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NEC Reason for 31%-2 Conductor Fill?

NEC Reason for 31%-2 Conductor Fill?

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.


RE: NEC Reason for 31%-2 Conductor Fill?

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

RE: NEC Reason for 31%-2 Conductor Fill?

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.   

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