Starting Circuit Fuse?

[Soundevice]

Good Morning,

I may be opening a can of worms here but thought I'd reach out to the community to see how folks are dealing with the following-

-what size of fuses are you using between the battery and battery switch?

Seemingly simple but as I've discovered, there is a wide swath of what is deemed acceptable in the boating community. ABYC standards call for a fuse within 7in from the battery EXCEPT FOR THE STARTING CIRCUIT. Since many boaters have a dual battery set up, one for starting, one for house, I'm curious on how you would fuse these circuits.
No fuse on the starting battery and a fuse on the house battery? 2 potential issues arise, in my view. First, with a stuck starter, you could burn through a long cable in not too long and with no fuse could be trouble. Second, if you use your house as a backup, do you get a trip on your fuse because of the large initial amp draw from the starter, which is why a fuse is not required on the starter circuit?
Boat electrical guru Nigel Calder has addressed this issue, somewhat vaguely, and recommends putting a 300amp slow blow fuse on the starting battery for outboard motors under 100 horsepower. Any thoughts on this as a solution? I'm thinking that I would install the 300 amp fuse on the starter and then a normal fuse for the house and if I were to trip it in an emergency, it would be relatively simple to bypass the fuse to start the motor if starter battery was dead.
After spending the winter learning as much as possible about 12v systems, this has been the biggest sticking point to completing the rewire of my 26X. I imagine there are more ways to skin this cat so I appreciate any thoughts and/or concerns. I will say that the starter on my Honda BF50A is quite small and I may be over thinking this a bit but I definitely would prefer EVERY cable to be fused on my boat, even if technically not required...

Thanks,

Ben

[Russ]

Interesting question.

I think my outboard is wired directly to the start battery.
My house panel has an inline breaker close to the positive post of the house battery. I used a breaker so I can cut power to the panel if I need to.

There is a combiner that joins them when one of the batteries reaches charging voltage (13.??). Both legs are fused.

Others that have an A/B/Both switch would likely have both starter and fused house on the central positive pole of the switch.

I suppose a 300a slow blow fuse would add more protection and wouldn't hurt. Our small outboards do not take much to start and I would think that's not going to blow it unless you had a dead short.

[Soundevice]

Russ, do you have any shutoff between the cranking battery and engine?
I ask because I'm thinking of installing a DC to DC charger between the battery banks to control the charging profile of the starting and house batteries. This actually isolates the house and in theory I would not need a A/B/BOTH switch. I don't like not having a shutoff, though, with the ability to quickly disconnect the batteries in an emergency.
When pulling my outboard last year, I discovered that the connection from the battery cables to the outboard cables was simply bolted together and electrical taped, separated, obviously, but still with only the tape separating the connection points. Because the outboard cables pos and neg are the same length, the exposed terminals line up right next to each other. I don't doubt that many people have a similar setup as the Mac doesn't have a covered terminal block astern to join the cables. My cables are laying on the rear birth and it would seem by the law of averages that at some point someone could throw something back there and potentially short the terminal attachment. This is pretty much why I want to fuse that circuit. I'm also mounting a covered terminal to secure the connection and clean it up a bit. If one was 'hardwired' directly to the battery and the above shorted, it would certainly be a panicked moment trying to unbolt the battery terminals!
I'd like to throw another related question out there- since the X and M are similar lengths, what size battery cables do you use to connect the outboard? I would think we all have similar length runs, approx 10ft, and sized motors, 50ish HP. The cables on my boat are 6ga which I feel are undersized but they have obviously served their purpose for 25 years. I'm thinking of installing 4ga even though much of the internet suggests 2/0.
Again, I appreciate any thoughts as I plan to not ever rewire my boat again and want to get it right!

PS-its a beautiful day in Pittsburgh and I'm heading out on the Alleghany. I saw on the Mac Map there is another local 26X owner...hope to see you on the water!

[Russ]

I'm pretty sure my outboard is wired directly to the start battery. Nothing inbetween. Both batteries are combined when either voltage goes to charge. I'm not familiar with DC to DC chargers.

