Cable Sizing
In an independent power system, you generally would find an
inverter and battery charger system working for the common
goal of providing power. What ties each of these together are the
cables to supply the power to run to or from the batteries or DC
distribution. Unfortunately, the most common installation error
is to under-size cables to the load/s or from the recharge sources.
Proper installation is primarily a matter of sizing a cable to match
its task, using the correct tools to attach terminals, and providing
adequate over-current protection with fuses and circuit breakers.
Cable sizing is simple enough. It is a function of the length of a
cable (measuring from the power source to the appliance and
back), and the current (amperage) that will flow through it. This
can be found by checking the label on the appliance in the circuit,
or the specifications sheet for the appliance. The longer the cable,
or the higher the amperage, the bigger the cable must be to avoid
unacceptable voltage losses. And there should always be plenty
of extra margin for safety because an appliance may actually use
more current than what it is rated for because of heat, low voltage,
extra load and other factors.
For 12V circuits, the relationship between cable length, current
flow, and cable size is given in the table below. Note that you have
two circuit types, Critical & Non Critical. The “critical” circuit is
based on a 3% voltage loss in the cable, while the “non-critical”
circuit is based on a 10% voltage loss. What this means is that when
the circuit is fully loaded (i.e. operating at rated amperage), the
voltage at the appliance will be 3% or 10% below that at the battery.
For example, if the battery is at 12.6 volts, the appliance will be
seeing 12.2 volts (3% loss), or 11.34 volts (10% loss).
Many appliances (notably lights) will run fine with a 10% voltage
loss, but others are particularly sensitive to such losses (notably
charging circuits, and some electric motors). In general, given the
harsh realities of the RV & marine environment, it’s better to use
the 3% volt drop table when sizing cables, rather than the 10%
table. There’s never a performance penalty if a cable is marginally
oversized; there is always a performance penalty (and possibly a
safety hazard) if it’s undersized.
Page 3 August 2024 (V1.1)