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.
The ABYC Standards On Bank Fusing:
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 but the wire should ideally be in a protective sheath or conduit.
If you're having trouble meeting the 7" rule, the Blue Sea 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.
If you prefer to cut to the chase I will give you the bottom line minimum bank fusing I like to see for any bank that could ever be called upon to start a motor:
Minimum fuse size for small sailboat diesel AUX engines 8HP to 35HP = 250A
Minimum fuse size for medium sailboat diesel AUX engines 35HP to 60HP = 300A
Above 60HP 80% of in-rush as minimum fuse size.
If you follow these guidelines you will not suffer "nuisance trips".
As a service to my readers I offer the Blue Sea battery terminal fuses, shown above, in the MarineHowTo.com web store, and at very competitive prices. Purchasing products through MarineHowTo.com keeps the site running and helps to keep it FREE! The Blue Sea fuses are sold as a package, fuse & holder together for just $29.95. My prices beat Defender, West Marine and just about anyone out there selling the Blue Sea fuses.
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 owner had just recently installed a 300A battery terminal fuse. This owner was very happy he had..
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 with a pair of MASSIVE Caterpillars, and these 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)
Recently I have had a lot of questions regarding fusing and I will try and answer them as best I can.
Question: 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.
Question: "What if my engine draws more than the ampacity limit the wiring is rated for?"
This is actually not uncommon. Many builders undersized starting wire for many years and got away with it due to the short duration starting circuits are loaded for. Today most builders have come up closer to where they should be. A good example is the original 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'..
Question: "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 fuses are not sized for the inrush, 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 it at this inrush capacity. The average starter load during the duration is closer to 150A.
In-Rush Yanmar Motor:
Trip Delay Curve
Question: If the starter can draw more than the fuse rating then why doesn't it blow?
Contrary to popular belief ANL, Class T or marine rated battery fuses (MRBF's) do not blow or trip at face value unless the duration is long enough.
As you can see below a 200A ANL can support 500% of it's rating for about .7 seconds, longer than average inrush by more than double. It can support over 300% for 1 second and 200% for as long as 5 seconds. You can push 150% through for up to 500 seconds.
Even if you figured an inrush that, by freak chance, lasted for 1 second, which should not happen, a 200 amp ANL can supply 600A which is almost double what the peak inrush is for the 2QM20 in the video above so in that scenario you'd still be fine.
Just as a point of reference a 1987 Universal M-25 draws roughly 225A +/- depending upon temp and other factors. This is the in-rush load though. All engines are slightly different. Some draw more and some draw less but the in-rush is still a very short duration on all of them. None of this changes the fact that you are still protecting the battery cable with the fuse and not the battery or other systems. The fuse is still sized to protect the battery cable and this in-rush data is simply a reference point..
NOTE: Photo taken of original image by Blue Sea Systems
What Type of Over Current Protection For My Bank?
Question: What type of over current protection for my bank/banks?
Generally speaking larger banks should be protected with the following types;
ANL CLASS T MRBF
These fuses are available from Blue Sea and others. They should however be "Ignition Protected" fuses if installed on a gasoline boat and Blue Sea is the only one I know of offering ANL fuses with ignition or spark protection for ANL's. Class T fuses are not IP rated however they are fully encased in a metal body. Class T fuses have simply not been tested for IP rather than do not meet IP. In speaking with Blue Sea systems I was told they have no documented cases of an IP breach on any Class T fuses.
Question: "What is AIC?"
AIC stand for Amperage Interrupt Current and Class T, ANL and MRBF fuses all have AIC suitable for decent sized house banks. If you have a large bank of Odyssey or Lithium batteries then a Class T would be best bet as the AIC rating of Class T fuses is nearly 20,000 amps. ANL fuse AIC is 6000A and MRBF is 10,000A.
The concern with AIC is that some breakers can literally weld shut before tripping, if the bank has enough short circuit behind it. For decent sized battery banks you ideally want an AIC rated fuse or breaker of 5000A AIC or greater. AIC is a greater concern for breakers but fuses are also AIC rated and can fail dangerously when subjected to shorting amperage greater than their AIC rating. The actual ABYC requirement for batteries is that any bank over 1100 CCA needs 5000 AIC rated protection or greater. Even two parallel group 27 batteries can supply more than 1100 cold cranking amps...
