Table one of Ch 9 says if we have one conductor we can fill the conduit to 53%, 2 conductor to 31% and more than 2 conductors the conduit can be filled to 40%. My question is why 31% for 2 conductors and 40% for more than 2 conductors? what is rational behind it.
An IAEI instructor once told me it is because of the heat dissipation. But he did not explain in details why?
It just has to do with how they pack into the pipe. Three takes up less room than 150% of 2 and it is volume we are speaking of. Heat really doesn't enter into it. You derate "x" number of conductors the same no matter what size pipe it is in.
Expounding on Tesla, 3 conductors can line up in a straight line so all the diameters add up. 4 or more and bundle collapses (the conductors don't ever twist up to line out in a row. Thus the binding issue for 3 conductors.