Limit on 90's

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Trevor,
The run I mentioned had a vertical section of about 70' feet with a few of the 90's. In that section of the run, the weight of the conductors made them literally feed themselves.
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That will make a big difference in the pulling force which is directly related to the sidewall pressure. With a more nomal run with that many 90s it is very likely that the sidewall pressure limit will be exceeded on the last few 90s. This damage may not show up at the time of installation, but may cause insulation failure in the future.
Each run is evaluated in the field based on length of run, conduit fill, etc.
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With the cost of copper, you really need to look at the pulling tension and sidewall pressure. In most cases the limiting factor is not the pulling tension, but the sidewall pressure. High sidewall pressure is one reason to use large radius bends as the side wall pressure is equal to the pulling tension at the end of the bend divided by the radius of the bend in feet.
There is a pulling calculator here. It is a bit difficult to use as you have to do each straight section and each bend as a seperate calculation.
Don
 
peter said:
For the minimun of three bends, allow one length vertical [z axis], one west [x axis] and one north [y axis]. And then perhaps another vertical drop to your destination box. Hence, three bends.
According to geometry, a straight line is the shortest distance between two points, so you should have no bends at all. But people would be tripping over it or ducking under it -- like Caution Tape -- and the boxes might have to be mounted at peculiar angles?!
Now two bends would also be possible. Vertical to floor, DIAGONAL across room and then vertcal to destination.

Celtic,
I've thought about this matter of large bending radius being easier to push. It would seem intuitive that a large, gradual bend would be easier to do. Just as you can take a curve faster on a freeway on ramp rather than the sharp, 90? corner at 2nd and B Street. In fact, carrying this reductio ad absudum to an extreme, you cannot pull at all through a plumbers 90? elbow.
To some extent, the flexibility of the cable or wire itself should have some affect. Is it not harder to bend 350 sq. mils to a 6" radius than a 24" radius?
But some mathematical geeks have contrived their sophisticaticated formulas which usually include some undefined element, a value that is in some book you don't have [shades of Groucho in "A Day at the Races"] and, of course, a Konstant which is whatever it takes to make the results conform to reality.
And they are right. The total force is the same for the entire pull [except for my quibbles]. But in the field, one man can only push so hard or pull so hard. I liken it to pushing a wheel barrow with a 100 kilogram gold brick in it up this 200' hill. Now the total amount of work done, any way you look at it, is going to be 44,000 foot pounds. However one side of the hill has a 45? slope and the other side has a 5? slope. I wager that your preference would be to push it up the shallower slope.
~Peter
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Do What!? :confused:
 
don_resqcapt19 said:
Trevor,

That will make a big difference in the pulling force which is directly related to the sidewall pressure. With a more nomal run with that many 90s it is very likely that the sidewall pressure limit will be exceeded on the last few 90s. This damage may not show up at the time of installation, but may cause insulation failure in the future.

With the cost of copper, you really need to look at the pulling tension and sidewall pressure. In most cases the limiting factor is not the pulling tension, but the sidewall pressure. High sidewall pressure is one reason to use large radius bends as the side wall pressure is equal to the pulling tension at the end of the bend divided by the radius of the bend in feet.
There is a pulling calculator here. It is a bit difficult to use as you have to do each straight section and each bend as a seperate calculation.
Don
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Nice link. Thanks I find those to be a nice little time saver:smile:
 
That link is where I've done my last few CEU's online. And holy crap, their priced are going up. It's more than doubled from the first time I found them.
 
peter said:
Celtic,
I've thought about this matter of large bending radius being easier to push. It would seem intuitive that a large, gradual bend would be easier to do. Just as you can take a curve faster on a freeway on ramp rather than the sharp, 90? corner at 2nd and B Street. In fact, carrying this reductio ad absudum to an extreme, you cannot pull at all through a plumbers 90? elbow.
To some extent, the flexibility of the cable or wire itself should have some affect. Is it not harder to bend 350 sq. mils to a 6" radius than a 24" radius?
But some mathematical geeks have contrived their sophisticaticated formulas which usually include some undefined element, a value that is in some book you don't have [shades of Groucho in "A Day at the Races"] and, of course, a Konstant which is whatever it takes to make the results conform to reality.[...]
~Peter
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We mathematical types prefer to use the term 'approximation'. Approximate is closely related to ass-u-me, however we try to make clear the simplifications that we make as we generate the approximation. The idea is that some of the variables in the 'real world' have much less effect on the outcome than other variables, so we intentionally ignore these small effects. If we are good, we _say_ which variables we are ignoring, and if we are bad, then these "unimportant" variables come and bite us on our assumptions.

The formulas for pull tension _approximate_ the real world by assuming that the cable being pulled is not stiff, in other words they assume that the cable is like a very flexible braided rope, ready to flop over in whichever direction it is pulled. Clearly this is _not_ reality, however for normal bend radius the stiffness of the cable is assumed to be much less of a factor than friction between the cable and conduit.

