Boat Circuit Fusing 101

Boat Circuit Fusing 101

The principles of properly providing an overcurrent protection device (Fuse or Circuit Breaker) on a boat circuit are rather quite simple. The simplicity slowly wears away as the rules, standards, and details come into play when it comes to fusing very specific type of circuits. The principals, however, remain the same.

And that’s what this article is all about. The core principles of properly providing an overcurrent protection device to a boat circuit. If you are looking for a more in-depth article about fusing a specific type of circuit, just scroll to the bottom of this article as I provided links to my other fusing-based articles.

To keep this article simple and clean, I am going to list and explain each core principle in bullet point format.

A quick note before we dive into it: You will see me use the words “fuse” and “fusing” quite often. For this article, those two words are used in place of “overcurrent protection device”. I generally find most individuals use the terms “fuse” and “fusing” quite generically and usually just imply an overcurrent protection device. Like I said above, my goal is to keep this as basic and simple to understand as possible. Hopefully, I succeed.

Let’s continue to the good stuff….

  • Always fuse as close as possible to the power source. The “power source” could be your battery, your fuse block, your circuit breaker panel, etc… The reason for this core principle is simple. The closer you fuse to the power source, the more wire you are protecting from an overcurrent situation. The more wire you can protect, the better.
  • Always fuse for the smallest gauge wire within that specific circuit. You must protect the weakest guy on the team. The reason for this core principle is that the smallest gauge wire will handle the smallest amount of current within that circuit. The smallest gauge wire will be the first wire compromised during an overcurrent situation.
  • When fusing a Motor Driven Load (bilge pump, live well, macerator, etc…) circuit, always fuse per the manufacturer's specification. ALWAYS! The manufacturer provides such critical information as it is needed to prevent a locked rotor condition within that pump. The manufacturer will supply the fuse size information on the package, manual, or the pump itself. You must also make sure the smallest wire within that specific circuit will be protected by the specified fuse size as well. If you are unfamiliar with “locked rotor condition”, I encourage you to read my article “How To Fuse A Bilge Pump”. The article fully explains what that condition is and why fusing to the manufacturer’s specification is so vital.
  • When fusing a non-motor driven load (lights, MFDs, stereo, etc…) circuit, you can see if the manufacturer provides a specified fuse for their product. Some manufacturers do, others do not. If they do, fantastic. You most certainly can use their specified fuse size. Just make sure that the specified fuse will also protect the smallest wire within that circuit.
  • Always know exactly what is in that specific circuit you are fusing. Almost every component within a circuit has an amperage rating. The wires, the switches, the terminal blocks, the devices, etc… all have a maximum amperage rating. Most of the time, the “pigtails” (wires on the device) will be the smallest amperage rating, thus you will fuse for such (unless it is a motor-driven load). But I have seen plenty of circuits on boats with a rocker switch rated for a maximum 15A be used in a much higher amperage circuit. You simply must know the components and specifications within that circuit you are fusing.

Well, that concludes the list of the Core Principles when it comes to fusing a boat circuit. At this point, some of you are most likely yelling at me: “Where the heck is the ABYC safety standards regarding this topic!?” and “How the heck can I fuse for the smallest gauge wire if I don’t know the allowable amperage through that size wire!?”. All great questions…

 I decided to keep the ABYC safety standards out of this article as I wanted to first tackle the fundamentals of fusing a boat circuit. I also wanted to keep this article relatively short and not overwhelming. Don’t worry though, the articles which focus on fusing a specific type of circuit will indeed have all associated ABYC safety standards listed.

And due to the multiple chart system of the allowable amperage through conductors (wire/cable), I decided to have a specific article just for that. Allowable amperage through a conductor changes based on how many conductors are bundled together, if it’s inside an engine space or not, as well as the insulation rating of the conductor. If I listed each chart in this article, it would be at a novel reading length. So please go check out the “Allowable Amperage Through Conductors” article for the charts.

Like always, if you have any questions, or see a glaring mistake within this article, please do not hesitate to reach out.

Articles about Fusing Specific Types of Circuits:

To Be Updated.

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