Fuses 101

Fuses 101

If I had to pick the most critical component of your boat’s electrical system, it would be, without a doubt, Overcurrent Protection. This topic is so important to me, it has made me quite flustered about how to best approach it. After many various rough drafts and ideas, I decided it would be best to break this topic into three articles. Fuses 101, Circuit Breakers 101, and How to Properly Fuse a Boat Circuit 101. Hopefully breaking up this topic into sections like this will make it easier to learn such critical information.

Basic Terminology:

Overcurrent Protection: A form of protection against excessive amounts of current traveling through conductors and equipment.

Overcurrent Protection Device: A piece of equipment designed to protect against an excessive amount of current traveling through a conductor(s) and or equipment. You may see this term listed as “OCP” or “OCPD.” Fuses and Circuit Breakers are the two major overcurrent protection devices on the market today.

Fuse: A type of overcurrent protection device. Fuses are a thermal device. Fuses are built with a conductive metal strip designed to melt and separate in the event of excessive current flowing through that specific circuit. View the fuse as the weakest link within a circuit. The term “fuse” is quite generic as there are many diverse types and styles.

Fuse Terminology and Acronyms:

I will start with what you typically see on the fuse and its packaging. I then will move on to the various types of fuses most utilized on boats today.

Voltage Rating: This rating specifies what the maximum voltage is for the fuse without affecting the Amperage Interrupting Capacity (AIC. See below for definition).

Rated Amperage: This is the nominal amperage rating of the fuse. This nominal rating is to inform you at what amperage the fuse will melt. We use the term “nominal” as all fuses can and do carry slightly more current than this rating. Usually, the melting point of a fuse is 125% to 130% above the nominal rating. So, a 10A fuse will melt at around 12.5A. This nominal rating is usually stamped on the fuse.

Amperage Interrupting Capacity (AIC): The max amperage the fuse can safely manage and interrupt (aka melt) at the fuse's rated voltage. If the amperage is to exceed this AIC, the fuse may become faulty and allow the current to “arc over” aka allowing current to still flow within the circuit. If the fuse is being used under the maximum voltage rating, the AIC will be higher by an unknown amount.

Melting Time: The time it takes the fuse to melt once the current rises above the minimum current rating (nominal amperage rating) of the fuse. The higher the current, the faster the fuse will melt.

Slow Blow Fuse:  A type of fuse that can tolerate more current than it is rated for, for a brief amount of time before melting. Very brief spikes in current will not melt this fuse. A sustained spike in current (think short circuit) will cause this fuse to melt. As the current rises, the melting time of this fuse slowly reduces.

Fast Blow Fuse: A type of fuse which is far more sensitive to the rise in current than the slow blow type. The melting time of this fuse drastically increases with small increases in current above its nominal amperage rating.

AGC Fuse: Automotive Glass Cartridge Fuse. Or an All-Glass fuse. These fuses are cylindrical and the shell of the fuse is glass. AGC fuses come in both slow-blow and fast-blow types. The nominal amperage rating is typically stamped on the metal ends of the fuse. You can typically find these fuses ranging from .05A to 30A. I have seen some AGC fuses rated up to 50A as well.

ATO Fuse: Automotive Type Open Fuse. Or Automotive Open Fuse. This fuse is commonly referred to as a “blade fuse” or “plug-in fuse.” It is made from two metal prongs and plastic housing. The word “Open” refers to the plastic blade housing being open on the bottom. The nominal amperage rating is stamped on the top of the housing. Also, the housing is color coded per the nominal amperage rating. You can typically find these fuses ranging from 1A to 40A.


ATC Fuse: Automotive Type Closed Fuse. Or Automotive Closed Fuse. The same fuse as ATO, but the plastic blade housing is closed on the bottom.

ATM Fuse: Automotive Type Mini Fuse. Or Automotive Mini Fuse. Same fuse as ATO and ATC, but a miniature version of such. You can typically find these fuses ranging from 2A to 30A. You may stumble across another type of ATM fuse manufactured by Mersen. Their ATM fuse stands for “Amp-Trap Midget” fuse and looks like the AGC fuse (minus the glass portion). I do not know much about them as I have never needed to use them aboard a boat.

MAXI Fuse: The largest type (both physically and by amperage rating) of blade fuse. Must use a specific fuse holder/block for this fuse. You can typically find these fuses ranging from 20A to 100A.

AMI Fuse: Not an acronym. “AMI” is simply a letter designation for this type of fuse. This is a bolt-in type fuse. Must use a specific fuse block for this fuse. The fuse housing is color coded per the nominal amperage rating. You can typically find these fuses ranging from 30A to 200A.

MEGA/AMG Fuse: Not an acronym. “MEGA” and “AMG” are letter designations. The reason for the two separate letter designations is because “MEGA” is manufactured by Littelfuse and “AMG” by Eaton (Cooper Bussmann). Physically they are the same bolt-in type fuse. The biggest difference lies within the AIC rating. “MEGA” AIC rating is at 2000A @ 32VDC while “AMG” is only at 1000A @ 32VDC. You can typically find these fuses ranging from 80A to 500A.

ANL Fuse: Not an acronym. “ANL” is a letter designation. Bolt-in-type fuse. Must use a specific fuse block for this fuse. You can typically find these fuses ranging from 30A to 750A.

Class T Fuse: The name Class T comes from a set of standards that agencies developed for some specific type of fuses. From these standards, different types of “classes” emerged. The different type of Classes I have seen to date are RK1, RK5, G, L, T, J, H, and CC. Do not let all those Classes overwhelm you as the most dominant Class fuse used aboard a boat is Class T. Must use a specific fuse block for this fuse. You can typically find these fuses ranging from 1A to 800A.

MRBF: Marine Rated Battery Fuse. This fuse is of a cube shape and clear (to easily see if the fuse is melted or not). These fuses are color coded per their nominal amperage rating. This fuse must be used with the Terminal Fuse Block. You can typically find these fuses ranging from 30A to 300A.


As you can see, there are a plethora of fuse options for your boat circuits. It may seem a tad overwhelming, but I assure you it is not. It is good to have options. With so many options, it is practically impossible to not be able to fuse a circuit.

 If I could only ask two things out of you, it would be:

  1. Please provide Overcurrent Protection to your boat circuits.
  2. When choosing a fuse, please read the included spec sheet of the fuse and base your choice on such. Do not base your choice on price and what is readily on hand.

Well, that concludes this article. I hope it was informative for you. If not, I hope it was at least an enjoyable read. If you have not already, I encourage you to go on and read the following two articles: Circuit Breakers 101 and How to Properly Fuse a Boat Circuit 101.

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