD. The centerline bend radius is Rcl=12"
An offset bend has a sweep of angle A, and then a sweep of angle A in the opposite direction.
The offsets are 2*Rcl*(1 - cos(A)) + straight*sin(A).
The total length of straight conduit within the bent region is 2*Rcl*A*pi/180 + straight.
The height of the swept section of conduit is 2*Rcl*sin(A) + straight*cos(A)
The maximum bend angle, with no straight section between bends, is 29 degrees. Sticking to common bend angles, you are looking at 22.5 degree bends, with a 3" straight section between the two bends. The total length of 12.5 inches of initially straight conduit used in these sweeps. The total height within the swept region is 12 inches.
You'll hear the term shrink, in regards to offsets. In this example, the conduit shrink is 0.5 inches. This is the initially straight length, minus the height.
Here's a picture of what this sweep looks like drawn to scale. ITS HUGE!
Chapter 9 Table 2 just state radius. I always took that to mean centerline radius, because that how all radii for conduit bends are given to my knowledge.
I didn't check your formula, but I'm sure someone will appreciate your work.
FWIW, the bender I prefer for this type of bend is a Greenlee Cam Track (881 series). The 2-1/2" EMT shoe has a 13-1/2" bend radius.
The maximum bend angle with a 3" straight section in between bends (quite likely the minimum you be able to do on an 881) is 21.6?.
Conduit length through bends is 13.18" with a developed length of 12.73", for a total shrink of 0.45".
BTW "shrink" is the difference between original straight conduit length and final length after bending, i.e. the length lost as a result of bending the offset.
Carultch
Senior Member
- Location
- Massachusetts
I wouldn't really say that it is lost. In the example below, you still need 62 inches of wire to pull through the conduit, both before and after bending. It is just length that gets put to another use, instead of just left-to-right distance. You can't pull just 40 inches of wire through it, and be complete.
It is my understanding that centerline arc length is conserved, when bending conduit.
It kind of makes sense that the inner part of the conduit is compressed, and the outer part is stretched. And to at least the amount that you can measure with ordinary tools, I would expect that the arc length of the center after bending would remain the same. You can also see that a conduit gets a little oval in cross-section shape, when bent. And that is how it can conserve material and stretch/compress the inner and outer walls.
This diagram shows what shrink is, which is how I described it as well.
40 inches is the distance of the conduit in the parallel direction, of a conduit that started as 62.8 inches of straight length.
Carultch
Senior Member
- Location
- Massachusetts
That's what I first thought. And coming from a mechanical engineering background, centerline radius in plumbing calculations, than the inner bend radius. So I first thought it was centerline radius.
It turns out that if you check standard factory sweep fittings, that the inner radius matches the bend radius table in the NEC. The datasheets of flexible LFMC and LFNC also match this concept.
I didn't check your formula, but I'm sure someone will appreciate your work.
Thanks.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
Not disagreeing with your math, didn't check it either, but is it physically possible with most benders to put two bends of 22.5 deg that close together on that size of a pipe especially when you need to rotate the pipe 180 degrees before making the second bend?
I didn't coin the slang term, but I agree it really isn't lost. It is only 'lost' from the 'run' length.
I think you better check your references. I don't have access to the full standards, but industry publications lead me to believe table values are centerline radius.
http://www.nationalpipe.com/document_files/1383848759_SCH40-80 Standard Sweeps.pdf
- Location
- Massachusetts
This picture is also a good example of a bend we cannot make in the field due to the limitations of the bending equipment.
It is not possible to make back to back bends that close together.
You can't get it from the factory or shop either! That's why I said OP'er was probably looking at not more than 10? bends in my first post to this thread.
Just out of curiosity, I used manual information to extract an approximate minimum straight section between bends of 6-1/2" on a Greenlee 881 using the 2-1/2" shoe.
As it works out, the maximum angle for a 3" offset of 2-1/2" EMT on an 881 is just over a 16?.
Carultch
Senior Member
- Location
- Massachusetts
Does there exist a table of standards for what bend arrangements *are* physically possible to make in the field?
Given conduit type and trade size, how does one who has never had practical experience with a bending tool, draw a conduit run with bends? What is the table that shows the minimum straight section that surrounds the bend, the distance between bends, and the bend radius with an objective definition of how it should be measured?
The best you can do if you don't have the bender is to 'extract' the information from the user manual.
Let me know if want to know how.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
There may be something published somewhere, but most have learned the hard way at some point - even if just with a 1/2 EMT hand bender and trying to bend to much offset in too little length of run.
Those pieces you mess up on are the ones used elsewhere if you find a need for a shorter piece. Even with simpler bends, we sometimes bend the wrong direction, mark pipe wrong, or use wrong mark on the bender and still end up with scraps for later use.
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
well, you are offsetting the thickness of the pipe, so you need to put a
dog leg level on the pipe, put a little scooch in it, flip it over, and put another
little scooch in it, and that'll be about 3 1/8" center to center. just eyeball it.
sight down it, and you want the bottom of the shoe to line up with the oppsite
edge of the pipe...
bend it long and cut to fit, and off you go.
and don't rent a bender to do it. i rented a bender to bend 2" GRC this
last weekend, and the only one i could find available was from these
crackheads in gardena. it made two 90's, and the primary sprocket
exploded in pieces on the third 90.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Well, THAT was a productive day. Hope you had a plan "B".
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
yeah, there was a plan B.
put the stuff in in pvc, pull the wire, and then change it out later.
that is what happened.... using factory GRC 45 degree bends later....
that aren't consistent 45's. picked up ten, and found the best match
of five of those, and used them...... 'cause i was having to go from an
LB to an existing hole, and make them match, with inconsistent kicks.
chinese pipe sucks.
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