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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.

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  • Hello, and welcome to Home Improvement. Be aware that in home construction, the physical limits of something is often less important than what code allows, and I'm pretty sure that's true here. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Dec 14 '19 at 13:56
  • Thanks for the quick response @DanielGriscom! That's exactly what I am trying to do - calculate what the NEC code allows. – John DiFini Dec 14 '19 at 14:00
  • Keep in mind that googling .conduit fil calculator" will bring up dozens of reasonably well-executed calcs of considerable complexity. No need to reinvent the wheel My only gripe with them is you cannot for instance, select 10/3 UF cable, which is treated as a round wire of the large dimension. – Harper - Reinstate Monica Dec 14 '19 at 17:11
  • I tried googling for conduit fill calculators, but I didn't like any of them for two reasons: 1) they didn't factor in the number of bends, and 2) didn't include how the calculations were being done. – John DiFini Dec 14 '19 at 17:32
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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.

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  • Thanks, @NoSparksPlease. That got me on the right track! In the 2017 edition, Ch 9, note (10), pg 70 –679 states the following. To save the brain damage, I am just going to roll with the worst-case CSA. The values for approx. conductor diameter & area shown in Table 5 are based on worst-case scenario & indicate round concentric-lay-stranded conductors. Solid & round concentric-lay-stranded conductor values are grouped together for the purpose of Table 5... – John DiFini Dec 14 '19 at 15:18
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2.08 mm^2 is for the conductor, and is relevant to geeky physics considerations like determining scrap value or computing wire resistance the hard way. You won't do much with this figure, as all the math is already done for you in 310.15 and 240.4.

6.258 mm^2 is for the wire, inclusive of insulation, and is relevant to practical questions of conduit fill. As an electrician you are far more likely to work with this figure.

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