One rule to rule them all: Ohm's Law - it cannot be violated

To a brand new electrical engineering student, the first thing you must learn is a law of physics called Ohm’s Law.  It was names after the German physicist Georg Ohm, who published the theory in 1827.  Very simply the law states:

V = I x R

Or, Voltage (V) equals Current (I) times Resistance (R).  It means that the voltage drop across a resistance is equal to the amount of that resistance (ohms) times the current traveling through the resistance (amps).

So what does this mean to the average boat owner?  Well… one application is that the higher current (amps) a load carries, means a higher voltage drop you will have in the cable.  Think about these steps as you look at the formula above:

1)      A large electrical load means high amps

2)      High amps means high “I” in the formula above

3)      With a fixed resistance (R), this means voltage drop (V) must increase proportionally with the current

4)      As voltage drop in the cable increases, it lowers the operating voltage of the equipment

What conclusion can we draw from this?  That to keep voltage drop (V) in your cable constant, while increasing current (I), you must decrease resistance (R).  And you decrease R by… wait for it… GET A BIGGER CABLE!  AH HA!  That is why larger loads need larger cable… large cable has less resistance.

A second application is that as cable length increases (R increases), with a constant current, the voltage drop also increases.  This is why when you run a long cable to a device far away from your battery; you should step up your cable size decrease resistance and voltage drop.