Fuses right out of the Battery?

[hriehl1]

I've read the Casey and Calder books so many times, I can describe the chemical chain-reactions that go on inside a battery... but I still do not know how to wire a boat. Maybe it is me, but some authors jump deep into the theoretical weeds and forget the practical.

I am making some changes with the essence as follows: Right out of the series-wired dual 6V bank I need to make a 3 foot run to my three 6-gang distribution panels and a 5-6 foot run to my outboard well for the 15 HP outboard starting circuit. So... do I need fuses immediately adjacent to the battery bank to protect the wire-runs to the outboard and distribution panels?

I have read conflicting accounts on whether a run to a starter needs to be (or should be) fused, and also whether a modest 3-foot run to the panels needs fuse protection. Your thoughts please.

Thanks

[Hirilondë]

I forget the maximum run to the panel for leaving that unfused but it is so simple to include an in line fuse holder right at the battery that I still used one on my boat. I used a 30 amp fuse and what ever wire was safe for carrying 30 amps the 3 feet or so it was to my panel. Your outboard manual will most likely tell you not to fuse that line as you will blow the fuse instead of starting the engine.

[Ryan]

You need the fuse for the run to the panels (within 7" of the battery). The ABYC code does not require a fuse from the battery to the starter (but my preference is to fuse EVERY wire at the battery).

[Hirilondë]

The ABYC code does not require a fuse from the battery to the starter (but my preference is to fuse EVERY wire at the battery).

The reason ABYC does not recommend a fuse in this line is because it would not work. The amps needed to crank an outboard would blow the fuse. Don't ask me how I know this. ;)

[Ryan]

To each his own, but you can indeed fuse the starter wire, I'm living proof! :) Regardless, it isn't required, so take your pick.

[Shoalcove]

I've got a fuse on my starter circuit as well. 3YM30 Diesel. Heavy cable and appropriate size fuse seems to work OK. I followed Mainesails discussion on this a while ago and am completely satisfied. I often start the motor on the house bank which was already fused and nothing bad ever happened! I do carry spare fuses...

[mitiempo]

ABYC has an exception for fusing starting circuits because a powerboat with large diesels cannot be easily fused if at all depending on engine size. But any diesel up to 75 hp and any outboard likely to be found on a sailboat certainly can. I routinely fuse starting circuits and there has never been a problem.

What you are fusing for is a short circuit. If a battery comes loose or a wire chafes through the fuse will blow. But if chosen properly the fuse should never blow in normal circumstances.

To fuse a battery you need a fuse with a high AIC rating. This means ANL,MRBF, or T class fuses. A battery or battery bank with over 1100 CCA requires a fuse with a minimum AIC of 5000 amps. What this means is that the fuse will not melt closed, leaving the connection intact. I use ANL and MRBF fuses. ANL fuses have an AIC of 6000 amps and MRBF an AIC of 10,000 amps.
ANL fuses can handle a large current relative to their rating and in the ranges used on a sailboat (250 amps or less generally) will blow at 160% or more of their rating after 500 seconds. So a 200 amp fuse will withstand 320 amps continuous for 500 seconds - over 8 minutes. Even a slow starting diesel or outboard is not a problem.

MRBF fuses are convenient for meeting the ABYC 7" rule. They do require extra height however and will not fit in some battery boxes. They can also be used on a bus for different loads.

As Shoalcove posted there is no problem if fused properly.

[Maine Sail]

The reason ABYC does not recommend a fuse in this line is because it would not work. The amps needed to crank an outboard would blow the fuse. Don't ask me how I know this. ;)

This is simply not the case. If the wire is sized properly and the fuse sized to protect the wire within its ampacity limits it can very easily be done and I have done it many times. If you blew your fuse it does not mean it won't work it just means it was not sized properly.

I have many boats where the inrush is twice or more the capacity of the fuse and they never trip and will not nuisance blow. Fuses such as MRBF's, Class T's and ANL's, all proper battery fuses, can easily withstand the 200 - 250 ms inrush. I personally have hundreds of starts on a 44HP diesel with a 225A ANL fuse and my wire can go to 330A @ 100% ampacity and 495A at 150% capacity, if necessary, but it is not..

