Hi,
My inverter output power is single phase, 240/120, 3W. In this case, is the neutral considered a "current-carrying conductor"? Also, would the EGC ever be considered a current-carrying conductor? (therefore, would I ever have to take it into account when doing conductor de-rating?)
Thanks in advance.
Regards,
Andy
In most cases with PV inverters, the neutral does not count as a current-carrying-conductor for derating purposes, unless it is immediately obvious that it has to carry the full load as in the case of single-phase-to-neutral units. Sometimes you can run an EGC-sized neutral, but you have to first confirm with the inverter manufacturers, what the maximum current on the neutral will be. Not something they usually document in datasheets or manuals.
Some examples of what the neutral is used to do:
1. Voltage sensing, effective grounding. Never gonna see a full ampere of current.
2. Internal power supplies
3. Return current for unbalanced loads, in the event that the inverter is built from multiple phase-to-neutral units. These do not necessarily all operate at once, so when loads are low, it can be the case that only two out of three are running.
4. Harmonic currents that accumulate on the neutral, instead of cancel.
In all but #4, of the above cases, the gross sum of the current among all 4 wires, never exceeds what the gross sum of what it would be if the full operational amperes were exclusive to only the ungrounded conductors. In the case of #4, this is rarely significant enough to render the neutral a CCC, for 310.15(B)(3)(a) purposes, because it is usually limited to a small minority of the total current. By "gross sum", I mean add up the magnitudes of the current, ignoring the fact that they are on different phases. What really matters is the sum of the squares, but we'll make a conservative assumption and ignore that.
