We have an 87' C27 with the M18 diesel and 2 batteries. Electrical is not my forte by the way. So, is it possible to determine the fuse size for each battery without knowing the wire size for the batteries and the load. It's pretty much the standard factory setup with the original main fuse panel and selector switch. I would like to add the Blue Seas MRBF fuses to the house and starting battery. I looked at Blue Seas web page and it has a calculation tool but I don't have the necessary information to use it and won't be at the boat for a few more weeks. Guessing I'll need more info!
Steve
Steve,
The battery fuses are always sized to protect the wire. It does not matter the length just the max ampacity rating for the wire. The ABYC allows for going to 150% of the max ampacity rating if your wire is slightly undersized, which many early Catalina's are.. These fuses are only to protect against catastrophic faults or dead shorts, which happen.
There is no reason to not protect starter batteries on small AUX sailboat engines. The ABYC "allows" an exemption but this is in no way a suggestion to not fuse. I have literally thousands and thousands of starts on batteries with fused banks and never once had a "nuisance trip" ever. In a properly sized fuse this simply will not happen unless it should happen.
This exemption only gets more complicated when we delve into a 1/2/BOTH switch. Which battery is the "start battery" and which is the "house". Neither is a truly "dedicated" start battery. When you consider that a large proportion of people using the 1/2/BOTH switch start and do house loads off the house bank, and the house bank is required to be fused if you want to be ABYC compliant, this gets into an even more gray area. Many of us involved in the ABYC, including the ABYC's own education director, have long argued to change the wording of the "exemption" for start batteries. It is one thing to exempt massive motors that require thousands of dollars in fusing to protect them it is another to allow an exemption on small motors that can very, very, very easily and very cheaply be fuse protected.
In 2005 we purchased a brand new Catalina (not used a brand new factory boat). I surveyed it myself as did a well known "surveyor" and neither of us caught the starter wire that was a tad too short. It was nearly impossible to see and when the motor was running and the wire jacket warmed and softened the wire chafed on the engine. Under a visual inspection it did not look at all as if this wire would cause a problem. In mid July it chafed through, shorted and blew the fuse. If I had not added the second fuse before taking delivery, Catalina shipped with just one bank protected, we could have easily burned our boat to the waterline.
The simple answer for fusing a start battery is that if you "nuisance blew" a fuse during starting then the fuse and or wire was not sized properly to begin with.
Any fuse on a battery bank is far safer than burning your boat down.. I have personally tested and blown a 300A fuse on 15' of 8GA wire with a 400A lithium battery bank that can supply approx 20,000 amp of short circuit current into a dead short.. I have tested LOTS of expensive fuses in my shop. That 300A fuse tripped in a split second with a dead short, as it should. The 8GA wire never even had a chance to get warm to the touch. Even if you go to the allowable 150% of ABYC you are still well protected. To put it in perspective my tests with 8GA wire were at 250% of the ABYC max allowable 150% amperage and the wire never even got warm...
8GA wire at 100% ampacity and 105C rated wire is 80A outside of an engine space. 150% is 120A outside an engine space. I went to 300A which is 375% of the 100% rating and 250% of the 150% rating.. There is a huge difference between a dead short and starting a motor. The smallest fuse we ever advise for any bank that may be bneeded to start a motor is 250A but 300A is a much better option.
For more information on this you might want to read this:
Battery Fuse Sizing - How?
