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NEC Reason for 31%-2 Conductor Fill?

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6yhnmju7

6yhnmju7

Electrical
Joined
Nov 16, 2008
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Location
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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
 
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Simple geometry and packing efficiency. Two circles inside another circle is far less efficient than three (or more) circles inside another circle.
 
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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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