conduit bend radius

[petersonra]

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.

I would be measuring them from the centerline of the conduit. Take a tape measure and run it on the bend along the centerline of the conduit. If it is an 18" radius bend, the bent section will be just over 28" long.

In any case if they are already there and the wire is pulled, it seems pointless to replace them. Maybe a bit of a rebate for not following the spec.

 

[don_resqcapt19]

I read what it says, but my back and arms do not believe.

My experience with very long radius bends and small conductors is that the pulling force required seems to be greater than with a standard radius. The amount of friction is a function of the sidwall pressure, the coefficient of fricton and the length of the inside radius of the bend. While a longer radius has less sidewall pressure, the inside radius length is longer. The friction remains approximately the same.

 

[ptonsparky]

My experience with very long radius bends and small conductors is that the pulling force required seems to be greater than with a standard radius. The amount of friction is a function of the sidwall pressure, the coefficient of fricton and the length of the inside radius of the bend. While a longer radius has less sidewall pressure, the inside radius length is longer. The friction remains approximately the same.

I have never put any type of tension devise on a pull so my 'arms and back' method is subjective that's for sure.

Why do they spec these on short pulls and and small wire if it really does not make any change. Recently bid a project that wanted large radius 1 1/4" rigid 90s on both ends of a 70' pull. Raceway was pvc otherwise. Three #3 thwn.

Next is to define "large". 'Not small' does not count.

 

[don_resqcapt19]

I have never put any type of tension devise on a pull so my 'arms and back' method is subjective that's for sure.

Why do they spec these on short pulls and and small wire if it really does not make any change. Recently bid a project that wanted large radius 1 1/4" rigid 90s on both ends of a 70' pull. Raceway was pvc otherwise. Three #3 thwn.

Next is to define "large". 'Not small' does not count.

I have also not used a tension device to measure the force. The pulls that I think pull harder were 3/4 or 1" with a number of #14s or 12s, but some of the bends needed more than a 10' length of conduit to make the 90.

They spec it because most people believe that the longer radius reduces the pulling tension. There are only two reasons why you need to specify a large radius bend. One if the cable or fiber that is being installed has a minimum bending radius that is larger than the minimum bend radius permitted by the NEC, or two, where the sidewall pressure would exceed what is permitted for that conductor.

As far a large, if you are talking about the bend radius, larger is anything larger than what you get when you make the bend on a standard conduit bender with a shoe for that size conduit.

 

[K8MHZ]

I had to do some big sweeps, like 45 inch radius of 1 1/2 EMT, for some fiber once. Although I never got a concise answer as to why the big sweeps, one of the communication dudes said it was to reduce the amount of force they needed to make the pull.

 

[Dan coulstring]

I have also not used a tension device to measure the force. The pulls that I think pull harder were 3/4 or 1" with a number of #14s or 12s, but some of the bends needed more than a 10' length of conduit to make the 90.

They spec it because most people believe that the longer radius reduces the pulling tension. There are only two reasons why you need to specify a large radius bend. One if the cable or fiber that is being installed has a minimum bending radius that is larger than the minimum bend radius permitted by the NEC, or two, where the sidewall pressure would exceed what is permitted for that conductor.

As far a large, if you are talking about the bend radius, larger is anything larger than what you get when you make the bend on a standard conduit bender with a shoe for that size conduit.

http://ecmweb.com/mag/electric_simple_calculations_cable/
This is an informative article with formulas for tension and SWBP and friction coefficients.

 

[jmellc]

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!

A curious thing I have seen a lot of. Many Chicago benders bend way too sharp on the 1/2" rigid conduit. Most I have seen were made by Greenlee. With electrical tools being their specialty, why would they make benders to do an illegal bend? I have never understood that.

 

[jmellc]

I worked 1 job where we ran conduits for fiber optics. They had to be wide radius, as the cable could not take sharp bends.

Interesting point about the increased friction for a wider 90. But most wire will follow a wide bend better than a tight one, in my experience, especially the bigger the wire. #2 copper for example will flex easily in your hand to a wide radius and still spring back a little. Bend it sharper & more bend stays in it. That increases resistance a lot.

 

[jmellc]

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.

