Because the voltages used in renewable energy
genenerating systems are quite low, usually from 12
to 110 volts, the currents (measured in Amps) drawn
by various appliances are proportionally higher compared
with 240 volt systems (Power = Volts x Amps). As a result,
the cables used for 240 volt systems are usually
unsuitable for use at lower voltages.
The cables used for low-voltage (or more correctly,
extra-low voltage) systems are often quite thick,
especially if they are to be quite long. This is because
while losing 6 volts from 240 volts is a mere 2.5% drop,
losing the same in a 12 volt system is a full 50% voltage
(and therefore power) loss in the cable! So, the cables
must be appreciably thicker to reduce voltage losses, even
though the cable's maximum current rating may be far in
excess of what will be drawn by the appliance.
A common example is lighting circuits. The cable often
used is 6mm², which has a current rating in excess
of 50 amps continuous. However, most extra-low voltage lamps
will only draw 2 to 3 amps, and the whole circuit might
only draw 10 amps. However, for a 20 metre long circuit
(ie, 10 metres to the lights, 10 metres back to the battery),
the voltage loss at 10 amps is around 0.6 volts, or 5% of
the voltage at 12 volts.
This is considered the maximum allowable voltage drop in
a system. Another problem with switching extra-low voltage
DC is that the considerable greater degree of arcing means
that switches and circuit breakers must be designed to cope
with this. AC designed switches may not last very long when
switching DC currents.