NEC Chapter 9, Table 1: percent conduit fill question

[stcalle]

In the NEC, I'm using 2005, for Chapter 9, Table 1 which refers to the percent conduit fill for a certain number of conductors, I wanted to know if someone could explain why the percent fill increases the way it does. What I mean is:

Number of Conductors Percent Conduit Fill
1 53%
2 31%
over 2 40%

So refering to the above information, I trying to figure out why the percent conduit fill decreases from 53% to 31% when going from 1 to 2 conductors then increases to 40% when having more than 2 conductors?

Is there a reason why the percent conduit fill does not just consistently increase?

Thanks

 

[electricman2]

Read the fine print notes. It has to do with ease of pulling conductors through the conduit, bends etc.

 

[al hildenbrand]

Is there a reason why the percent conduit fill does not just consistently increase?

In a nutshell, a 5# bag only holds 5#.

On the theoretical side, three or more conductors will "nestle" into each other, somewhat, but there is always lost cross sectional area where the conductors don't dovetail. A group of small conductors don't even come close to "approaching" the 100% cross sectional sum that a single conductor represents.

On the real world "hands-on-wire" side, the wires will not lie as smooth against each other as one can imagine as being theoretically possible. The conductor stiffness at raceway bends, individual conductor kinks, wire irregularities from manufacturing, and tangling during installation all contribute to the need for upper limit of 40%.

My engineer side still marvels at the difficulty, my hands and shoulders feel as the installing electrician, when everything is right at the "theoretical" maximum. Any one raceway pull, with close to 360? of bends, regardless of long or short distance, can have enough drag during conductor pull to damage the insulation during installation without careful practices.

 

[bphgravity]

I believe 2 conductors form a "double-helix" (think DNA) arrangment when pulled through a raceway. This creates an outward force along the length of the pull whereas more than 2 conductors lay flat on the raceway or bundle toward the center.