April 2

Understanding Marine Electrics: Do You Need a Fuse at the Output of Your Solar Panels or Alternator?

One of the most common questions about solar panels is whether you need a fuse at the output of the panel. In this blog post we’ll answer this question and provide insights into the technical requirements and exceptions to overcurrent protection (OCP) rules.

Technical Requirements for OCP

Boat building standards require an OCP device at all sources of power. Solar panels are a source of power. In which case, a fuse is necessary at the output of solar panels. It’s a simple Aristotelian syllogism!

Exception to the Rule

However, there are exceptions to this general OCP rule. There is one in particular that applies here. If the ampacity of the conductor coming from a source of power (the solar panel) is higher than the maximum possible output of the source of power, there is no way the source of power can melt down the conductor. In this case, OCP is not needed at the source of power. However, OCP is absolutely needed at the other end of this circuit where it connects to the batteries because in a short circuit situation even a small battery has the potential to melt down the conductors from a solar panel.

In general, if solar panel conductors are sized to keep voltage drop below 10%, and preferably lower, the ampacity of the conductors will always be higher than the maximum possible current out of the solar panel, in which case OCP is not required at the solar panel.

Overcurrent Protection for alternators

Alternators are another source of power with similar considerations. But here the ABYC and ISO have different requirements. For the ISO, if the alternator is internally regulated and is wired back to the solenoid on the starter motor with a conductor that is less than half a meter in length, it does not require overcurrent protection. If these conditions are not met, which is the case with almost all high-output alternators, OCP is required at the alternator. In practice it is rarely installed. If installed, because of the high ambient temperatures in the vicinity of alternators being run hard, is likely to nuisance blow, potentially destroying the alternator! For the ABYC, if the maximum possible output of the alternator is less than the ampacity of the output conductor, no OCP is required at the alternator (same as for a solar panel). Once again, with both the ISO and ABYC, OCP absolutely is required at the battery end of this circuit.

The Limits of Technology and Conductor Sizes

Some of the devices being put on boats these days are pushing the limits of conductor sizes and ampacity, notably 12V alternators with outputs rated at up to 360 amps, and 24V inverters with AC outputs rated up to 8 kW which requires DC inputs above 300 amps. If conductor ampacity has to be derated because of installation in an engine room, or for bundling, it can be hard to comply with ampacity and OCP requirements in both the ISO and ABYC standards. 

An understanding of the details of these standards is essential for a safe and reliable installation. 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!

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.

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