1- You are not protecting the "load" with battery fuses you are protecting the wires in the event of a short so you are on the right track.
2- The allowable amperage for a single 2 GA conductor per ABYC E-11 Table VI is 210 amps outside engine spaces, and 178.5 amps inside engine spaces. This is for marine battery cable with a 105C jacket temp rating. You can safely rate this fuse at 150% of the above ampacity ratings, if you absolutely need to, and the fuse blow specs are already considered into this 150% number. I always prefer to run bigger wire and size my fuses at 100% or less but have on occasion gone to 150% with boats like Catalina's where the owner did not want to spend the $$ on new battery cable. Our fuse on the current boat is sized at less than 100% and is a 250 ANL on 2/0 wire.
Start Battery Fusing?
The ABYC does not specifically require fuses on dedicated start batteries but this is because on big engines they just don't make typical fuses big enough to handle both cable protection and starter loads, but we are talking BIG engines. I have fused up to 330HP with zero issues and no nuisance blows. It has nothing to do with the in-rush loads as the fuses will not trip on those short durations anyway unless their is something drastically wrong and the in-rush lasts longer than it should..
I have spoken at length with both John Adey and Eric Johnson (who recently left the ABYC) regarding this matter. They both feel personally that fusing smaller engines is a good practice, and many ABYC techs already do this. The standard was written around start batteries being close to the engine and short wire runs. They do require a batt switch as a safety mechanism but this is a poor safety mechanism in the locations many of them are installed in IMHO. We know short wires runs to the start battery are just not the case in MANY installations these days so I almost always fuse them on small aux engines.
It is very hard to write a standard to meet all applications thus the starter battery exemption, which I tend to disagree with, even though I understand the difficulty in applying this to all motors. They are working on changing this but we don't yet know what it will look like and it may then only apply to certain battery types such as LiIon. This subject, fusing starter batteries, is discussed in nearly every E-11 PTC meeting..
The ABYC also specifically do not address starter cable sizing because the industry is using rather outdated meausres for rating starting current loads. In-rush is also not a good measure because it would be extremely difficult to size either wire or most fuses for that. You basically want it sized for average cold cranking starter load and some manufacturers provide this and some don't. We have chuckled many times over how some builders, Catalina is one example, size starter cables. Unfortunately they can do this because it is not specifically addressed in E-11. I can assure you though that every time I convert an owners boat from 4 AWG to 2/0 the motor starts and cranks like a bat out of hell compared to the original factory wiring.
3- Proper marine rated ignition protected fuses, such as ANL fuses from Blue Seas, will not blow on in-rush current if sized correctly for your small motor. I personally ran a 200 amp ANL on our 2005 Catalina. Our current boat runs a 250 amp ANL on a 44HP Westebeke 4 cyl diesel. I have installed LOTS of start battery and house battery fuses and never had one "nuisance blow". Most folks I know start off the house bank anyway and house banks are required to have fuses, at least on new boats.
ANL's will handle up to roughly 150% of their rating for approximately 500 seconds. For a 200 amp ANL this is 300 amps for up to 500 seconds, or about 8 minutes. For in-rush they can handle up to 600% of their rating for about .5 seconds. This is substantially longer than in-rush currents generally last, which is usually under .2 seconds. For a 200 amp ANL this is 1200 amps for half a second. They can also do 300% for up to 1 full second which is 600 amps for a 200 ANL. In-rush currents are usually measured in milliseconds and rarely last any longer than .2 seconds.
I can't count the number of bank fuses that I have installed but I have never once had one "nuisance blow". Depending upon your engine you may be just fine with a 200 amp ANL but 250 is not an unsafe fuse size based on the ABYC ampacity tables.
-Maine Sail / CS-36T
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