SMA say there are 2 reasons for needing the 6mm2 cable instead of the 2.5mm2 as per 17th edition.
1 - Volt drop needs to be limited to 1% along the cable to avoid the inverter having to kick out at too high a voltage relative to the grid voltage to avoid it tripping out the G83 over voltage settings too often.
2 - The 17th edition cable sizings are based on the maximum load only being used for relatively short periods of time, whereas a 4kW PV system could be kicking out 4kW continuously for 5-6 hours on end, so the cable would be more prone to overheating.
it's sheer incompetence IMO to be using 2.5mm cable when both the manufacturers written instructions and their design tool state that this isn't acceptable, though I know a lot of companies seem to have been doing it.
this is an issue that will become more and more problematic the more other systems get installed on the same phase from the same transformer, as in peak sunlight levels they will push up the local grid voltage, and it will be the inverters on the cable run with the highest resistance that will end up dropping out first. From your figures, it looks as if you're will cut out when the incoming voltage is around 246, which is utterly ridiculous given that the 10 minute cut out voltage SMA use is 253V.
get them back.
