NEC Reason for 31%-2 Conductor Fill?

[6yhnmju7]

Why are 2 conductors in conduit/tubing limlted to 31% fill while 3 conductors with the potential of jamming is allowed 40%. I work overseas on a USAF base where usually a 4 conductor cable is run with a smaller ground wire.

Jim

 

[davidbeach]

Simple geometry and packing efficiency. Two circles inside another circle is far less efficient than three (or more) circles inside another circle.

 

[dpc]

If you put the ground wire inside the cable, then you can treat the cable as a single conductor for fill calculations. The ground wire is better inside the cable anyway.