A circuit breaker is placed in a circuit to open that circuit if too much electrical current flows. So, how does one choose a breaker? And what does "too much current" mean?
The appliance and equipment manufacturers will often suggest or even demand that a certain size breaker be used to serve their equipment. But this kind of thinking can lead to error.
You will not go wrong if you think about it this way:
Not the equipment. The breaker should always be sized to prevent the wire from carrying more current than it should.
Every wire which is carrying electrical current is a heater, according to the formula:
In a confined space (such as in a wire bundle or inside an insulated wall) this heat can build up to dangerous levels.
One might think that a house, with its 110/220V wiring would be the greater risk. Not so. On a boat, where many circuits are 12V, the currents can easily be many multiples of those found in household wiring. Voltage does not appear in the formula - it is all about the current. And in that formula, you will notice that the current term is squared - meaning that tripling the current makes 9 times as much heat.
The ABYC makes these recommendations for wire current carrying capacity:
I think that these recommendations should be taken as the absolute extreme maximum current carrying capacity of a wire. I prefer to size breakers according to the National Electric Code (which governs virtually all shore-side wiring, including that in your house). The NEC says this:
It's always OK to have a breaker sized smaller than the current carrying capacity of the wire.
So what's a boat owner to do, when wiring in that brand new shiny 12V toaster? The manufacturer's brochure says to use a 30 amp breaker. So install that 30 amp breaker at the panel, and then hook up the toaster using 10 gauge wire, because a 30 amp breaker will adequately protect 10 gauge wire.
How about that new GPS? It comes with an 18 gauge pigtail with an in-line fuse. Suppose you already have a 15 amp breaker on your panel labeled "Instruments", feeding a 14 gauge wire that leads to a terminal strip where a bunch of other instruments are attached. You can attach the GPS to that terminal strip, confident in the fact that even if all the instruments should suddenly go berserk and draw more than 15 amps, the breaker will still protect the 14 gauge feed wire.
If the GPS does not have an in-line fuse in the pigtail, install one. A short in the pigtail wiring could draw as much as the power feed to the terminal strip allows - 15 amps in this case. Despite what the ABYC says, that 18 gauge pigtail will be smoking if it is handling 15 amps. In fact, you should think of that terminal strip as a branch power panel, with each attached circuit protected by its own fuse, sized appropriately for its wire size.
Please note that any wire between a power source and a breaker or fuse is unprotected. If something should happen to cause a short in this section of wire, the breaker will not see it. Always install breakers or fuses as close to the power source as possible.
Next time: Wire and connections