April 2

Understanding Marine Electrics: Fuses for Starter Motors – Are They a Good Idea?

Boat owners know how important it is to have safe and reliable electrical systems. Critical to safety are overcurrent protection devices (OCP) - fuses and circuit breakers. In this blogpost we discuss a hotly debated topic - do we need this OCP for starter motors? BoatHowTo expert Nigel Calder explains why he thinks it's important to add a fuse to the starter motor circuit.

The Standard's Perspective on Fuses in Cranking Conductors

The American Boat and Yacht Council (ABYC) and International Organization for Standardization (ISO) standards do not require a fuse in the positive cranking conductor. It is pretty much the only positive conductor on a boat that is not required to have OCP.

This is an inheritance from the automotive world, where the battery is typically located near the motor, and the conductor runs are short, minimizing the likelihood of a short circuit. Boats are different. There are commonly longer conductor runs, with the positive and negative conductors running together (exacerbating the risk of short circuits), and in any case there have been fires started by short circuits in starter motors. OCP is advisable.

But now we run into a problem. Because cranking only takes place for a second or two, so heating effects in the conductor are minimal, and starter motors are tolerant of significant voltage drop, in comparison to other conductors cranking conductors are typically undersized from an ampacity perspective. If a fuse is put in this circuit based on the conductor's ampacity, there is a risk of the fuse nuisance blowing whenever the engine is cranked. 

Importance of Oversizing the Fuse

We can still have a fuse so long as it is sized to not nuisance blow. For small engines below 100 horsepower, Nigel suggests a 300 amp slow-blow ANL fuse. For larger boats, he recommends a 500 amp slow-blow fuse. It is important to note that slow-blow fuses have a time delay characteristic. The fuse blows at 130% or more of its rated current, but at 130% this must be sustained for several minutes: the fuse will handle the inrush current into a starter motor and will not blow when the engine is cranked under normal conditions (Nigel has wired his boats like this for decades and has never blown a fuse). It will blow, however, if there is a short circuit in the system or in the starter motor, and may blow if the engine is seized, or filled with saltwater, overloading the starter motor.

The Bottom Line

Sized like this, the fuse will typically not comply with the ampacity tables of the ABYC and ISO, but since it is not required by these standards, that is OK. What the standards do require, which is frequently omitted by boatbuilders, is additional sheathing or protection on positive battery conductors that are not protected by a fuse. This additional protection is required up to the first fuse in the circuit.

If you want to delve deeper into the topic of OCP, check out Nigel's article on Overcurrent Protection on Boats and of course our courses on Marine Electrical Systems!

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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