What Size Rigid Metal Conduit Holds Six 8 AWG THHN Conductors?
This is a classic journeyman exam question and the answer is 3/4 inch rigid metal conduit. It is also one of the tightest calls in the whole fill table, passing by four ten thousandths of a square inch, so it is worth walking through slowly.
Running the numbers
Chapter 9, Table 5 gives the approximate area of 8 AWG THHN or THWN as 0.0366 square inches. Six of them is 0.0366 times 6, which equals 0.2196 square inches of conductor.
Chapter 9, Table 1 sets the fill limit at 40 percent for three or more conductors. Chapter 9, Table 4 gives the internal areas for rigid metal conduit. Half inch RMC has a total area of 0.314 square inches, and 40 percent of that is 0.125 square inches. That is nowhere close to 0.2196, so half inch is out. Three quarter inch RMC has a total area of 0.549 square inches, and 40 percent of that is 0.220 square inches. Compare: 0.2196 needed against 0.220 available. It fits, with 0.0004 square inches to spare.
Table 4 actually prints the 40 percent column for you, so on an exam you can skip the multiplication and read 0.220 directly out of the over 2 wires 40 percent column for 3/4 inch RMC. That saves a step and avoids a rounding mistake, and rounding matters a lot on a margin this thin.
Where the answer changes
Swap the wire type and the answer moves. Six 8 AWG XHHW is 0.0437 square inches each, or 0.2622 total, which blows past 0.220 and forces 1 inch RMC. Six 8 AWG RHW with an outer covering is larger still. Insulation type drives conduit size as much as conductor size does, which is exactly why Table 5 is organized by insulation rather than by gauge.
Swap the raceway and the answer moves too. For 3/4 inch EMT the 40 percent fill is 0.213 square inches, which is less than the 0.2196 needed, so six 8 AWG THHN in EMT requires 1 inch. For 3/4 inch PVC Schedule 40 the 40 percent fill is 0.203 square inches, which also fails, so PVC needs 1 inch as well. Rigid metal conduit has thicker walls than EMT but a larger nominal inside diameter at the 3/4 inch trade size, which is what lets it squeak by.
In the field, most electricians would pull this in 1 inch anyway. Six 8 AWG at the theoretical maximum fill is a miserable pull, especially with any bends, and Chapter 9 Note 4 reminds you that where conduit nipples not exceeding 24 inches are used the fill may go to 60 percent, which is a hint that the 40 percent number was set with pulling tension in mind. Pass the exam with 3/4 inch. Order 1 inch for the truck.
One more detail for the six conductor case: if all six are current carrying, 310.15(C)(1) applies an 80 percent adjustment factor. THHN 8 AWG at the 90 degree C ampacity of 55 amps derates to 44 amps, then terminations pull you back to the 75 degree C column value of 50 amps under 110.14(C), so the usable ampacity is 44 amps.