Boat owners know how important it is to have safe and reliable electrical systems. Critical to safety are overcurrent protection devices (OCPD) - fuses and circuit breakers. But when you look at the cranking circuits for most boats, there is no OCPD. Why is this? Let’s look at this first from a standard’s perspective and then from a practical perspective.
The Standard's Perspective
The two principal organizations that write electrical standards for recreational boats are the American Boat and Yacht Council (ABYC) in North America and the International Organization for Standardization (ISO) in Europe. Neither organization requires a fuse in the positive cranking conductor. It is pretty much the only positive conductor on a boat that is not required to have an OCPD.
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 amp-carrying (‘ampacity’) perspective. As a result, 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, especially in cold weather when the engine is hard to turn over and the cranking amps initially go through the roof. What to do about this?
The Practical Perspective
We can still have a fuse so long as it is sized to not nuisance blow. Since the fuse is not required by the ABYC or ISO it does not have to comply with their sizing requirements, which are based on the ampacity of the cranking conductors. We may have a conductor that is rated to carry 200 amps and which, in normal circumstances, would require a 200 amp fuse, but we decide to install a 300 amp fuse.
For small engines below 100 horsepower, I recommend a 300 amp slow-blow ANL fuse. For larger engines with higher cranking loads, I recommend up to a 500 amp slow-blow fuse. The ‘inrush’ currents for starter motors, especially in cold weather, can be well above these levels so you might think there will be nuisance blowing. There is not. To see why we need to explore the characteristics of fuses.
Fuse Properties
It is common to think that a fuse rated for 100 amps will blow if the current exceeds 100 amps. However, most fuses will not blow until the current is around 130% of the fuse’s amp rating. And even then, this 130% current must be sustained for quite some time, often ten minutes or more, and maybe the fuse still won’t blow until the current goes a little higher. Once the current rises above 130% the fuse starts to blow faster and faster. Some fuses are designed such that small increases in current rapidly reduce the time taken to blow (fast blow fuses) and some are designed such that it takes considerable increases in the current to cause rapid blowing (slow blow fuses).
Given the few seconds that cranking currents are sustained, it takes an amperage that is several hundred percent above a slow-blow fuse’s nominal rating to blow the fuse. In all the years I have been putting fuses in cranking circuits I have not had any issues with nuisance blowing. In the event of a short circuit (for example, in the starter motor, which has been the cause of a number of fires on boats) the amperage will rise high enough and fast enough to blow the fuse before the conductors melt down, despite the fact the fuse is oversized for the cranking conductors.
The Bottom Line
A fuse in the cranking circuit that is sized to prevent nuisance blowing 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 it will do is protect against short circuit conditions that have, over the years, set a number of boats on fire. It’s a cheap insurance policy.
If the fuse is not added, for obvious reasons the standards require additional sheathing or protection for positive cranking conductors. This is frequently omitted by boatbuilders. Even with a fuse, this additional protection is required between the battery and the fuse.
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!
