Question: I’ve recently added a pretty extensive solar panel array to my new Leopard catamaran. I also installed a solar charge controller inside the boat. My question is about providing a fuse or circuit breaker for the output of the panels. I oversized the cabling between the panel array and the controller to minimize any voltage drop from the panels, but I’m at a loss as to what size fuse or breaker I should use in this cable run.

Solar panels are “self-limiting,” so as long as their cabling capacity exceeds the maximum possible amperage flow, no fuse is required by standards.

Solar panels are “self-limiting,” so as long as their cabling capacity exceeds the maximum possible amperage flow, no fuse is required by standards.

The panels are rated at 80 watts each and I have mounted four of them on the boat. The way I calculate it that’s 320 watts @ 12 volts nominally. Doing the math tells me I end up with something on the order of 25 or more amps running through the cable. Should I just install a 30-amp fuse in the circuit and call it a day?

Answer: Excellent question. You’ve thought this all out pretty well, and your calculations are correct. Assuming you have properly sized the wiring, which really needs to take into consideration the length of the wire run, which you have not told me, a 30-amp fuse or breaker could do the job. Dividing watts by volts is the basic equation used to find amperage draw in an electrical circuit. Fuse or breaker size is based on a wire’s designed ability to conduct X amount of amperage. The ABYC refers to this as a wire’s “ampacity,” a word you won’t find in any dictionary.

In your case there is another consideration, however, that centers on a notable exemption to the over-current or fuse requirement for a circuit. Your solar panels are what are known as “self-limiting” devices, meaning that no matter what, the amount of amperage they can produce is limited to the rating you have already established. So, as long as the cabling you have installed has a basic “ampacity” that exceeds the 25-30 amps you have mentioned, you are not required by standards to install a fuse or breaker. In fact, the breaker or fuse would never trip in the event of a short circuit between the panels and your charge controller. Remember that what blows fuses and trips circuit breakers is amperage that is excessive for the circuit in question. Also, fuses and circuit breakers won’t actually trip at whatever their nominal rating is. A 30-amp fuse, for example, won’t actually trip until it is exposed to current between 125 and 160 percent of that nominal value, depending on the design of the specific fuse in question.

For more info, read Solar Power for Boats and Solar Panels Benefit From Ventilation and Good Charge-Control.