Conduit Fill

[TacoMancer]

the question came up in my class about conduit fill. In the note(s) to table 1 in the 2011 NEC. with one conductor a conduit is filled 53%,two conductors 31%, three conductors 40%. the question is how these percentages came about and why? the best answer i could find was that the percentages are not based on the conduit volume or heat, but knowing what it is not doesn't really help all that much. I've heard theories about how the fill is different due to magnetic fields canceling each other out but i can't find any reference to support that. can anyone help? my whole class would appreciate some direction...

 

[iwire]

Nothing to do with heat, heat is handled by derating requirements.

Nothing to do with magnetic fields, that is handled by code sections like 300.3(B).


It has to do with the space in the conduit and the likely hood of the conductors jamming while being pulled in. (Jamming, the conductors crossing over each other and getting stuck in the conduit).

A single conductor can not jam against another conductor so we can fit it tighter in the conduit.

Two conductors in a conduit has the most chance of jamming so we can't fill the conduit as tight as we can with a single conductor or multiple conductor dc

Three or more conductors is less likely to jam than two so we are allowed to fill a bit more but not as much as a single.

 

[TacoMancer]

Great!

Great!

Awesome! now we are getting somewhere.

So you are saying that the volume of the conduit can be filled up to 53% of its total volume with a single conductor because the conductor has a very small chance of jamming. But I don't see how three conductors can jam less than two.

i guess the most confusing thing about it is that you would think that as the number of conductors increases, the percentage fill should be a direct correlation. but %fill for two is less than the %fill for three

 

[Dennis Alwon]

There was a thread on this where someone, I think Smart, did a graphic showing why the 2 wire scenario is worse than the 3 wire fill.

 

[Ponchik]

Here is link that may be helpful.

http://www.polywater.com/jamming.html

There is another one I will find.


http://ecmweb.com/content/simple-calculations-cable-pulling

 

[iwire]

So you are saying that the volume of the conduit can be filled up to 53% of its total volume with a single conductor because the conductor has a very small chance of jamming.

Almost, change it to 'can't jam' because there is only one so it has nothing to jam against.

But I don't see how three conductors can jam less than two.

Tons of info if you Google it but here is a start.

http://m.ecmweb.com/content/simple-calculations-cable-pulling

 

[Dennis Alwon]

Here is the thread with the graphic I mentioned http://forums.mikeholt.com/showthread.php?t=151674&highlight=conduit+fill