We see more and more solar arrays on boats, some of them with numerous panels. These panels may be arranged in series to create relatively high voltages, and then stepped down to battery charging voltages by a voltage regulator, or wired in parallel at a relatively low voltage, sometimes with individual controllers on each panel. This raises some interesting questions as to where in these systems an overcurrent protective device (OCPD – typically, a fuse) is required and what should be its amp rating.
Technical Requirements for OCP
Boat building standards require an OCPD at all sources of power. Solar panels are a source of power. In which case, it would seem a fuse is necessary at the output of solar panels. It’s a simple Aristotelian syllogism! But there’s a complication.
The function of a fuse is to prevent the melting down of conductors in the event a short circuit causes high amperages to flow. For the conductors to melt, the amp flow must exceed the amp-carrying capability (the ‘ampacity’) of the conductor. If a solar panel is wired to minimize voltage drop between the panel and its controller and/or the battery to which it is connected, the ampacity of the conductors will be such that it exceeds by a long shot the worst-case short-circuit amps the solar panel can produce. No solar panel properly wired can output high enough amps to melt down the conductors attached to it. As such, no OCPD is required at any solar panel.
There is an exception in the boatbuilding standards which recognizes this. So long as the ampacity of the conductors attached to a solar panel exceeds the maximum possible output of the solar panel or panels, no OCPD is required at the solar panel end of the circuit.
It’s completely different at the other end, the battery end, of the circuit. Given a short circuit in any solar panel wiring, even a small battery has the capacity to melt down the conductors. The melt down does not occur just at the point of the short circuit but along the full length of the conductors from the point of connection to battery positive up to the short circuit and back to the point of connection to battery negative. The boat is likely to catch fire. There absolutely must be an OCPD at the battery end of the solar panel circuit, with its amp rating no higher than the ampacity of the smallest conductor in the circuit.
Series versus Parallel Connection
At this point things can get a little complicated. Let’s say we have series connected a number of panels to boost the voltage. This is done because for a given level of output from a solar array the higher the voltage the lower the amps and the smaller the necessary conductors. But now we feed this higher voltage output to a controller which steps it down to battery charging voltages. The lower the voltage the higher the amps. This low voltage, high amp output will require larger conductors than those from the solar panels to the controller. The OCPD at the battery end of this circuit may well have an amp rating above the ampacity of the smaller, high voltage, conductors. In which case an additional OCPD is required where the smaller conductors connect to the controller. It is often omitted, in which case these conductors are a potential fire risk.
Let’s flip this around the other way. We connect a number of panels to individual controllers (which, from a performance perspective, is the optimum way to install solar panels on boats). We parallel the outputs from the controllers at a positive and negative busbar, with a single set of conductors from the busbars to the battery. Once again, the conductors from the busbars to the battery, which are carrying the aggregated output of the solar panels, are likely to be considerably larger than the conductors between the individual panels and controllers. The OCPD at the battery end of the circuit is likely to have an amp rating above the ampacity of the conductors between the panels and controllers. There needs to be an additional OCPD where each panel connects to its controller. I almost never see this OCPD installed, in which case the conductors to each panel are a potential fire risk.
Complex Installations
Solar panel installations are becoming ever larger and more complex, especially on catamarans (both sail and power) and larger trawler yachts. The associated wiring harnesses may pose challenges which are frequently overlooked in terms of OCPD. The bottom line here is that every positive conductor in the installation needs to be protected by an OCPD at the battery end of its point of connection to the system. If the conductors between panels and controllers are small than the conductors between controllers and the battery, then OCPD may well be required at the point of connection to the controller as well as at the point of connection to the battery. In all cases the OCPD must have an amp rating no higher than the ampacity of the smallest conductor being protected.
Check out our courses on Marine Electrical Systems for detailed coverage of these issues.
If you want to read more about OCP on boats, check out Nigel's article on Overcurrent Protection on Boats!