Here's a great quote from Calder...

"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."
Nigel Calder Cruising Handbook

If I was in the OP's shoes I'd fuse both wires. The panel wire can be fused off the battery switch and a main bank fuse, like an MRBF, for the motor wiring.

[hriehl1]

I feel better in that the differing views here confirm there is no one absolute right answer. Let me boil it down to what I think is my immediate problem. Once I have fuses specced, I can insure my wiring gauge is adequate or I can upgrade it.

From a 200 AH battery bank, I (think I) want:
1. An immediate fuse, then
2. An immediate Battery On-Off switch (always on, there for emergency turn-off only), then
3. Split into two downstream lines to:
3a. 7-foot run to 15 HP 2-stroke outboard starter,
3b. Immediate Blue Sea SafetyHub 100; fused feeds to:
three 35-amp 6-gang distribution panels, and
two always-hot bilge pumps w/ floats.

Can anyone provide guidance on:
a. what size fuse to place just downstream from the battery, and
b. the "safe" wire size for the 7-foot run to the outboard.

I have searched for over an hour and cannot come up with amperage or wire-size recommendation for the starting circuit for a Mariner 2-stroke 15 HP outboard. My Seloc outboard repair manual gives no clue.

Thanks for any further help you can provide.

[mitiempo]

Hi

Not sure how many amps the starter uses, but 4 awg will handle 75 amps with 3% voltage drop over a 14' run. If it is a bit large it will only lead to easier starts. 4 awg will handle 160 amps outside an engine spaces - 135 amps inside engine spaces.

Fusing - either ANL or MRBF - 120 amps is a good choice I think.

The on/off switch can be placed where convenient as the wire is fuse protected. Blue Seas 6006 would be a good choice.

I wouldn't use the safety hub.

I'm not sure what you mean by 35 amp 6 gang distribution panels. I can't find any Blue Seas fuse panels rated that low.

Are they switched for each circuit?
How many circuits do you thing are necessary?

You could go from the switch to either fused or breaker panels like Blue Seas 4306 (90 amp 6 circuit) or 4376 (45 amp 6 circuit) or my preference a traditional panel like Blue Seas 8023 ( 8 circuit 100 amps).

[hriehl1]

The 3 distribution panels are Sea Dog 422110 six-circuit AGC fuse panels each rated for 37 amps total and 10 amps max per circuit. They are in the boat today and I admit I'm attracted to fuses (they seem simpler) rather than breakers. I will use 15 of the 18 available panel circuits, leaving three for expansion.

The Blue Sea SafetyHub 100 seemed like a nice tidy way to split-out the battery switch output into three fused 30 amp runs to the three panels and a couple fused lower-amp hot runs to the two bilge pumps. Why do you not recommend the SafetyHub?

The 3 panels are rated at 37 amps each but I won't load any one of them beyond 25 amps max; would you fuse the run to the panels at 30 (under-fused) or 40 (10% over-fused) amps?

[mitiempo]

The Sea Dog panels are ok. The run to them can be fused and they can be linked with an appropriate size wire as they will be very close I would guess, probably within inches. This is the same way the buses are connected on a large Blue Seas panel, usually with 4 or 6 awg wire, I forget which. That makes the Safetyhub unnecessary.

I would first decide on wire gauge to the total of the expected amperage of the 3 panels. Say 75 amps, figure the distance (there and back) using this calculator http://boatstuff.awardspace.com/awgcalc.html at 3% voltage drop. Then fuse the wire at a higher amperage than the expected draw - say 80 amps. You are fusing to protect the wire not the loads.

The bilge pumps should be wired direct to the battery before the switch. This allows you to keep them active when the main switch is off and you are not there. And as I posted, I would install the switch in a convenient place, not at the batteries.

[Ryan]

The 3 panels are rated at 37 amps each but I won't load any one of them beyond 25 amps max; would you fuse the run to the panels at 30 (under-fused) or 40 (10% over-fused) amps?

Choose your wire size depending on the loads you intend to supply, and then choose your fuse size to protect the wire. The primary job of your distribution fuses are for the protection of the wire runs, not the loads, so size them accordingly.