# Conduit sizing - above and beyond the NEC



## Dennis Alwon (May 9, 2009)

I don't think there is a way to figure it. I think first you would have to understand that any fill with 4 90's is a bit difficult so if you are doing larger size wires with a 40% fill and 4 nineties I would upsize the conduit 1 size-- For me it is always a guess. I think with experience it gets easier.


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## Pete m. (Nov 19, 2011)

I believe some of the equation for pipe sizing involves the pulling tension on the conductors themselves. I know that I've seen posts from one of our members, I think it was http://www.electriciantalk.com/members/don_resqcapt19-13840/, on the same subject.

Other than that I've always looked at it like Dennis.

Pete


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## don_resqcapt19 (Jul 18, 2010)

You can run the pulling calcs here. 
One of the statements on that website is:


> *The size of raceway does not effect the pulling tension or the sidewall pressure. This assumes that the raceway is not overfilled and is in compliance with the NEC.*


*
*The only thing that changing the conduit size can do is to create a very small reduction in the friction. With a smaller conduit and the same size conductors, there would be slightly more surface area of the wire in direct contact with the sidewall of the raceway as a result of the reduced radius for the smaller raceway, but it would have a very very small effect on the required pulling tension.


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## chicken steve (Mar 22, 2011)

Isn't there an engineered tension one can pull wire? The reason tuggers have gauges?

~CS~


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## Cow (Jan 16, 2008)

Carultch said:


> My question is, how do you know? How can you calculate what size larger than the NEC you would need, when considering the complexity of the pull?


Experience. 

Learning the hard way, and telling yourself you'll never run the minimum again. Knowing that 30-35% fill goes a lot easier than 35-40% fill. Especially with multiple bends.


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## Carultch (May 14, 2013)

Cow said:


> Experience.
> 
> Learning the hard way, and telling yourself you'll never run the minimum again. Knowing that 30-35% fill goes a lot easier than 35-40% fill. Especially with multiple bends.


Being the person whose work is mostly at a desk specifying sizes, I don't get much opportunity to learn the hard way, feeling the pull tension in action. But I do hear about the consequences when a major mistake happens from my design.

Is there a certain percent fill, which you can confidently know you have specified a practical solution? (And obviously not look like an ignorant "CYA" engineer who puts a 20A circuit in a 4" pipe)

Would I be able to say that as long as I remain below 30% fill, that I would always be in good shape? And maybe on simpler pulls, I can be as ambitious as 35% fill.

Or are there ever reasons to limit yourself to 25% fill or even lower?


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## ponyboy (Nov 18, 2012)

You can do the math every time you plan a run and it'll shock how much you can fit in there. The math will tell you you can compliantly fit (4) 4/0s in a 2" EMT but experience will tell you that's just not a good idea


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## hardworkingstiff (Jan 22, 2007)

IMO, how the conductors and being pulled in is a factor that the NEC does not address. If you are using a tugger, then pretty much you will be OK with the 40% fill.

If you are using manual labor, then it could be difficult at times.


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## Carultch (May 14, 2013)

chicken steve said:


> Isn't there an engineered tension one can pull wire? The reason tuggers have gauges?
> 
> ~CS~


There are such calculators, but I can't find any that assist in providing feedback on conduit size increases.

I do find ones that inform you of the "jam ratio" and whether that problem can happen.


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## don_resqcapt19 (Jul 18, 2010)

Carultch said:


> There are such calculators, but I can't find any that assist in providing feedback on conduit size increases. ....


That is because increasing the conduit size over what is required by the rules in the NEC has almost no effect on the pulling tension. 

The pulling tension is a function of the weight of the conductors and the friction between the conductors and the inside wall of the conduit. When you increase the size of the conduit you do not change the weight of the wire and you only make a very very small change in the amount of friction, so there is almost no change in the amount of force that is required to install the conductors.


