3/8 MC Fill Calculations 12/3 with Ground

[2Big2Ride]

I needed about 35' of 3 conductors with ground to go between two three-way switches. I had some empty 3/8" MC and solid 12-gauge THHN in black, red, white and green so I filled the MC with the four conductors and roughed in the assembled wire and MC. ...I would swear I saw 12/3-3/8" MC at my local big box hardware store. For some reason I decided I better check to see if that fill was allowable and now am confused and concerned. Which information is correct?

Southwire Conduit Fill Calculator = 38.41% fill (yes, fill is okay to use)

Or my books?

12 gauge THHN = .0133 sq/in each
.0133 * 4 = .0532 sq/in
3/8" MC at 40% fill = .046 sq/in maximum fill (no, fill greater than allowable capacity)

 

[Norcal]

3/8" flex is not allowed for your purpose, 1/2" is minimum for FMC*, Flexible Metal Conduit, & it is not MC, Metal Clad,see 348.20(A).

Metal Clad is a listed cable assembly, w/ the conductors installed at the factory.

There are exceptions to the 1/2" minimum rule but they do not apply to the OP's situation.

 

[2Big2Ride]

I will pull out what I put up and go buy the right stuff. Thanks for the help NorCal.