If you want extra safety, you could install one of these between the motor and start battery. Not a bad idea to be able to cut power. Although one more thing to forget or go wrong.

[Be Free]

Good Morning,

I may be opening a can of worms here but thought I'd reach out to the community to see how folks are dealing with the following-

-what size of fuses are you using between the battery and battery switch?

Seemingly simple but as I've discovered, there is a wide swath of what is deemed acceptable in the boating community. ABYC standards call for a fuse within 7in from the battery EXCEPT FOR THE STARTING CIRCUIT. Since many boaters have a dual battery set up, one for starting, one for house, I'm curious on how you would fuse these circuits.
No fuse on the starting battery and a fuse on the house battery? 2 potential issues arise, in my view. First, with a stuck starter, you could burn through a long cable in not too long and with no fuse could be trouble. Second, if you use your house as a backup, do you get a trip on your fuse because of the large initial amp draw from the starter, which is why a fuse is not required on the starter circuit?
Boat electrical guru Nigel Calder has addressed this issue, somewhat vaguely, and recommends putting a 300amp slow blow fuse on the starting battery for outboard motors under 100 horsepower. Any thoughts on this as a solution? I'm thinking that I would install the 300 amp fuse on the starter and then a normal fuse for the house and if I were to trip it in an emergency, it would be relatively simple to bypass the fuse to start the motor if starter battery was dead.
After spending the winter learning as much as possible about 12v systems, this has been the biggest sticking point to completing the rewire of my 26X. I imagine there are more ways to skin this cat so I appreciate any thoughts and/or concerns. I will say that the starter on my Honda BF50A is quite small and I may be over thinking this a bit but I definitely would prefer EVERY cable to be fused on my boat, even if technically not required...

Thanks,

Ben

Ben,
Great questions!

The first thing I'd like to point out is that that ABYC standards are a minimum, not maximum. They do not forbid fuses on the starting circuit, they simply do not require them.

The second point is that the fuse is there to protect the wire, not the equipment on the other end. Again, the fuse needs be be less than or equal to the ampacity of the wire. Making it significantly less may be wasteful but it is not unsafe. You will generally pick a fuse that is above what you expect the loads on the wire to be and less than the ampacity of the wire.

The starting circuit is one of the exceptions. Your starter will draw a lot of amps in the first few milliseconds and a significant amount while the engine is turning over. It is likely that the cable is not large enough to carry the cranking load for more than a few seconds (a "big" few, but not minutes). That's why Nigel Calder gives the recommendation to use a 300 amp slow blow fuse on the starter circuit. The inrush current is not going to last long enough to blow the fuse and a small outboard is not going to crank long enough to blow it. On the other hand, a dead short in the positive cable will blow the fuse before the cable melts.

So, my recommendation:

Fuse the house bank a little higher than the combined house loads but lower than the maximum that the cable connecting the battery to the boat. Obviously you will do the same to all of the branch circuits.

Fuse the starter cable with a 300 amp slow blow fuse per Nigel Calder's recommendation. I personally do not like switches between my engine and my start battery. The fuse takes care of disconnecting the battery in an emergency (just like the fusible link does in your car) and I just pull the negative cable while I'm working on it, again, just like a car.

My personal preference is to have a start battery and a house bank that can share charging sources but not loads. That way if I kill my house bank I can still start the engine the next morning (and charge from the alternator) but there is no way for the house loads to kill the start battery. If the start battery is dead for some reason I can still use jumper cables to the house bank or use a portable jumper battery (I keep both on the boat for long trips).

[Russ]

I tend to go along with this guy's assessment.

If your battery is low, you are very likely going to blow that starter fuse because of the draw on the circuit increased greatly in this condition.
As he points out, that might be when you need to start the engine fast. If that fuse blows when the boat is drifting, you don't have time to test the fuse and replace it.

His suggestion is to insulate and protect that starter circuit so it won't have a short. My cables to the outboard are HUGE. VERY thick insulation and are grommeted where they pass through the deck.

I don't know..it's your boat and if you feel better fusing it, then do so. Just use the advice above and use a slow blow 300+ amp fuse. I've been there when my boat (not my Mac) was drifting and the starter wouldn't start (it was a solenoid) and it was frightening. Not a time to begin troubleshooting.

