Principles behind conductor fill: 53, 31??, 40

[broundtree]

Everytime I use the conductor fill tables in Chapter 9, it bugs me that I don't understand why the allowable fill for 2 conductors is less than that for 3 or more (31% and 40% respectively). Why is this? I've asked many seasoned masters and engineers, but have never really received a concise, principle-based answer.

 

[iwire]

Two conductors have a much higher chance of jamming in corners when being installed in the raceway than three or more conductors.

 

[broundtree]

Two conductors have a much higher chance of jamming in corners when being installed in the raceway than three or more conductors.

Please forgive my ignorance, but why? I clearly understand why 3 conductors jam easily given the right raceway ID to conductor OD ratios (as mentioned in FPN No.2 of Table 1) as I can imagine a conductor crossing between the other two causing a side by side cross-section rather than triplex or cradle, but why are two more likely to jam than three?

 

[iwire]

but why are two more likely to jam than three?

I would think that is because the two will be side by side and when dragged around the corner they will be pulled into the narrow part of the conduit.

But I could be wrong, it is what I was always told.

 

[winnie]

Conductor fill is calculated based upon the cross sectional _area_ that the conductors take up in the conduit. However once you have a very few number of conductors nearly maxing out a conduit, you need to consider the linear _diameter_ of the conductors.

Rather than dealing with the proper amount of open space needed for conductors to freely move, consider the area of conductors that just barely fit in the conduit. I'll also ignore 'trade sizes'; the hypothetical 1" conduit that I am describing has a real internal diameter of 1"

Start with a conduit that is 1 inch in diameter. This has a cross section of 1000000 circular mils. A _single_ conductor that just _fills_ this conduit would also have 1 inch diameter, and thus a cross section of 1000000 circular mils.

Now look at two conductors in this same conduit. These conductors would have to have a diameter of 0.5", because two side by side would exactly fill 1". The combined cross section of these two conductors is 500000 circular mils. Thus for _two_ conductors, just barely packing a conduit full, the maximum cross sectional area is only 50%

Now look at three conductors in the same conduit. These conductors would have a diameter of 0.464", and three laying in a 'triangle' would just exactly fill the 1" conduit. The combined cross section of these _three_ conductors is 646171 circular mils. So with three conductors and maximum packing, you get to 64.6% fill.

The ratio of 50 to 64.6 is equal to the ratio 31 to 40, give or take a bit of rounding. Unfortunately this doesn't quite work for a single conductor, where keeping the same ratio would permit 62% fill, but code only permits 53%.

-Jon

 

[bphgravity]

I have been told that 2 conductors pulled through a raceway tend to form a "double-helix". (Think of DNA) The conductors cross each other, extend to the inside surface of the raceway, and then cross again over and over. Throughout the length of the pull, this can result in a lot of surface friction. 1 conductor and 3 or more conductors don't have this tendency and only experience surface friction on bends.

 

[broundtree]

Thanks to all and especially Winnie, your explanation got me thinking so I sketched two scenarios of 2" RGS @ max fill with two and then three conductors/cables. It became very clear at that point.

(2) cable max fill for 2" RGS is 1.056in2. A little math yeilds max OD of identical cables is 0.82".

(3) cable max fill for 2" RGS is 1.363in2. Math => max OD = 0.76".

The widest dimension of the (2) cables side by side is 1.64". The widest dimension of the (3) cable bundle is 1.52". So even though (2) cable fill is 31%, the cross section is wider than the (3) cable bundle. And its that widest dimension that'll cause problems while pulling through bends. Makes sense to me now. Thanks again.