A series of unfortunate (yet preventable) events

I was recently called to a client’s boat in an emergency.  The boat has two alternators, one feeding the start bank and one feeding the house battery.  They noticed that their house bank wasn’t powering the system when devices started shutting down, and after checking the battery monitor, confirmed the bank was dead.  They checked the thermal circuit breaker installed between the batteries and the alternator, and sure enough, it had popped, but after flicking it back on, there was still no charge.  In fact, the alternator was toast.  Being internally regulated, as many older alternators are, once there was nowhere for the output power to go, it fried itself.  The thermal circuit breaker in question was rated at 70A, just above the stated maximum output for the alternator.  Being thermally regulated, all it needed was a little extra resistance at peak power, and it performed a nuisance trip, cutting off the path for the output voltage.  According to ABYC, in most cases, internally regulated alternators are not actually required to have a fuse.  You can still elect to install one to protect the cable, but in this case, it was the wrong type of fuse, and it was much, much too small.  In this scenario, like many others, improper fusing can do more harm than good.  That’s why I’m going to let you in on the secrets of fusing that most boaters, and seemingly many electricians as well, haven’t heard about.

A brief interruption

First, a quick piece of vocabulary that’s crucial to understanding fuses.

Interrupt capacity (AIC): is the maximum fault current a fuse can safely break without failing to stop the arc. Or simply, it’s the fuse’s breaking strength during a short circuit.

If a fuse has a low AIC and experiences a high load, instead of popping the fuse and disconnecting the circuit, the electricity will be able to arc across the fuse, or in some cases, melt the metal parts of the fuse together.

If you have a battery bank over 500Ah, you’re likely required to have a fuse with 20,000 AIC.

Biggest and Bestest

The most common fuse type that has the highest AIC rating is the Class-T fuse.  At around 20,000A AIC, they can handle a very high amperage short-circuit safely.  They are also generally slow-blow fuses, meaning they can handle rushes of high current for short durations without tripping, making them ideal for devices that surge.  Another notable high AIC fuse is the MRBF terminal fuses at around 10,000A, also slow-blow.  These are the types of fuses we want to use as DC mains protection on the battery banks, or for large devices that surge, like inverters or alternators.  Most inverter manufacturers will state specifically that they want a Class-T fuse.

The ANL type fuses are also high AIC at around 6,000A, but these are not slow-blowing, so they should be used for large devices that have constant, steady current draw.

Small but significant

The most common small-sized fuses we see on boats are either AGx glass type fuses, such as AGC, or ATx blade type fuses, such as ATO/ATC.  Both are quite similar in performance, but with a couple of key differences.

ATx fuses being flat and plastic, are much easier to handle, and they are vibration resistant (something that, as boaters, we care quite a bit about).  They are also color-coded, so it’s easy to see what size you need or what size you’re holding when you grab one from a mixed bag.

AGx fuses are an older style that, for the most part, are less popular now.  They are made of glass, which allows you to easily see if the fuse is blown, but it makes them prone to breaking and vibration damage.  If you have a boat from the early 80s or older with stock electronics, you will most likely have lots of AGC-type fuses on board.  I personally will never use these unless I am looking for a very specific or very small amperage, as they generally have more options that are more readily available than ATx fuses.

Both AGx and ATx type fuses are ideal for almost all of our medium to low amperage devices and electronics.  This includes, but not limited to, bilge pumps, navigation, solar panels, battery monitors, electronics, etc..

Now for something completely different

Circuit breakers, while not being fuses, are also valid and valuable overcurrent protection devices.  Whether it’s larger 187-series thermal type breakers, or smaller A-series magnetic breakers like we see in breaker panels, they act very similarly to our fast-blow fuses.  We tend to use these in applications where it’s convenient, like when we want to be able to turn the circuit off without the hassle of removing a fuse, or if it’s prone to tripping and we want to be able to reset it easily.  Of course, aesthetically, a full panel of uniformly labeled breakers looks much nicer than a row of 40 blade fuses in various colors.  Most boats use circuit breakers as overcurrent protection for all switched circuits.  That is, at least, until new installations start getting added with inline fuses.

Eyes on the size

Now that we know the different types and where we use them, the last piece of the puzzle is how to find the right size.  The most important answer here is to consult the device manufacturer.  Most devices will list the exact fuse they want you to use in the manual or spec sheet.  This is especially important for devices like bilge pumps, which often look for a much different-sized fuse than the following rules I’m about to share, but if a bilge pump is inadequately protected, it can cause the motor to burn out and the device to fail, or worse, start a fire.  It is important that when installing a fuse to the manufacturer’s specs, the wire or cable feeding that device is at least capable of handling that rating.

If we don’t get a recommended fuse from the manufacturer, a general rule of thumb is to use a fuse rated at 125% of the devices continuous current load (if the device pulls 8A, use a 10A fuse).  Again, always make sure the wire is upsized to handle more amperage than the rating of the fuse.

All “un-grounded conductors” (current-carrying conductors such as DC positive and AC hot) must be fused, with few exceptions.  One exception being cranking conductors such as starters.  Although starters aren’t required to have a fuse, it’s still a good idea, but you will need to go big.  We want to go big to avoid nuisance tripping, and we are only ever worried about a complete dead short in cranking circuits, not overload.  

This is the same with alternators, like in my intro.  Often, they don’t require overcurrent protection, but it’s still a good idea.  For alternators, I generally use an MRFB slow-blow fuse, and I rate it at 150% of what the alternator is rated at (making sure the cable is sized accordingly).  

– –

This is exactly what I did for my client.  His new alternator is still in perfect working order, like his last one would have been if there had been an oversized, slow-blow fuse installed instead of that darn breaker.  Hopefully, after reading this article, you’re now armed with the tools you need to make sure the devices on your boat are fused properly and prevent the same fate from happening to your alternator and other devices.  Now you too can join my fight to save our boats’ electrical systems, one less toasted alternator at a time.

   ~Leif Norquay-Pahl, 

        ABYC-certified technician and owner of ODIN Marine

If you have any questions about fusing or other electrical-related queries, you can send them to info@odinmarine.ca, and I will try and get to as many as I can.  I will also be selecting my favorite questions to put in a Q&A section of the next electrical corner, so everybody can benefit from the answers.