Can I downsize a neutral on a PV system? And where in the code can I find this? If I am connecting to a 120/208v system I have to run a neutral just for my production meter. I am told that the only reason the meter needs the neutral is for a reference point between phases and that there is no current on it. As long as it's a balanced system I should be able to downsize, correct?
You should also make sure that you study the inverter manufacturer's requirements in depth to confirm that you can do this. Some inverters do require it, some inverters don't require it. Some utilities and AHJs may still require some form of "effective grounding", and commonly the easiest way to do this is via the neutral.
It may be that the harmonic currents of the inverter adds up, and is carried on the neutral. I've always run a full size neutral out of ignorance for understanding what is going on. Some inverters do have the possibility for the full neutral current, such as the Fronius IG Plus 12kW WYE. If any of its individual inverting units (connected in a wye manner) fail, the imbalance will be carried back no the neutral. These inverters also have a mixed mode operation setting, which allows the inverting units to run at different power ratings to optimize efficiency, in which case any imbalance is carried by the neutral.
Most often it is the case, that if there is any serious current on the neutral for an inverter that only actively connects to the phases, the inverter will shut off with its grid sensing features.