Does NEC 2011 Chapter 9 Table 5 have the correct conductor properties?

[kevinbur]

I was creating a spreadsheet to calculate conduit fill using the calculations given in Chapter 9 of the 2011 NEC and I think I may have found some incorrect values in table 5. After I finished my spreadsheet, I tried to back check the calculations with the values found in Table C.1 and it didn't work out correctly. I've taken into account the 0.8 remainder that will allow for one more conductor if all the conductors found in the conduit are of the same size and the different percent fill allowable per number of wires.

My issue seems to be with the conductor properties of THW/THHW #14 - 8 AWG conductors in Chapter 9 Table 5. Their properties match those of XHW/XHHW (same sizes); however, they do not have the same maximum number of conductors in EMT on Table C.1. My tables calculate all other values correctly except for these four THW/THHW wire sizes. Am I missing something in my calculations or do one of these tables need to change?

- Kevin

 

[Dennis Alwon]

Table C is for information purposes but is often incorrect. I have seen at least 2 or 3 instances of discrepancies between them. Just use Chapter 9

 

[kevinbur]

Ok gotcha.

 

[ZZDoug]

I noticed something similar recently during a masters exam I was taking. The answer choices given on a question didn't include the numbers given in the Annex C conduit fill tables. So I did the calculations using the Table 4 & 5 data and surprise surprise surprise, something was wrong. I don't remember the details, maybe it was #1 THHN in rigid, but it was the numbers in the Annex C tables that didn't match up.

In any event, you would think the NFPA would have used the data from Tables 4 & 5 to get the numbers in the Annex C tables. And these days we have these machines called computers that do that kind of stuff really well. But even that would depend on Table 4 & 5 data being correct to begin with. Considering the amount of time and money that can ride on those tables being correct, you would think more effort would have been put into making sure they are correct.

 

[Dennis Alwon]

I wish I knew why they leave that table as it is filled with errors. If it is informational then it should be correct.