If sizing for safety
: The total maximum current
of all panels
connected to the network without benefit of fusing ... Sizing for safety
usually results in wire on the small side of reasonable. It is a jacket temperature issue.
If sizing for efficiency: one really needs a "policy" regarding the acceptable loss. Yachties tend to be foolish here and greatly oversize wires, wasting money
while achieving a very small benefit.
If sizing for voltage drop seen by the consuming device (eg: SSB
, refrigeration): Use the difference between the expected supply voltage and the voltage specification of the device.
Complicated enough? That's how us retired engineers earned the big bucks and retired early ...