I have a solar system design for a class project. It shows that we have a PV system connected with microinverters to a 3 phase service. I have calculated everything except i cant figure out per the NEC how to size the neutral since its a balanced 3 phase system. So my 100amp PV system connected to 3 phase, how do i calculate the neutral conductor size required. its not clear to me in the NEC. thanks
Smallest allowable neutral, is to match its size to the EGC.
Largest neutral you would ever have, for solar in general, is one that would match the size of the ungrounded conductors. If in doubt, this will cover you worst case scenario.
Check with the inverter manufacturer to determine if NEC705.28(C), formerly 705.95(B) can apply. If the inverter only uses the neutral for instrumentation, and voltage/phase detection, the neutral current is negligible. The neutral can therefore be as small as the EGC, or whatever other green or bare wire would take the EGC's place on the line side of a disconnect.
In your case, determine if the microinverters are connected phase-to-neutral, or phase-to-phase. If they are connected phase-to-neutral, then there is a chance that the neutral carries the full load. Either if all of phase A are not running, or all of phase A and B are not running, then the neutral would have to make up the difference.
By contrast, if they are wired phase-to-phase, there's a good chance that they may not even touch the neutral. The example I'm thinking of, has the inverters wired phase-to-phase on each pair of phases staggered throughout the system. The inverters themselves never touch the neutral, and only the monitoring system uses it as a reference point. That would be an example where 705.28(C) applies, and it can be as small as the EGC.