-Jon
 
Thanks everyone. Whats happened is my brother-inlaw is doing most of the work (he is not an electrician). I have only been able to give him advice by email and phone. His house is so far away I can only go occasionally. Then when I get there I want to cry. But do not want to hurt his feelings. This would not have even been an issue if he would have stubbed the conduits up through the slab (LIKE I TOLD HIM)!
 
Barbeer,
I don't understand your comment. I was just implying that it would be easier to push the wheelbarrow up a shallow slope than a steep one.
The point is that I, or anyone else can only push the wires into the conduit with so much pressure. If the head reaches a sharp 90? bend, then this pressure is resisted and the push cannot procede further. However, if the bend is a super bend, then that one push will go in and then another push will push it further and so on until it is through.
Pushing is an important factor in difficult "pulls".
Winnie,
You must be an engineer or something. I was just pulling your leg. I've been reading "Building Scientific Apparatus" and they sure make a mess out of their formulas.
Celtic,
The gold brick is actually a lead ignot which was painted with gold paint. I actually did this some years ago and it did fool some guys who were rather high at the time.
~Peter
 
Peter it is interesting that once you start doing pulling tension calculations you find that the radius of the bend does not change the pulling tension.

A large radius does change the side wall pressure the conductors experience but will not lessen the pulling tension.

This of course assumes your minimum radius was already code compliant.

Obviously pulling through an LB will be harder than pulling through a large radius sweep.:wink:

I think the thing to remember is that once you get into large pulling tensions the force to bend the wire around the corner becomes such a small percentage of the total force that it becomes insignificant.

This is not the case when you have a 3/4" EMT with three 90s in 10' and try to pull six solid 10s through it.

In this case the bending of the conductors becomes a much higher percentage of the total force needed.

By the way, twisted conductors (as in triplex) etc) pull better around corners.
 
peter said:
Barbeer,
I don't understand your comment. I was just implying that it would be easier to push the wheelbarrow up a shallow slope than a steep one.~Peter
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reductio ad absudum = ?

[shades of Groucho in "A Day at the Races"] = ?

[except for my quibbles]. = ?

Just what does it all mean!? I am unclear of these references is all I mean. I agree about the shallower of the two slopes.
 
peter said:
However, if the bend is a super bend, then that one push will go in and then another push will push it further and so on until it is through.
Pushing is an important factor in difficult "pulls".
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There is a very important truth in the above.

Friction depends upon the force applied between the cable and the wall of the conduit. For a long horizontal run, this force is caused by the weight of the cable pushing the conduit against the wall.

But at a bend, the force is caused by the tension on the cable itself as it is pulled around the bend. Imagine pulling on both sides of a rope wrapped around a tree; the force of the rope against the tree is caused by how hard you pull.

The side force on a bend is determined by the tension of the cable going into the bend, the tension of the cable going out of the bend, and the total angle of the bend. There is a small deviation from this caused by the stiffness of the wires; pulling calculations ignore this deviation.

The tension going out of the bend is the sum of the tension going into the bend and the friction of the bend itself.

The friction of the bend itself depends upon the side force. (Yes, as I've written it it sounds circular, but the equations can actually be solved.) What this tells us is that if you can reduce the tension on the cable going into the bend, you reduce the side force and thus the friction in the bend, which reduces the tension coming out of the bend, again reducing the side force.

The net result: a bend is a big multiplier of the tension coming into it. With zero tension going into a bend, there is essentially _no_ friction produced in the bend itself.

This is why pulling forces are lower if you start from the side closest to the bends, and why it pays to have someone push wires in if they are close enough to the first bend that they can effectively reduce the tension at the bend to zero.

This is also how you can control a descending load by wrapping the support rope around something stationary; if you pull on the rope then you create the tension that causes the friction that causes the rope to grip your support bar; but when you release your tension you release the friction and the load descends.

There is also a cool device known as a torque amplifier that works by using cord wrapped around a pair of spinning drums. The drums spin in the opposite direction, so there is a clear 'input' and 'output' side of the cable. Since the cable is stationary and the drums spinning, 'input and output' are the reverse of pulling a moving cable through a stationary conduit. But the same result of 'input' tension adjusting 'output' tension occurs. The output shaft is held stationary by the balanced large forces caused by cable tension, caused by the friction of the cable on the spinning drum. This friction is adjusted by relatively small forces on the 'input' shaft. The net result is that the output shaft will spin in synchronism with the input shaft, but with much more torque behind it.

peter said:
Winnie,
You must be an engineer or something. I was just pulling your leg. I've been reading "Building Scientific Apparatus" and they sure make a mess out of their formulas.~Peter
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I do hope that I took your comments in the tone intended and tried to respond in kind :) Thus "ass-u-me" a the core of the explanation of what we mathematical types do :)

-Jon
 
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