How many 12/3 cables in 1/2" (trade) Greenfield?


I am having difficulty understanding and applying the NEC tables I have seen online to cables containing more than one conductor.
Is there any simple rule that says how many 12/3 Romex cables you can put in flexible conduit (Greenfield)?
Specifically, what trade size flexible conduit would I need to carry 2 12/3 Romex cables? What size for 3 12/3 cables?
Thanks!
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OOPS - just to clarify I meant 12/2 (plus ground) not 12/3.

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The flexible metal conduit is only being used to "protect" the wires in the vertical drop from their run along my basement ceiling joists to the waist-level outlet boxes -- approximately 4 ft total length. The rest of the run is pure Romex (and appropriate bushings are used to protect the wire where it enters the flex conduit and where the flex conduit attaches to the 4x4 outlet box).
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writes:

I'm guessing that 3/4" for two and 1" for three 12/2's. Keep in mind that the greenfield will need to be terminated at one end in a grounded metal box as all metal conduit carrying live wires needs to be grounded.
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On Tue, 2 Sep 2008 07:26:01 -0400, "John Grabowski"

You are probably guessing one size too small. When you use an oval cable you need to calculate it as a circle the diameter of the widest dimension. They assume the cable may be twisted Since a 12/2 is about .4" across the flat side it would look like a #1 single THHN conductor in your calculation. That means one in a 1/2", one in a 3/4" and one in a 1" (table C3). You could surely get more in there if you carefully stacked and taped them but it would not be code compliant.
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12/3 (round) cable actually has a smaller diameter than 12/2 (flat) cable. So you may consider using 12/3 for that reason.
Cheers, Wayne
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On Tue, 02 Sep 2008 18:28:20 GMT, Wayne Whitney

Excellent advice.
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