Why Does The AIC Rating Matter?
Question: Why does the AIC rating matter?
Here is an example of what can happen to a cheap ANL fuse. These are not ignition protection (IP) nor AIC rated fuses. I searched and could not find any sort of data for them other than "Made in China".
The fuses were connected directly to a LiFePO4 lithium battery bank and then the circuit was shorted. The fuse trip was so violent, it literally blew the windows out of the fuses. This is an UNSAFE failure mode for a fuse. It failed for both AIC and ignition protection safety.
Running this same test with Blue Sea Systems ANL fuses I could not get a single fuse to fail in an unsafe manner. The quality and brand of fuse you choose matters. Not all fuses are created equal.
So which fuses are safe for battery bank fusing:
How Do I Determine My Wires Ampacity Rating?
Question: How do I determine the right size fuse for my wire?
This is Table VI from the ABYC E-11 Electrical Standard (click it to make it larger). It is for single conductor wires not bundled or sheathed together. The table is organized by jacket temperature rating.
UL1426 Marine wire should generally be 105C rated. Manufacturers like Pacer, Ancor, Berkshire, Cobra and others all build marine battery cable to UL1426 standards and most any chandler will have it.
The ABYC standard does allow you to go to 150% of the Table VI ampacity rating, if necessary, but if you do this always round down to the next size fuse rather than up. My personal preference is to size the wire correctly so that use of the "150% rule" is not necessary.
So, if you were using 2/0 wire, and it was outside the engine space, then you could use a fuse up to 330A @ 100% of the ampacity rating.
If you needed to go bigger with your fusing you could use the 150% rule and use 330A X 150% = 495A fuse, or rounded down to the next commercially available size.
What do manufacturers suggest?
Here are some minimum manufacturer suggestions for battery/starter cable from Westerbeke & Universal.
It does not take much to have 10' of wire length even with batteries just a few feet from the engine. Keep in mind these numbers are wire length along the conductor, not an "as the crow flies" distance.
Data Table Courtesy ABYC E-11
This Vessel is Protected With A 300A ANL Fuse:
This is another prime example of "inrush" vs. average starting current and what will blow the fuse and what will not.
In this screen capture we can see that the starter drew slightly over 640A during the peak inrush. It should be noted that this 4 cylinder diesel motor has been started in excess of 1000 times with a 300A ANL.
When sizing for starting loads always use the largest fuse you can for the wire, so as to eliminate any chance of "nuisance trips".
Starting Load At 20F
In this screen we can see the "average" starting load was 286A. This was measured on a day when the air temp was 20F and the batteries in the bilge were at 32F. As such these currents were slightly higher than they normally would be on a typical small sailboat AUX motor.
Question: "How does a 300A fuse deal with an average starting current of 286A and not trip? Don't you want to always be under 80% of the fuses rated trip point?"
The secret is in the duration. This engine starts, from a loaded starter to unloaded, in just 0.75 seconds, as evidenced by the; "TIME 765mS".
If you look back at the trip delay curves for an ANL you'll see why this 300A ANL fuse has never blown despite over 1000+ starts on this engine since the installation.
Exceptions To The "Rule" vs. Common Sense
I hear it stated over and over and over that the ABYC standards make an exception for starting motor circuits, and this is 100% true. The reason it is true is to accommodate HUGE engines that can not easily or simply be protected with over current protection. The vast majority of marine engines in the world today can easily be protected with over current protection. If you own a massive yacht, with massive engines, please put your starting motor conductors in a protected conduit if they can not be fused.
Ungrounded conductors shall be provided with overcurrent protection within a distance of seven inches (178mm) of the point at which the conductor is connected to the source of power measured along the conductor.
1. Cranking motor conductors."
On engines sub 300HP +/- there is no good excuse for not fusing all battery banks on-board including starting circuits. If your start bank can not be fused then please take the time to protect the wire in a conduit for your own safety.
What do we know about this fire on a 15' Boston Whaler?
1- Electrical fire 2- Single Group 24 battery 3- Multiple children under the age of 9 on-board seconds before the fire! 3- NO OVER CURRENT PROTECTION!!!!!
Use the ABYC exception, or common sense? You decide.....
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