60 degree conduit bends

[hbeery10]

I had a fellow electrician tell me the other day that it is easier to pull wire through a conduit with 90 degree bends than with 60 degree bends due to the way the wire drags on the inside of the bends. He said he would never do an offset with 60 degree bends. Can anyone verify this? I’ve used a 60 degree offset before in a tight spot and didn’t have any issues.


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[480sparky]

Never heard of such a thing.

 

[GoldDigger]

There might possibly be some effect on jamming when non-recommended wire size combinations are used with three wires. But that is just speculation.
Overall, I think it is a bogus issue.
Especially if the 90 and the 60 are made with the same bending radius.

 

[infinity]

I agree with Goldie it's bogus. For me 45° is the maximum bend I would go with on an offset unless there were special circumstances.

 

[MNSparky]

I would think the wire would rub on more surface area in the 90 than it would the 60, maybe not by much though. I definitely wouldn't say it's easier to pull through a 90 over a 60 however, I'd say they are pretty close to the same.

 

[MAC702]

 

[LarryFine]

I'd agree that two 60's are harder to pull through than a single 90.

 

[hbeery10]

Thanks guys, I’m with you. I was pretty skeptical when he said it and told him so but probably could have been a little more tactful about it. I just wanted some second opinions to make sure I wasn’t missing something. Thanks!


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[kwired]

with two 60 degree bends you have a total deflection of 120 degrees, makes sense that will likely pull harder than through a single 90 degree bend.

If you use the same bender for each bend and only make "one shot" bends you have the same bending radius for all bends.

 

[junkhound]

Total angle pulled around is the basics physic principle.
T₂ = T₁ e^μθ,

T2 is your pull on the wire, T1 is the resisting force (e.g the force needed to pull wire off the spool - why it helps so much to have someone PUSH at the feed end)
^μθ is the coefficient of friction times the total number of degrees in radians (1 r = 57 degrees)

Have your 'knowledgeable' electrician run a test to show you if he insists on the 60 deg version of physics. Perhaps their experience is with a wire spool directly above the conduit entrance, and a 60 deg pull scrapes the edge of the entrance, which increases T1.

For educational purposes, also note that the coefficient of friction is AN EXPONENT in the equation, which is why lube seems to 'help so much more' on high number of degrees pulled than on just one 90 degree pull.

 

[hbeery10]

So I talked to this electrician again to verify what he had said. His claim is that it’s easier to pull wire through two back to back 90’s than it is through a 60 degree offset. I’m not trying to prove him wrong but I want to know if I get to a situation that a standard 45 degree offset is too big what do I do.


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[MAC702]

"Back to back" 90s? So like a U instead of an offset? That's different! At least it seems like it would be.

 

[hbeery10]

"Back to back" 90s? So like a U instead of an offset? That's different! At least it seems like it would be.

No, I think he means bend two 90’s in opposite directions to offset the pipe over.


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[kwired]

So I talked to this electrician again to verify what he had said. His claim is that it’s easier to pull wire through two back to back 90’s than it is through a 60 degree offset. I’m not trying to prove him wrong but I want to know if I get to a situation that a standard 45 degree offset is too big what do I do.


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You still allowed 360 degrees of bend between access points. More bend you have the harder it will typically pull.

If you make the offset with two 90's I bet it pulls at least about as hard if not harder than the same offset using 60 degree bends - assuming all are made with same one shot bender.

 

[hbeery10]

You still allowed 360 degrees of bend between access points. More bend you have the harder it will typically pull.

If you make the offset with two 90's I bet it pulls at least about as hard if not harder than the same offset using 60 degree bends - assuming all are made with same one shot bender.

Yes, we’re using the same shoe for all bends.


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[MAC702]

An offset will get harder to pull as bend degrees increase. Back to our original programming...

 

[ActionDave]

It's harder to pull through bends that are closer together than bends that are spread out for instance I can tell when I hit a three point saddle and it will be much harder to get through than any of the nineties at the start and end of the run.

 

[winnie]

The 'capstan equation' mentioned by junkhound in post 10 is basic physics. This means 2 things: 1 it has been tested and found to be reasonably accurate in many situations and 2 it is an approximation which will fail if the basic assumptions are not correct.

All other things being equal, I would expect shallower angles for an offset to equate to an easier pull. In other words doing the same offset with 2 75s would be harder to pull than with 2 60s and even easier would be 2 45s. But it is _possible_ that some additional factor (such as conductor jamming as suggested by GoldDigger or some weird effect caused by the particular bending shoes used) would change this ordering and make 60s particularly bad.

Remember 'in theory there is no difference between theory and practice'; real data always trumps theory.

-Jon

 

[kwired]

It's harder to pull through bends that are closer together than bends that are spread out for instance I can tell when I hit a three point saddle and it will be much harder to get through than any of the nineties at the start and end of the run.

And those three point saddles also depend on how many degrees the bends are. I was initially taught to make a 45 degree bend and two 22.5 bends when making such saddles, but over the years I have sort of gotten to where I often only make a 30 or even 22.5 degree bend for the center of the saddle, especially when crossing smaller obstructions. Requires a little more guesswork that sort of ends up coming more naturally the more you do it at finding the center of the bend on the bender shoe, often only the center of a 45 is marked on the shoe and probably the reason they teach you to make a 45 as the center bend.