harmonic and non linear load

[newb]

do i need to count the neutral of a 3 phase 4wire transformer secondary of a delta-wye system as a current carrying conductor per 310.15(b)2. their is a code right under the 310.15(b)2 table (i dont have the code right in front of me) that states if you have a 3p 4w wye system where the majority of the load consistes of non-linear loads, harmonics are introduced, and the grounded conductor shall count as a current carrying conductor. so more then 50% of the loads i will be supplying are LCD computer screens(just the screen, not a computer) and low voltage LED (light emitting diode) lighting, which is powered from class two power supplies and electronic contol boards.
my load is about 450 amps. Is the non linear load something i should be concerned about with a load of this type and size? I will problably count the neutral anyway and derate, but i would like to here what others have to say.

 

[infinity]

I think that either way you would be OK. IMO the entire issue of harmonics has been a bit overstated. In your scenario it would be very difficult to predict if your harmonic loading would exceed 50%. If it makes you feel safer than go ahead with counting all 4 conductors as current carrying.

 

[dsteves]

Avoid any estimations which would allow you to undersize the neutral based on an assumption of perfectly balanced L-N loads reducing the neutral burden. Although unusual, it is possible for neutral current to theoretically reach sqrt(3) of the line current in purely theoretical systems. Conservatively, you can oversize the neutral conductor by two AWG sizes over the phase conductor size, and in any instance I have encountered that will avoid any issues; however, you should understand that approach is the ultimate conservative design, and that it may be (a) unnecessary and (b) costly if not justifiable.

Regardless of any calculation you may use, you must consider the neutral in that system as a current-carrying conductor, regardless, unless you are building a delta-connected load. You state that the system is 3ph 4w, and I see no evidence that you are not using the neutral at utilization equipment.

There is no substitute for knowing the intended load, but if you have to guess, guess high by 1.732 times phase load in a wye system and you'll never burn a neutral.

Dan