tables 4 chapter 9

[JeffD]

I was wondering if anyone can tell me why in the tables 4 of chapter 9 you are allowed less conduit fill with 2 wires than over 2? I would naturally think that 2 would be between 1's of 53% and 2's of 40% but it is only 31%. Just something I always questioned and never heard an explanation for.

 

[bphgravity]

See the FPN's to Chapter 9 - Table 1. These percentages are based on the dynamics of conductors being pulled through a raceway. Two conductors tend to jam when filling more than 31% of the raceway.

I have been told that 2 conductors form a double-helix (think DNA) when being pulled which exterts outward force onto the interior conduit wall. This is less likely to occur with over 2 conductors.

 

[Pierre C Belarge]

Those pesky little FPNs. The only reason the print is not any smaller, is the printers cannot make it any smaller.
Hence it is easy to pass right by them when reading the NEC. Some of them have some very good info contained within them, such as the note that Bryan pointed out.

 

[JeffD]

Thanks Bryan,

I'm an engineer and don't pull wire so I wasn't aware that 2 wires jam easier than more than 2.

 

[ryan_618]

I have been told that 2 conductors form a double-helix (think DNA) ...

That makes it much easier. I'll have to remember that the next time I am looking at DNA in my microscope! JK