Solution for a large voltage drop AC In for Multiplus 12/3000/120

Hi,

I have installed a Multiplus 12/3000/120 on a sailing yacht and I notice a large voltage drop on the AC In side.

- All cables from shore plug up to AC In are 2.5mm2
- Cable distances are from shore plug to yacht = 15m / from yacht in to Multiplus = 5m
- I have one RCD between shore and the AC In (also one after AC Out)
- Voltage from shore without load is 218V
- Voltage AC In drops from 218V to 205-210V when charging at 90Amps 12V
- I notice a quite large negative power reading on the AC Out (up to -100W)
- When decreasing the charger output to <30 Amps 12V the AC In voltage rises to around 216V and the AC Out power reading rises to around -10W
- When not connected to shore, the inverter gives a normal 230V to the AC Out

You can find a screenshot below of the Victron Connect which shows all figures.

Is this issue purely depending on the Marina I'm berthing (old cabling, trafo issues...) or are there any internal/installation aspects I might be looking into?
Is the AC Out negative reading a side effect of the voltage drop on the AC In?