[Russ]

And then there is Nigel who I respect

[Be Free]

Some real world examples:

Honda outboards (like the OP is asking about) from 35-50 hp (IIRC) all use the same battery cables. It is a pair of 6AWG cables molded together. They are well insulated but they are susceptible to a sharp object penetrating the insulating and connecting the two since they are running side-by-side. A dead short across the two cables is a small but real possibility. It has happened.

The ampacity of 6AWG cables is at most 135 amps. For the sake of argument lets assume that Honda used the very best cable (but I suspect it is actually 80 amp cable).

The no load draw on a starter for a BF50 (and 40, not sure about the 35) is 75 amps. You are not even trying to start the engine yet and you have used up half of your cable's capacity.

The BF50A calls for a 420cca battery and the BF50D needs 582cca. The CCA (simplified) is the amount of current the battery can supply for 30 seconds without dropping below a specified voltage at a specific temperature. Think of it as the most current the battery can supply trying to start your engine under the worst conditions you are likely to be in. Those numbers fall off rapidly as the battery ages.

Your engine is not going to be pulling anywhere near those amps unless there is already something wrong with it. Just for the sake of argument, let's assume it does.

A 300 amp slow-blow ANL fuse will hold for 5 seconds at 500 amps and 500 seconds at 450 amps. I've never needed anywhere near five seconds of cranking for my outboard to start. In fact, Honda specifies that the starter should not be run for more than five seconds. If it has not started in the first two seconds there is probably something wrong. If it's drawing 500 amps there is definitely something wrong. Even at 450 amps I'd still have to crank for over eight minutes to blow the fuse. I'm certainly not going to be doing that. The starter would be long gone (if the battery cable did not melt down first).

Another way to look at it is that it is more likely that the battery is going to run down than it is for a properly sized fuse to blow trying to crank an engine that was ever going to start.

Next, let's consider a dead short across the battery cables. CCA numbers are out the window with a dead short.

A group 24 or 27 (the most likely start battery in our boats) will have at least 3000 short circuit amps. There is no question that the 6AWG cable is going to melt and probably start a fire. That same short circuit will blow the 300 amp fuse instantly and protect your cables.

tldr: If the engine was ever going to start the 300 amp slow-blow is not going to trip; without it your boat may burn up.

Bonus advise: don't use a circuit breaker instead of the fuse. Most will weld themselves closed under a 3000 amp short circuit load.

[Soundevice]

Wow, excellent info!
Russ, the above 2 videos, which I'd seen, are exactly why I reached out to this forum. 2 guys that I've found great info from and both offering different recommendations!
I was actually looking into breakers this morning because it would serve as a switch and also easy to flip in a nuisance trip situation. I'd read that a thermal breaker may act as a slow blow but didn't even know that they could fuse with high enough amperage and stick. I did feel it was getting too complicated so was going to just get higher gauged starter cable and delete the 300 amp slow blow. Now after reading Be Free's breakdown, which I had real difficulty breaking down from a fuse chart online, I will put a 300amp slow blow in, just to be safer.
2 questions-

-any recommendations on procuring the actual slow blow fuse? I didn't see any on the marine electrical supply places and others I found I wasn't sure they'd be suitable for the marine environment.

-in the above parameters listed, BF50A motor in a Mac 26X with approx 10ft run, what size cable would you suggest. At 150amp draw entered into a cable calculator I got 2/0 sized battery cables. This seems a bit excessive, especially since its functioned with 6AWG cables, as stated before. The fact that one shouldn't be cranking the starter for more than 5 seconds really skews the numbers since most ratings will be for a constant and longer draw. Would 2WG be a sufficient upgrade, especially with a now fused circuit? Is too large a cable possible?

I really appreciate your responses. While some of this may dip into the academic, the real world applications of this are important and I find it interesting that there is not more definitive information available or maybe even existing. Obviously, there are many different parameters and designed electrical systems on these boats but why a particular motor company can't give a chart with the recommended cable size to length from battery seems strange. This is not my field so I imagine they have their reasons though the cynic in me thinks lawyers may be involved!
Thanks for taking the time to discuss these items, what a great resource!