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## Pharon (Jan 20, 2014)

Carultch said:


> Being the person whose work is mostly at a desk specifying sizes, I don't get much opportunity to learn the hard way, feeling the pull tension in action. But I do hear about the consequences when a major mistake happens from my design.
> 
> Is there a certain percent fill, which you can confidently know you have specified a practical solution? (And obviously not look like an ignorant "CYA" engineer who puts a 20A circuit in a 4" pipe)
> 
> ...


As the specifier, you can really only guess at which means and methods the contractor will use in the field. As such, I typically specify what the NEC requires and don't try to guess at how difficult the pull will be, counting the ground as the same size as the phase conductors (even though it's smaller) just to err on the conservative side.

The installer can always choose to use the next size larger if it's determined it will make the pull easier.


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## 360max (Jun 10, 2011)

...on smaller conductors, keep in mind stranded pulls a whole lot easier than solid


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## RIVETER (Sep 26, 2009)

If you are pulling a certain amount of conductors into a conduit that the NEC allows, then, knowing that the NEC is the minimum, anything above that is better and possibly conjecture.


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## BBS (Aug 19, 2009)

If you're concerned about it, use the Raceway Pulling Tension Calculator linked to above. 
You're right, it doesn't tell you to increase conduit size, but it does tell you to use greater radius sweeps.
It's possible you might not be able to locate a higher sweep radius in the conduit size you want to use. In that case going to a higher conduit size might enable the use of larger radius sweeps.


Specify the correct conduit size, specify large sweeps. If the guys in the field don't follow your instructions it's their own fault.


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## don_resqcapt19 (Jul 18, 2010)

BBS said:


> ...
> You're right, it doesn't tell you to increase conduit size, but it does tell you to use greater radius sweeps.
> ...


Changing the radius of the bends does not change the pulling tension, but it does reduce the sidewall pressure. In most pulls with bends, especially bends at the pulling end of the run, the sidewall pressure, not the pulling tension is the limiting factor.


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## chicken steve (Mar 22, 2011)

We've a history of long UG lateral services were we are. The poco's spec long rad sweeps. Some spec GRC long rad sweeps , the UG conduit being pvc. 

Although i've never had it happen, i guess the pull rope can _burn _it's way through a pvc sweep on the far end. 

We rig it all with greenlee baskets & sheaves, the reels are all set up, etc. It's just my _(unproven) _opinion that, pulling anything over a 1/2 ton of URD triplex into _any _conduit is a _bad _idea.

We'll often break a run up in ground boxes for this reason

~CS~


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## Southeast Power (Jan 18, 2009)

The only useful information I can agree with here is that larger conduit = longer sweeps.
All of the calculators will assume the standard sweep measurements unless you can change them. Straight runs are really not much different.
Put one or two 90s in a run and then you have to so some planning.
If you have the opportunity to bend your own 90s, figure out how to bend them in segments that will mimic the next size up conduit. If you are using factory 90s, consider using two 45s where you can if practical.


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## pete87 (Oct 22, 2012)

Carultch said:


> I completely understand the standard NEC requirements for sizing conduits based upon wire fill area. 9 applications out of 10, it means that you can fill a conduit to 40% fill, which intuitively makes sense.
> 
> What I do not understand, is how to make the decisions about sizing above and beyond the NEC, that often is required for practical pulling of the wire. My question is, how do you know? How can you calculate what size larger than the NEC you would need, when considering the complexity of the pull?
> 
> ...


Increasing the pipe size is not the Main Factor in Planing , layout of the pipe run . Besides the obvious , i might use a Larger Pipe for future use in getting somewhere .
It must really suck when using different crews that are not responsible for their work . 
I have only seen 2 pipe runs in my life that the pulls failed . One the Equipment was pulled off the wall ... the other , the nose caught in a coupling . Experience is needed with your pipe crew leader .



