Solar Power DIY Blog

So why all the fuss about ensure the cables are big enough?  Is the cabling that is used in normal house wiring good enough?

To answer these question we need to bring up the issue of voltage drop.  See cables, even copper cables are not perfect conductors hence there is a very small amount of resistance.  This results in a voltage drop along the cable, the amount of the drop is directly proportional to current that the cable is carrying, the length of the cable across the whole circuit (length of positive cable plus length of negative cable) and the thickness of the cable.  The voltage drop is not dependant upon the supply voltage.

With higher voltages, the voltage drop is such a small percentage, the cable runs can be very long without much relative loss.  At 120 volts or 20 volts a 1.5 volt loss represents not much more than 1 percent.  Take the same current at 24 volt and 1.5 volt drop is almost a 7 percent loss, maybe just acceptable.  At 12 volt this amount of voltage drop will probably present problems.

What compounds the problem due to ohm’s law is that lower voltages, currents in the cabling will be generally higher.  A 12 watt light is a very small light but at 12 volts will require 1 amp current to run it, the same light designed for 120 volt will take 1/10 of an amp – not considering other inefficiencies such as inverter losses and inherent inefficiencies in the higher voltage AC lights).  The same thing applies with a laptop computer.  A 120 watt laptop will be 5 amps at 24 volts, 10 amps at 12 volts, 1 amp at 120 volts, 0.5 amps at 240 volts – again not considering other influencing factors when running off AC versus running of DC.

So in summary you must be aware that the lower voltages need to have by nature heavier cabling.  That is one of the reason’s why I chose a system voltage of 24 volts rather than 12 volts for my system.  I am building a moderately sized 2 storey family house where some cabling is a little longer than for a smaller house or weekend cabin.   There are cabling size charts around which guide you concerning sizes versus cable lengths.  Of course you need to ensure that even for short lengths that the capacity of the cable is sufficient.  But the bottom line of what I am saying here is DON’T JUST CONSIDER CURRENT CAPACITY.  I am yelling this because it is often the cause of system problems.  You MUST consider the length’s of the cable runs.

Enough rambling and ranting for one day.