I have found that stranded 14 AWG THHN is 6.258 sq mm, but I am having a hard time finding the value for solid 14 AWG THHN. I keep seeing 2.08 sq mm, but I find it hard to believe that the cross-sectional area of stranded THHN is ~3 times larger than solid THHN.
BTW, I am using these figures to calculate maximum conduit fill as allowed by the NEC specs. If you are interested, I have created a conduit fill calculator using Google Sheets. You are more than welcome to use it. Just make a copy of the spreadsheet first.
Best Answer
NEC Chapter 9, Table 8 shows 2.08 mm² is the CSA of bare solid #14 awg, and stranded is 2.68 mm².
But those numbers are just the CSA of the copper. You need to use the CSA including insulation, which varies with type of insulation. Those are found in Chapter 9, Table 5, in which you find #14 varies from 3.548 to 18.90 mm².