What derating factor applies to 8 current carrying conductors?

70 percent. Table 310.15(C)(1) applies 70 percent to 7 through 9 current carrying conductors in a raceway or cable.

Do I derate from the 90 degree or the 75 degree column?

Start at the column matching the conductor insulation, which is 90 degrees C for THHN. Apply the adjustment, then limit the result to the termination temperature rating under 110.14(C).

Are neutrals and grounds counted as current carrying conductors?

Equipment grounding conductors are never counted. A neutral carrying only unbalanced current is not counted, but the neutral of a three phase four wire wye circuit with major nonlinear load is counted per 310.15(E)(2).

What is the ampacity of 10 AWG THHN with seven other conductors in the conduit?

28 amperes. Eight current carrying conductors falls in the 7 through 9 range of Table 310.15(C)(1), which applies a 70 percent adjustment factor. THHN is a 90 degrees C insulation, so you start at 40 amperes from the 90 degrees C column of Table 310.16, and 40 times 0.70 equals 28 amperes.

Why you start at the 90 degree column

Section 110.14(C) limits the final conductor ampacity based on the temperature rating of the terminations, which for most equipment is 60 or 75 degrees C. That limit applies to the finished number, not to the starting point. Section 310.15(B) permits conductors with higher temperature ratings to be used for ampacity adjustment and correction calculations, provided the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination.

So the sequence is: start at 40 amperes for 90 degrees C THHN, apply the 70 percent adjustment to get 28 amperes, then compare to the termination limit. The 75 degrees C value for 10 AWG copper is 35 amperes and the 60 degrees C value is 30 amperes. Both are higher than 28, so 28 amperes is your answer either way. Skipping the 90 degrees C start and adjusting 35 amperes down to 24.5 amperes is the most common mistake on this calculation.

Counting current carrying conductors

Section 310.15(E) tells you what to count. A grounded conductor that carries only the unbalanced current from other conductors of the same circuit is not counted, which is why the neutral of a three wire single phase circuit or a four wire three phase wye circuit with linear loads is excluded. Section 310.15(E)(2) reverses that for a three phase four wire wye circuit where the major portion of the load is nonlinear, since harmonic current makes the neutral a current carrying conductor.

Equipment grounding conductors and bonding jumpers are never counted, per 310.15(E)(3). Section 310.15(C)(1) Exception 1 also exempts conductors in a nipple not exceeding 600 mm, or 24 inches, in length from the adjustment factors entirely.

Choosing the overcurrent device

28 amperes is not a standard rating in 240.6(A). Section 240.4(B) permits the next higher standard device, which is 30 amperes, provided the conductors do not supply more than one receptacle for cord and plug connected portable loads and the next higher rating does not exceed 800 amperes. Section 240.4(D)(7) caps 10 AWG copper at 30 amperes anyway, so 30 is the ceiling from two directions.

If the circuit does feed multiple receptacles, 240.4(B)(1) blocks the rounding up and you drop to a 25 ampere device. Many contractors just use 25 amperes on any derated 10 AWG homerun to avoid the argument.

Ambient correction stacks on top

If the raceway is not at 30 degrees C, apply Table 310.15(B)(1) as well. At 41 to 45 degrees C the factor for 90 degrees C insulation is 0.87, so 40 times 0.87 times 0.70 equals 24.4 amperes, and the device drops to 25 amperes. Rooftop raceways get the additional consideration in 310.15(B)(2).

Field takeaway

Start at the insulation column, adjust, correct, and only then apply the termination limit. Eight current carrying 10 AWG THHN conductors give you 28 amperes, protected at 30 amperes for a fixed load or 25 amperes on a multi receptacle circuit.