If a circuit supplies a fluorescent, mercury or metal halide do the neutrals still have to be counted as current carrying? This neutral will carry a third harmonic.

Thanks for any replies

Greg

Yes, any non-linear(harmonic) load creates a current carrying nuetral.

Only on 3 phase systems.
Don

... and only when that nonlinear load is the major portion (over 50%) ....

See the 2005 NEC .....

Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.

Article 310

(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 shall therefore be considered a current-carrying conductor.
FPN: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.

You can still induce out of phase currents in a single phase neutral (multiwire circuit) but you won't get the overload effect you get in 3 phase. It can still make measurememnts not balance if your meter isn't true RMS.