DC Voltage Drop
For Solar PV installations, the standard to follow would be from Australia called AS/NZS 5033:2014. It states, “For LV PV arrays under maximum load conditions (Imp), the voltage drop from the most remote module in the array to the input of the inverter should not exceed 3% of the Vmp voltage (at STC)“. In simple terms, the allowed DC voltage drop limit is 3%.
AC Voltage Drop
The standard to follow would again be from Australia called AS/NZS 4777.1:2016. It states, “The overall voltage rise from the point of supply to the inverter AC terminals shall not exceed 2% of the nominal voltage at the point of supply”. In simple terms, the allowed AC voltage drop limit is 2%.
AC voltage drop/rise [i.e. between the inverter and the switchboard] should be kept as low as possible. The purpose of this is to keep the voltage rise to a minimum – this is to prevent voltage rise in the local grid. High levels of voltage in sections of the grid may lead to over-voltage tripping in grid connected inverters. This will avoid problems with inverter disconnection at times of peak generation, especially where the supply voltage in the area is high.
Of course the best way to minimize voltage drops in your solar system is to use cables that are as short and as thick as practicably possible. The thicker the cable, the lower the voltage drop across the cable. So if you absolutely need a long run of cable – you can usually compensate with thicker wire.
Summary
My recommendation
Have a maximum system level voltage drop of 3% (including DC and AC). It is recommended to have up to 2% voltage drop at the DC side while only 1% is accepted at the AC side of the system for a total of 3% in voltage drop for the entire system.
The recommendation depends on the type of system (Enphase, SolarEdge) and capacity as well. So for Enphase systems (where there is no DC cable as such) and SolarEdge (where there is no DC voltage drop upto 300m), we can take 2% as the permissible AC voltage drop. Similarly for smaller systems where there are only 3-4 panels in a string and hence lower DC voltage, we can take 1% as the max permissible DC voltage drop.