Pete


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## Carultch (May 14, 2013)

pete87 said:


> Increasing the pipe size is not the Main Factor in Planing , layout of the pipe run . Besides the obvious , i might use a Larger Pipe for future use in getting somewhere .
> It must really suck when using different crews that are not responsible for their work .
> I have only seen 2 pipe runs in my life that the pulls failed . One the Equipment was pulled off the wall ... the other , the nose caught in a coupling . Experience is needed with your pipe crew leader .
> 
> ...


Often times what happens, is we specify a conduit run to be built by the project's general electrical contractor. Someone who will be there during the time when a building still has open framing, and who can coordinate with trades such as the roofer to furnish the penetration seals.

Then, once the building is ready for my equipment, it is time for the electrician who does the work specific to my scope, to pull the wire and connect it to my equipment.


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## pete87 (Oct 22, 2012)

Understood Carultch , Proably the best youcan do is have a pull string in and waiting for you .






Pete.


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## jimmy21 (Mar 31, 2012)

A lot of it also has to do with where it will be pulled from. If you have a 300' run with 6 90s and a C in the middle, its a lot different if you are going to pull through the C vs actually using the C. Sometimes that can't be figured out ahead of time, though. Who knows where the other trades will stack their crap and who will have priority when it will come time to pull. 

Also, other things come in to play, copper vs aluminum. Is it going to be pulled by hand, or is it a tugger. Where your pull points will be and how awkward they are. Its not so cut and dry but I think you could set up a rule like anything longer than x number of feet with more than 3 90s will be be up sized to 30% fill


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## don_resqcapt19 (Jul 18, 2010)

jimmy21 said:


> ... Its not so cut and dry but I think you could set up a rule like anything longer than x number of feet with more than 3 90s will be be up sized to 30% fill


If a 30% fill is four 350s in one size conduit and if a 40% fill is four 350s in a smaller size conduit, there will be almost no difference in the required force to install the conductors.


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## Dennis Alwon (May 9, 2009)

don_resqcapt19 said:


> If a 30% fill is four 350s in one size conduit and if a 40% fill is four 350s in a smaller size conduit, there will be almost no difference in the required force to install the conductors.


Don, I hear what your saying but I can say from experience a run with 4- 90° 
bends as oppose to 2 or 3 bends takes a lot more manpower to pull. I don't understand how this doesn't change the force


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## pete87 (Oct 22, 2012)

Carultch , the best benefit of installing Larger pipe is when your crew pulls in the wire and they do not feed the conductors in properly and cross them ,
they will pull better in larger pipe .



Pete


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## don_resqcapt19 (Jul 18, 2010)

Dennis,
I didn't say adding 90s doesn't change the required pulling force. I said increasing the conduit size or the radius of the 90s does not change the required pulling force.


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## BBS (Aug 19, 2009)

don_resqcapt19 said:


> I said increasing the conduit size or the radius of the 90s does not change the required pulling force.


Generally I agree with most of what you're saying, but did you not just contradict yourself?

As stated before, increasing bend radius will decrease sidewall pressure. In a conduit run with a number of 90s sidewall pressure will be a limiting factor in the force required:


don_resqcapt19 said:


> Changing the radius of the bends does not change the pulling tension, but it does reduce the sidewall pressure. In most pulls with bends, especially bends at the pulling end of the run, the sidewall pressure, not the pulling tension is the limiting factor.


Friction is directly proportional to the force between the objects, in this case the sidewall pressure acting between the conductors and the conduit. As sidewall pressure goes up, the friction increases, requiring higher pulling tension to pull past the bend.
Increased bend radius will result in a lower sidewall pressure, resulting in less friction, requiring less pulling tension.


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## Carultch (May 14, 2013)

BBS said:


> Increased bend radius will result in a lower sidewall pressure, resulting in less friction, requiring less pulling tension.


In theory, friction force on a cord pulled against a fixed cylinder is independent of the radius of the cylinder. The formula contains the friction coefficient, the angle of how much the cord is wrapped, and the two tension values. It doesn't contain the radius. Force of friction is independent of surface area, in the general sense. A fine point pencil and a dull point pencil, both when pressed and dragged against the same paper with the same compression force, will experience the same friction force.

