Neher-McGrath calculations provide an alternative method for computing derating of conductors in conduit.
While an engineer is required to oversee the computations. This shows how I would do the computations.
My goal is to place as many 12g wires in a conduit as possible while maintaining 20amp capacity.
12g THHN (90 degree) has 30amp capacity. 7 to 9 current carrying conductors in conduit are derated to 21amps. 9 current carrying conductors (c) plus 4 grounds will fit in 3/4" PVC-40. 3/4" PVC-40 has an inside diameter (id) of .824" and a wall thickness (t) of .113".
From the Neher-McGrath calculations (or general heat transfer solutions) one can deduce that id/t/c is a constant. In this case the constant is .81.
To compute the number of conductors allowed in a different size PVC-40 conduit use the formula
c=(id/t)/.81.
2" PVC-40 conduit has an 2.067 and a wall thickness of .154.
(2.067/.154)/.81 = 16 current carrying wires.
Fill computations allow up to 99 12g wires in 2" conduit. So fill in not a concern. 8 two wire THHN circuits plus grounds will fit. There might be enough space to use 8 12-2+G NM circuits in 2" conduit.
While I chose 12g wire for my starting point, an engineer is free to start where he wishes. He can be more agressive than I.
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There is one detail. Neher-McGrath requires the conduit spacing be the lesser of 3 times the conduit diameter (center to center) and the wall thickness.