[Be Free]

Look for a Blue Sea 300 amp ANL fuse. You will also need to add the holder. You could also use a 300 amp MRBF. The ANL fuses are a little less expensive if you blow one. In either case stick to good, name brand versions. Don't buy cheap knockoffs.

You don't need to change out the cabling that came with the Honda. Mine has been in place for over 20 years. You won't overload under normal conditions and the fuse will protect you in abnormal conditions.

[OverEasy]

Hi Soundevice!

Great question!

When going through the existing electrical system on Over Easy soon after purchasing her I also found some deficiencies with the engine start circuit similar to what you describe. In our case the cable splice was in the bilge under the aft berth, positive & negative splice bolts were bolted opposite each other, common steel zinc bolts, no washers of any kind and wrapped in 6 layers of separating black ‘vinyl’ tape that showed obvious signs of having been submerged in water (not only on the outside but for several feet under the sheathing as evidenced by the corrosion and moisture found). 🫣 While there is now way for me to determine when this was done or by whom I have a significant dislike for the perpetrator(s). The cable was also hard wired to the battery and there was no battery isolation switch or circuit protection device.

Note: There are no formal regulations regarding any construction aspect of boat building that manufacturers can utilize or refer to.
This is in part due to the industry lobbying and a desire for a lack of accountability. That’s why the ABYC are only recommendations adopted in part because the individual boat owners insurance companies decided to generate them to protect themselves when dealing with property and subsequent injury claims.

While I agree wholeheartedly with Russ and Be Free on the larger wire gage and also on the incorporation of a circuit protection device.
2/0 wire (multstrand) is always a good choice for an engine start circuit. Just be sure to get marine grade wire (which has the individual strands tinned to avoid copper corrosion) with the water resistant jacket such as:
> THHN (Thermoplastic 167°F. Wet Locations. Flame Retardant, Heat Resistant. (194°F. Dry Locations.)).
> THWN (Thermoplastic insulation, 167°F. with outer nylon (or equivalent) jacket; Flame Retardant, Heat and Water Resistant.)

When it says ‘Thermoplastic’ for the insulation it means:
TPE which is thermoplastic elastomer or TPR. Thermoplastic rubber has mechanical characteristics of thermoset rubbers yet is a thermoplastic. They have excellent ozone and chemical resistance, excellent electrical properties and low water absorption. Temperature range is -70°C to 125°C.

If you can’t get the marine grade wire, you could place the cables in a water tire conduit with an “in-ground” rating when routing under the aft berth and bilge to protect the wire jacket. At the junctures where the wire strands are exposed you can apply an electrically conductive grease like NO-OX-ID to protect the exposed material.


A properly rated circuit breaker is every bit as good as a fuse.
If properly rated it will not arc closed even if there were a dead short.
It is what they are specifically designed to protect against.

The advantage is that a properly rated circuit breaker can be quickly reset WITHOUT having to fumble around finding spares to install.
You don’t need to provide for more than 150% of your actual start cranking amperage.
While the primary objective of either a fuse or circuit breaker is to protect the wire it can and does provide the means to protect the device power is being supplied to. The two aspects are not exclusive to each other but are actually complementary.
The benefit in doing this is to protect your engine started from burning out should something hang up and jam the Bendix gear or interface engagement with the flywheel.

Having an excessive capacity on a start circuit doesn’t help you in a crisis, it only allows the circuit to cook things.

ABYC standards are a minimum, not maximum.
ABYC Standards refer to NEC (National Electrical Codes) and good industry practices.

The reason engine manufacturers don’t provide electrical cable charts or recommended lengths is that each boat manufacturer and model is different as to where batteries are located in relation to the engine and how the manufacturer chooses to route the wires.