Good idea. I want to try that. Not just for 90's but maybe bend some 45's, 30's & 15's from scrap pieces. Good to have handy & makes good use of scrap. I think some supply houses used to carry 15's & 30's in some conduits but none around here do.

 

[don_resqcapt19]

A curious thing I have seen a lot of. Many Chicago benders bend way too sharp on the 1/2" rigid conduit. Most I have seen were made by Greenlee. With electrical tools being their specialty, why would they make benders to do an illegal bend? I have never understood that.

I know that the radius for 1/2" conduit is very tight on those benders and almost always bend the 1/2" conduit in the 3/4" shoe, but I don't think that the radius is illegal. Chapter9, Table 2 permits a radius of 4" for 1/2" conduit.

 

[tkb]

I know that the radius for 1/2" conduit is very tight on those benders and almost always bend the 1/2" conduit in the 3/4" shoe, but I don't think that the radius is illegal. Chapter9, Table 2 permits a radius of 4" for 1/2" conduit.

The Greenlee 1800 bender has a bend radius on the 1/2" shoe of 2-5/8".
Table 2 in Chapter 9 calls for a 4" radius in 1/2".

The 1/2" shoe is only good for making kicks and offsets. If you make a 90? bend, it would be a violation.
I have always used the 3/4" shoe when using this bender.

 

[don_resqcapt19]

The Greenlee 1800 bender has a bend radius on the 1/2" shoe of 2-5/8".
Table 2 in Chapter 9 calls for a 4" radius in 1/2".

The 1/2" shoe is only good for making kicks and offsets. If you make a 90? bend, it would be a violation.
I have always used the 3/4" shoe when using this bender.

Tim,
I did not know that. Thanks.

344.24 Bends ? How Made. Bends of RMC shall be so made that the conduit will not be damaged and so that the internal diameter of the conduit will not be effectively reduced. The radius of the curve of any field bend to the centerline of the conduit shall not be less than indicated in Table 2, Chapter 9.

I assumed that since the code rule applies to all bends no matter how many degrees are in the bend that the bender would comply with the code rule.
I do see that the instructions for that bender say:

* To meet electrical code, bend 1/2" diameter conduit in the 3/4" position to produce a bend with a radius greater than 4".

 

[tkb]

Tim,
I did not know that. Thanks.

I assumed that since the code rule applies to all bends no matter how many degrees are in the bend that the bender would comply with the code rule.
I do see that the instructions for that bender say:

I don't know why they manufacture this bender like this but it has been this way forever.
I will admit that I have used the 1/2" shoe a bunch of times because of space limitations at terminations. Kinda like those explosion proof elbows that look like plumbing 90's.

I'm not sure if those 90's are ok to use per code or listing but I remember that they could only be used at terminations. I may be wrong though.

 

[jmellc]

I don't know why they manufacture this bender like this but it has been this way forever.
I will admit that I have used the 1/2" shoe a bunch of times because of space limitations at terminations. Kinda like those explosion proof elbows that look like plumbing 90's.

I'm not sure if those 90's are ok to use per code or listing but I remember that they could only be used at terminations. I may be wrong though.

I think sharp 90's used to be allowed for a short termination where wire was pushed by hand, say from an LB to a machine, or J box to machine. But that is not the usual 90 either.

Never made sense to me for a mfr to make a machine that way. Someone may forget to remind the helper or someone used to hand benders may be using a Chicago bender for first time, etc. Instruction booklets are usually the first thing to get lost too. I came on a job once & saw 1/2 90's in the runs, asked foreman if he knew of the short radius. He looked at me funny, like "don't challenge me already". He said, "I thought they didn't look right but I wouldn't think of a bending machine doing illegal bends". From then on, he used the 3/4 shoe for 1/2. Once we know that, we work around it. Before that, we are set up for problems.

 

[jmellc]

Watch for this too. It doesn't keep the tail of the pipe level. We had one with no protractor built in & had to use levels. Had to shim up the rear of the bender to level it out. Check that out if you do critical bends on that type of bender.

Best bender I worked with was the Greenlee Hurricane bender. Hydraulic, with protractor & notes telling how much to allow for spring back. Was accurate to 1 or 2 degrees, quick & easy to use. Had shoes for EMT & rigid up to 2 inch. Not sure if you could get larger shoes. I never used the bigger hydraulic benders much, helped with them a little. Nearly every job had 1 or 2 small crews that did all those bends. Someone really had to know those benders, with having to move pins & inserts for all different angles & sizes. I just never had the chance to use them much. I wouldn't try to use 1 today without help. Too easy to damage something or waste expensive conduit.

