Cable Definition

[GoldDigger]

Twisting with one in the center, the outer six will be on an angle. That'll make the cross sectional area of the six each somewhat elliptical. As such, they will not fit perfectly around the seventh. The circumscribing circle will be larger... making the density percentage lower.

True. As the pitch of the twist approaches zero, the ratio approaches the maximum. If you do not keep one in the center it gets worse a lot faster and the density will vary along the length.

 

[mwm1752]

The way identically sized conductors pack into a circle has a density peak at 7 because six conductors fit perfectly around the seventh in the middle.
Kind of like the increased stability of atoms with full outer electron shells.
If you twist the seven without keeping one in the center you lose the otherwise small advantage you get at 7.

I know its no surprise but confusion has now set in -- Are you suggesting one can just make a cable instead of using a factory cable set?

 

[GoldDigger]

I know its no surprise but confusion has now set in -- Are you suggesting one can just make a cable instead of using a factory cable set?

Not at all. Just discussing the details of the close packing, whoever does it.
I am not convinced that the twisted wire bundle is a cable as the NEC uses the word in the fill rule in the first place, so I will leave the distinction, if any, between factory and field twisting to others.
I will also note, though, that if you consider the twisted bundle a cable for fill, you also need to consider it as a unit for bend radius. And without an outer jacket the wires could separate where the bundle bends. And there may be greater twisting forces as it is pulled.

 

[MasterTheNEC]

Hey guys I have really enjoyed watching the debate on this. I really had a motive when I posted it as a response to Don but it took on a life of it's own. Ironically, the goal was to show how a manufacturer looks at "cable" versus how the NEC may look at it. I know for a fact the cable and wire industry look at it as a cable without an overall jacket but the NEC looks at it totally different and I understand that. The fact that Chapter 3 does not actually express any of the "cables" within that chapter as simply twisting "cabling" individual conductors I do believe it is hard (and an uphill battle) for AHJ's to consider it as such as use the 53% fill.

As I stated, we have never done this and always sell it assuming the end user will work off the 40% fill but others who produce wire actively market it as..well someone posted their information so I wont go into that. In the end, we have our opinions and I have mine as well but alas being NEC Geeks it is hard to get from point A to point B in the NEC without assuming the AHJ will listen to an educated argument.

So this is why we do not promote it as already being a "cable", even if our tests support it (and the radius part does not factor since they all bow at bends) the NEC does not make it 100% clear so it is up to the AHJ to make the call. We would most certainly not want to tell someone who buys 500,000' of multiconductor 4/0 that you can use the 53% as a single cable only to have it installed and failed by an AHJ who believes it should use the 40% fill ....we sell product, not expectations.....

However, I figured I would throw it out their, try to lead the conversation (sorry for jacking the thread Don) and see what floated to the surface. Thanks Guys.