It was my understanding any circuit supplying fluorescent lighting loads the neutral was counted as current carrying. It was also my understanding that over 9 conductors in a raceway that conductor could only carry 50% the total ampacity.
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The distinction is made for nonlinear loads, but the extent to which a particular fluorescent ballast acts as a nonlinear load will vary.
If the Code feels that any degree of nonlinearity triggers the exception, then you would have to count it. If the actual harmonic content of the particular luminaires is low enough, it should not be a practical problem.
If the nonlinearity is bad enough, triplen harmonics in particular, you might have to provide an oversized neutral and it would definitely be current carrying.
From definitions, Article 100, Nonlinear Loads:
Informational Note: Electronic equipment, electronic/electric- discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
But from 300.15(B)(5)(c):
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.
Which does not seem to allow as much leeway for interpretation IF you automatically consider fluorescents to be non-linear.