 

[stevenje]

I worked 1 job where we ran conduits for fiber optics. They had to be wide radius, as the cable could not take sharp bends.

The first fiber optic job I worked on was a project for IBM back in the 90's. We ran 1-1/2" EMT, no bends larger than 22 1/2? and 24" x 24" x 6" pull boxes where ever you had to turn 90?. Along with many other restrictions, it was a challenging job finding a path above the existing drop ceilings.

 

[PEDRO ESCOVILLA]

to the original post, refer to Table 2 in chapter 9, of the 2008 NEC. page 671

 

[jmellc]

The first fiber optic job I worked on was a project for IBM back in the 90's. We ran 1-1/2" EMT, no bends larger than 22 1/2? and 24" x 24" x 6" pull boxes where ever you had to turn 90?. Along with many other restrictions, it was a challenging job finding a path above the existing drop ceilings.

IBM was where I did this project too, in Research Triangle Park, NC. IBM was always a challenge. Along with the normal challenges of the jobs, they usually had their own criteria as well. If you could satisfy IBM, you could satisfy anyone. I think we ran 3/4 and 1 inch EMT for those cables. Seems the cables were to be pulled by someone else. I wasn't full time on that job, went there nights & Saturdays, so I don't remember as many details.

The good thing about IBM was that we sometimes saw products for the first or only time. They used a lot of high end, high dollar lights, etc. Had a dimmer panel I never saw anywhere else. Some chance to learn out of the way products.

Bad thing was dealing with their odd ways. We had hallways we couldn't enter at certain times of day, regardless of traffic volume. Project managers would pick someone over smallest details and demand things impossible to accomplish. We did a briefing center where 1 of our crews had to use shallow boxes over a counter against a concrete wall. 2-3 layers of drywall. Devices barely fit boxes. Standard plates barely fit right over them. Then some IBM hot shot didn't like the plates & ordered some special metal plates with grainy finish, etc. PLATE HAD A FLAT BACK, NO HOLLOW OR "SHELL" AT ALL. Plates did not fit. 1 of their guys demanded I fix it when I was there on another job. I took a look & told him to call my boss, that I could not help him. "But I want it fixed right now". I said "you can't have it right now. You outsmarted yourself with these plates you should have known would not fit. I am here to work on lights. Call my boss & complain to him about plates. I can't help you." He complained that I was rude to him. The boss backed me up & said he'd tried to tell him same thing before he ordered the plates. Some people are very intelligent but not very smart.

 

[stevenje]

IBM was where I did this project too, in Research Triangle Park, NC. IBM was always a challenge. Along with the normal challenges of the jobs, they usually had their own criteria as well. If you could satisfy IBM, you could satisfy anyone.

I worked at the IBM facility outside of Boulder, CO. The company that I started out in this trade with (early 70's) had a full time crew at IBM. One of my jobs as the new "shop boy", was to spray paint ivory steel Stemco cover plates with "IBM Blue" paint. I wish I had a nickle for everyone of those I painted. I'd be a rich man. I smile when I think of those days gone by. Great jobs, great people and good money.

 

[jmellc]

I worked at the IBM facility outside of Boulder, CO. The company that I started out in this trade with (early 70's) had a full time crew at IBM. One of my jobs as the new "shop boy", was to spray paint ivory steel Stemco cover plates with "IBM Blue" paint. I wish I had a nickle for everyone of those I painted. I'd be a rich man. I smile when I think of those days gone by. Great jobs, great people and good money.

Yes, I can imagine IBM wanting the covers painted like that. I started in 1976. Did electrical for 2 years, went to school, did electrical, tried other things a few years. Went back to electrical in 1995. If I were doing it over again, I'd have stayed with it 1st time. $ was not that good where I worked then (eastern NC), but the co. I worked for got all kinds of work, from lights in the outhouse to industrial plants. A good place to learn. I could have started my own thing in the early 80's & made pretty good. I worked with some good people I'd like to connect with again. I was young & foolish then, older & less foolish today. Today's market is intense like never before, as I see it.