conduit fill

[dennis ryland]

What is the basis for 2 conductors being allowed only 31% fill and over 2 conductors are allowed 40% fill. As I understand it (from cable mfg literature) jamming is the special case of 3 conductor diameters equaling the diameter of the conduit and 'jamming' under tension. As an example, 2-350 kcm RHH* (no outer jacket) over fill a 2" conduit per table C8 however,if 1 or 2 #10,THHW are pulled in along with them,calculating areas by using table 5, I am allowed to use the over 2 wires 40% fill column of table 4 and it is allowed.

 

[bphgravity]

Re: conduit fill

It is my understanding that two cables within a raceway will tend to twist and form a OVAL type patern throughout the run, whereas one wire or three or more form CIRCULAR patterns. This circle takes less space than the oval shape and EE mathematics show this to be the appropriate percentage fills to ensure the disapation of heat off the conductors.