What is the cross-sectional area of solid, 14 AWG THHN wire?
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.