In NO CASE should ANY ELECTRICAL POWER CABLE NOT BE PROTECTED with a current limiting device, EVER!
The readily apparent and significantly destructive and inherently dangerous risk TO PERSONNEL AND PROPERTY is not EVER worth the risk.
If anyone disagrees they can take it up with the aircraft industry and automotive industry which both have regulations requiring start circuit current limiting devices. (Having to restart an aircraft engine is a whole lot more dire than that of a boat (in the vast majority of cases) and not having a current limiting device is a patently dangerous thing to do, especially on a boat.

After having spent a successful career designing and fabricating a wide variety of equipment and having been requested to conduct multiple failure investigations for FAA as will as commercial clients I’ve yet to see where a properly rated circuit current protection device caused injury, damage or fatality.

BUT I have seen multiple instances where an unprotected electrical circuit, or a circuit with an over rated circuit protective device, HAS CAUSED injury, damage and fatalities.

Best to put in a properly rated circuit breaker (or fuse) that is always readily available and accessible.

It is also best to put a battery isolation switch in place as close to the positive terminal as possible.
While this may be inconvenient and require some reworking/modification of things it is well worth it vs. the potential consequences.
You want to always have plainly identifiable as well as ready and unobstructed access to the battery isolation switch(es).
(While you might know it’s hidden away in side the unmarked battery box under the lid that you secure the cooler on top of your potential crew may not know or remember that in the moment of need (or have the time).
It sounds like you want to do what is right and I support you in getting it done right when you have the opportunity!

Best Regards
Over Easy

[Soundevice]

I'm familiar with all of the above posters and respect your thoughts on this. While I don't post often as I'm not an expert on anything related to boating, I know that you all post solid information. The same goes for Jeff Cote and especially Nigel Calder. The fact that there is no perfect answer makes this equally fun for the flexibility and frustrating for the lack of certainty! The fact there are many opinions and some contrary to each other kind of makes me draw the conclusion that it really depends on how safe you want your boat systems to be and to some it may be overkill while to others it would be the 'right' way to do it. As I am not an expert, I will generally err on the side of extra precautions.
To that end, I will plan on using 2/0 tinned battery cable since I'd like to replace the existing anyway. It'll cost an extra 50 bucks or so compared to the 2AWG so not a huge price dif for the piece of mind.
A 300amp breaker as opposed to a fuse is definitely attractive for the obvious reason of not having to keep a spare handy and the ease of flipping a switch if in an emergency though if I can't find a suitable breaker a fuse will work too. It sounds like a nuisance trip isn't as much of an issue since its rated way over any normal usage on that circuit.
Over Easy, would you have any recommendations as to what type of breaker and where one would procure a, hopefully, marine grade breaker from?

Thanks for all the info, its been enlightening for sure!

[Starscream]

My boat has the two batteries wired directly to the battery selector. From there, I have a fused line to the main switch panel, another fused line to a second switch panel on the starboard side, and an unfused line to the motor.

I had the standard BF50A motor cables and when I changed to the Etec 90 I had all kinds of starting problems because the (now bankrupt) dealer didnt change the wires, or even check them. I had the cables changed out, I forget what gauge but I'm they are huge, heavy and now run in conduit. I want to say 00 but I'd have to check. No more starting problems.

[Be Free]

Soundevice,

If you go with a breaker make sure it has an AIC (Amperage Interrupt Capacity) greater than the short circuit amperage of your battery (or battery bank). Even for a start battery that is usually going to be above 3000 amps.

Starscream,

The dealer should have replaced those cables when the engine was installed. I'm not familiar with the specifications of the ETEC starter but since it it turning over an engine with almost twice the HP of the one the cables came with I'm sure it was struggling with the #6 cable that the dealer left in there. The power requirements of the starter are essentially fixed. The amps required to produce that power will go up as the voltage drops (due to the undersized cable). The increased amperage draws the voltage down which in turn increases the amps. It's a vicious cycle.

[OverEasy]

Hi SoundDevice!

West Marine has a generally good supply of marine breakers to choose from as do the other marine supply dealers like wholesale Marine (and others).

Blue Sea Systems, EGIS Mobile Electric and Sierra are three well respected and recognized marine grade breaker manufacturers.
Each have manufacturer websites and all are happy to help you get what you need (even though I don’t believe they sell direct to public ).

Best Regards
Over Easy