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_simpl...lations_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.