How many 8 AWG THHN fit in 2 inch rigid conduit?

36, using the 1.342 square inch 40 percent area from Chapter 9 Table 4 and the 0.0366 square inch conductor area from Table 5.

Does the ground wire count toward conduit fill?

Yes for fill. Every conductor counts in the Chapter 9 area calculation. It does not count as a current carrying conductor under 310.15(E)(3).

What is 8 AWG worth with 36 conductors in the pipe?

About 19 amperes. 55 amperes at 90 degrees C times the 35 percent adjustment factor from Table 310.15(C)(1).

How many 8 AWG conductors fit in 2 inch rigid metal conduit?

Run the fill math first, then run the ampacity math, because the second one is what actually limits the job.

Conduit fill

Chapter 9 Table 4 lists trade size 2 rigid metal conduit with an internal diameter of 2.067 inches and a total internal area of 3.356 square inches. Chapter 9 Table 1 sets the fill limit at 40 percent for more than two conductors, which Table 4 already computes for you as 1.342 square inches.

Chapter 9 Table 5 gives the approximate area of 8 AWG THHN, THWN, or THWN-2 as 0.0366 square inches.

1.342 divided by 0.0366 is 36.7. Chapter 9 Note 7 says to drop the fraction when the result is a decimal of 0.8 or smaller, so the answer is 36 conductors.

Insulation type moves that number a lot. 8 AWG XHHW at 0.0437 square inches gives 30 conductors. 8 AWG THW at 0.0510 gives 26. 8 AWG RHH or RHW with an outer cover at 0.0819 gives 16. Always check which row of Table 5 you are reading.

Why 36 is not a real answer

Section 310.15(C)(1) requires the ampacity adjustment factor to be applied when more than three current carrying conductors are in a raceway longer than 24 inches. Table 310.15(C)(1) drops you to 80 percent at 4 to 6 conductors, 70 percent at 7 to 9, 50 percent at 10 to 20, 45 percent at 21 to 30, 40 percent at 31 to 35, and 35 percent at 36 to 40.

8 AWG THHN starts at 55 amperes in the 90 degrees C column of Table 310.16, and 310.15(B) permits you to start the adjustment from the 90 degrees C value as long as the final result does not exceed the 75 degrees C termination rating under 110.14(C).

At 36 current carrying conductors, 55 amperes times 0.35 is 19.25 amperes. You just filled a 2 inch pipe with 8 AWG copper and each conductor is worth less than a 12 AWG circuit. Add an ambient correction from Table 310.15(B)(1) for a hot rooftop or a boiler room and it drops further.

Counting current carrying conductors

Not every wire counts. Section 310.15(E)(1) exempts the grounded conductor of a balanced 3 wire circuit derived from a single phase 3 wire system. Section 310.15(E)(3) exempts equipment grounding conductors and bonding conductors entirely. But 310.15(E)(2) does count the neutral of a 4 wire wye circuit where the major portion of the load is nonlinear, which covers most LED lighting and electronic loads.

Section 310.15(C)(1)(a) also exempts you from adjustment where the raceway nipple is 24 inches or less, though Chapter 9 Note 4 then allows 60 percent fill in that nipple.

A practical loading

Twelve 8 AWG current carrying conductors plus grounds in 2 inch RMC lands you in the 10 to 20 conductor bracket at 50 percent. 55 times 0.50 is 27.5 amperes each, which supports 25 ampere circuits and leaves headroom. That is a sane pull. If you need more than that, go to 2 1/2 inch RMC or split the run into two pipes so each stays under the adjustment cliff.

Field takeaway

36 is the Chapter 9 answer for 8 AWG THHN in 2 inch RMC. The number you should design to is closer to 9 or 12 current carrying conductors, because 310.15(C)(1) is what determines whether the circuits are usable.