When you represent the compression force from the cord onto the cylinder on a per-unit-area basis (i.e. pressure), that is when radius matters. And this is what sidewall pressure means. It is a per-unit-area basis concept. Too much sidewall pressure, and you damage your insulation or possibly your conduit. The same pulling force occurs on both a small sweep and a larger sweep of an otherwise identical setup, but on the larger sweep, the force is better distributed onto the wire and conduit surfaces, so as not to cause as much damage.

Force measured in pounds, isn't affected.
Pressure measured in psi, is affected.


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## don_resqcapt19 (Jul 18, 2010)

BBS said:


> ...
> Friction is directly proportional to the force between the objects, in this case the sidewall pressure acting between the conductors and the conduit. As sidewall pressure goes up, the friction increases, requiring higher pulling tension to pull past the bend.
> Increased bend radius will result in a lower sidewall pressure, resulting in less friction, requiring less pulling tension.


With a smaller radius there is a higher sidewall pressure and higher friction, but over a shorter length. With a longer radius there is lower sidewall pressure and lower friction, but over a longer length.

The total friction is the friction per inch times the number of inches. The result is almost the same friction for both the short and long radius bends.


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## BBS (Aug 19, 2009)

don_resqcapt19 said:


> The total friction is the friction per inch times the number of inches. The result is almost the same friction for both the short and long radius bends.


Thank you. That explains it well.


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## butcher733 (Aug 4, 2012)

pete87 said:


> Carultch , the best benefit of installing Larger pipe is when your crew pulls in the wire and they do not feed the conductors in properly and cross them ,
> they will pull better in larger pipe .
> 
> 
> ...


This strikes me as an extremely practical reason to upsize pipe in high fill situations.


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## don_resqcapt19 (Jul 18, 2010)

butcher733 said:


> This strikes me as an extremely practical reason to upsize pipe in high fill situations.


You need to look at the "jam" calculations. There are cases where increasing the conduit size increases the probability of a "jam", however jams are unlikely where 4 or more conductors are being installed in the raceway. Chapter 9, Table 1, Informational Note #2


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## dronai (Apr 11, 2011)

Carultch said:


> Often times what happens, is we specify a *conduit run to be* *built by the project's general electrical contractor*. Someone who will be there during the time when a building still has open framing, and who can coordinate with trades such as the roofer to furnish the penetration seals.
> 
> *Then, once the building is ready for my equipment, it is time for the electrician who does the work specific to my scope*, *to pull the wire and connect it to my equipment.*


This is part of the problem. If the installer had to pull in the runs after running the conduit, he would plan very carefully to avoid what is happening. 
As an installer, I would use my own judgment and upsize if needed, or provide pull boxes, and condulets as needed. The wire puller is getting screwed with the installer sticking strictly to the plan.


Interested what Don is saying about a 36" sweep not being easier to pull into than a tighter 90. Experience tells me I don't agree with the theory. Also upsizing above and beyond seems to always make the run much easier to pull in.


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## Cow (Jan 16, 2008)

dronai said:


> Interested what Don is saying about a 36" sweep not being easier to pull into than a tighter 90. Experience tells me I don't agree with the theory. Also upsizing above and beyond seems to always make the run much easier to pull in.


I have the same thoughts as well.


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## ponyboy (Nov 18, 2012)

Cow said:


> I have the same thoughts as well.



Yeah I don't need math or formulas to tell me what's going to pull like butter or what's going to pull like a dead cow. No offense.


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## Cow (Jan 16, 2008)

ponyboy said:


> Yeah I don't need math or formulas to tell me what's going to pull like butter or what's going to pull like a dead cow. No offense.


