For conductor sizes No. 14 to No. 8 for insulations THHW, THW, and THW-2 the fill tables are incorrect. These conductors should have the same fill numbers as TW since they have the same cross sectional areas as TW. This was confirmed by using a computer program that recalculates the fill based on areas from Tables 4, 5, 5A, and 8 located in Chapter 9 of the NEC. The program is the raceway and trough fill program from Code Calculators downloadable for free at:
I have submitted the following proposal to change the 2011 NEC.
For Informative Annex C, Tables C.1 to C.12 add THHW, THW, and THW-2 to the TW section for No. 14, No. 12, No. 10, and No. 8 and retain the fill numbers as shown for TW. Remove THHW, THW, and THW-2 from Sections for No. 14, No. 12, No. 10 and No. 8. Redo RHH*, RHW*, and RHW-2* into single section for No. 14, No. 12, No. 10, and No. 8 and retain fill numbers shown for these insulations.
Table 310.104(A) shows dimensions of TW insulation thickness for No. 14, No. 12, No. 10 and No. 8 as equal to THHW, THW, and THW-2. Table 402.3 shows TW as having the same insulation thickness as XF and XFF for No. 14.
Chapter 9 Table 5 page 716 bottom shows that TW has same area for No. 14 AWG as XF, XFF, THHW, THW, and THW-2. On Page 717 top Table 5 shows TW as having the same area as No. 12, No. 10, and No. 8 AWG as THHW, THW, and THW-2. However, the Tables in Annex C, C.1 to C.12, show the TW fill section separate from these insulations with different fill numbers. Either Table 5 is incorrect or the tables in Informational Annex C are incorrect. From the information in Tables 310.104(A) and 402.3 the areas shown in Table 5, Chapter 9 are correct but the fill sections for THHW, THW, THW-2 in Annex C Tables C.1 through C.12 are incorrect. Since THHW, THW, and THW-2 conductors have same cross sectional areas as TW for No. 14, No. 12, No. 10 and No. 8 they should be placed in the same category as TW. By moving insulations THHW, THW, and THW-2 to the TW section and regrouping RHH*, RHW*, and RHW-2* for sizes No. 14, No. 12, No. 10, and No. 8 this will correct the problem. It should also be noted that the present Tables C.1 to C.12 omits THW-2 for sizes No. 12 and No. 10, but Table 5 gives the area for these sizes and Table 310.104(A) lists these as valid sizes.