Lately I have had a number of questions about battery bank fuse sizing and I wanted to give a brief explanation of how I size these fuses and why they are a good safety feature..
*Simply put you size the fuse to protect the wire, provided your wire is sufficiently sized.
#1 The ABYC Standard:
The ABYC requirement is for a battery bank fuse to be within 7 wire inches of the battery bank. In the real world of boats and battery compartments this is often hard to do. If you can't get within 7" then the next best thing is to be as close as possible.
If you're having trouble meeting the 7" rule the battery terminal fuses shown in this photo are an excellent option. These fuses are excellent but they do add nearly 2", in height, to a battery post so measure your battery compartment height carefully. The limiting factor with the battery terminal fuses is that they only go to 300A so if trying to fuse a bigger engine you may need a Class T or ANL fuse.
Why I Believe In Battery Bank Fusing
Here is a prime example of where battery bank fusing can help save a boat. A plastic wire tie in an engine compartment let go on this owner. The battery cable fell against an engine pulley and began to chafe. When the metal of the engine and the copper of the wire made contact the fuse went POP.
This boat had just recently installed a 300A battery terminal fuse. This owner was very happy he had..
#2 Exceptions to the ABYC Fusing Standard.
The ABYC has an "exception" to the bank fusing rule for cranking motor batteries. This exception however is more broad based and written to include for large engines which have massive amounts of starting current being drawn. These engines are very expensive to fuse properly hence the "exception" to the rule.. Think big sport fishing boats and our engines are about as far away from a small sailboat AUX engine as can be... Small diesel or gas AUX engines on sailboats are often well served fused, rather than unprotected.
I quote Nigel Calder here:
"The net result is that nowadays, electrical shorts are probably the number-one cause of fires on boats. There is simply no excuse for not protecting all high-current circuits, including the cranking circuit." (From the Nigel Calder Cruising Handbook)
#3 What exactly am I protecting?
Over current protection (OCP) or over current protection devices (OCPD's) are sized to protect the wire not the devices they are powering. This is often misunderstood. You can always go smaller with OCP, than the wires ampacity rating, but ideally should not exceed the ampacity rating. The OCPD is there to prevent the wire from overheating, melting and starting a fire.
#4 What if my engine draws more than the ampacity limit the wiring is rated for?
This is NOT uncommon. Many builders undersized starting wire for many years and got away with it due to the short duration of starting circuits. Today most have come up closer to where they should be. A good example is the Universal M-25 as shipped on Catalina Yachts.
Catalina used to ship the Universal M-25's with 4GA wire. They now ship that same engine, M-25XPB, with 2/0 gauge wire. That is a HUGE difference. If you have small gauge wire an upgrade to larger wire can be a very good investment and your engine will start a lot quicker and the starter will see a lot less voltage drop. Nearly every sailboat I went aboard during the last boat show was using 1GA or larger wire with 1/0 and 2/0 being the most popular in boats over 30'..
#5 Won't the starters inrush current blow my fuse?
First, what exactly is "inrush current"? Inrush current is the very brief spike in current that the starter undergoes to get the motor to begin turning over from a stopped state. The inrush duration is usually about 200ms to 250ms long and not long enough to blow a properly sized fuse. ANL, MRBF or Class T are not sized for the in-rush, they are sized to the wire they are protecting.
This video below shows the absolute peak inrush as captured by a Fluke 376 meter. The engine is an older 2QM20 Yanmar. The absolute peak current draw, perhaps 2/10th of a second, is 316 amps yet this motor is protected by a fuse rated well below the inrush. It has never blown nor will at this inrush capacity. The "average starter" load during the duration is closer to 150A.