Coming from a dairy background, I have actually dragged(or is it drug?) dead cows. And I would not want my wire pulls to be anything like that! :laughing:


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## ponyboy (Nov 18, 2012)

Cow said:


> Coming from a dairy background, I have actually dragged(or is it drug?) dead cows. And I would not want my wire pulls to be anything like that! :laughing:



That's what the JD 60 is for.


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## don_resqcapt19 (Jul 18, 2010)

Cow said:


> I have the same thoughts as well.


I have not really noticed a difference as most pulls that I have been involved with where this would be an issue used a tugger. 

However I have hand pulled some very long radius (took over 10' of conduit to make the 90) 3/4" and 1" conduits with #10 and smaller conductors, and those seemed to pull much harder than a conduit run with standard 90s.


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## Dennis Alwon (May 9, 2009)

dronai said:


> Interested what Don is saying about a 36" sweep not being easier to pull into than a tighter 90. Experience tells me I don't agree with the theory. Also upsizing above and beyond seems to always make the run much easier to pull in.


Our Power company must not agree either. When we have to run the conduit for them they insist on the longer 90's being installed. I can only assume that they believe it makes the pull easier. That does not mean they are correct.


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## mitchellelectric14 (Dec 5, 2014)

Hey I'm new and have nec questions


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## ponyboy (Nov 18, 2012)

mitchellelectric14 said:


> Hey I'm new and have nec questions



Ask away. I'm pretty much the brains of this operation.


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## BBS (Aug 19, 2009)

mitchellelectric14 said:


> Hey I'm new and have nec questions


If you are an electrical professional then please ask your questions in a new thread that you create in the relevant section of the forum, unless they are directly related to the subject of this thread. Then you can take part in this discussion.


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## mitchellelectric14 (Dec 5, 2014)

So my dad owns his own electrical firm and I've worked with him from time to time when I have summer break winter and spring but I'm lost....I want to show him I'm really interested in the work witch I am but I don't know what to ask like the knowledge behind the work running circuts hooking up transformers ect do u have any references or questions I could ask
??


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## Carultch (May 14, 2013)

mitchellelectric14 said:


> So my dad owns his own electrical firm and I've worked with him from time to time when I have summer break winter and spring but I'm lost....I want to show him I'm really interested in the work witch I am but I don't know what to ask like the knowledge behind the work running circuts hooking up transformers ect do u have any references or questions I could ask
> ??


I can point you in the direction of this video, for hooking up transformers:
https://www.youtube.com/watch?v=tNVoJbhH764

If you want to talk about selecting a conduit size, that is what we are discussing here.

The NEC has a bare minimum sizing procedure, which most applications (3 wires or more) means you can fill the conduit to 40% fill by area. This represents about 3/4 of the interior diameter of the conduit.

From what I've heard, this can be a lot tighter than you really want to build it. And that is what I was trying to understand, is just how do you determine how much larger to you need to exceed the bare minimum requirements, in order to have peace of mind that you've specified a practical solution.

A lot of my work is on paper, specifying an installation in concept, rather than in the field doing the construction. So I don't exactly get the best "feel" for when building exactly to the NEC is a good practice, and when it is better to make a judgement call and do better than the NEC.


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## Carultch (May 14, 2013)

dronai said:


> This is part of the problem. If the installer had to pull in the runs after running the conduit, he would plan very carefully to avoid what is happening.
> As an installer, I would use my own judgment and upsize if needed, or provide pull boxes, and condulets as needed. The wire puller is getting screwed with the installer sticking strictly to the plan.
> 
> 
> Interested what Don is saying about a 36" sweep not being easier to pull into than a tighter 90. Experience tells me I don't agree with the theory. Also upsizing above and beyond seems to always make the run much easier to pull in.


Maybe I'm old-fashioned, but shouldn't "the plan" account for the sizes that should actually be built?

Such that each crew can stick strictly to their scope of "the plan", and not screw the other crew. I'm the one who makes "the plan", therefore I need to know a method to figure this out.


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