Main Fuse on Battery

[TomTom]

I need some advice on what size fuse to put on the main wire coming off the battery.

Do I size this based on the engine starting amperage?

Or is this normally dealt with by the engine fuse and I don’t need to worry about a fuse on the battery cable?

In which case do I just add up the total amperage of everything that will be running on the boat at one time and use this to determine the fuse size?

[fallguy1000]

Starters are not fused.

If you have other loads coming off the same battery; those would be fused.

A typical easy way to fuse for a smaler boat would be to install a mrbf fuse atop the battery and fuse it to need and send it to a fuse panel.

Terminal lugs in boats are limited to four terminals or wires.

The ideal way to do it is to run an mrbf fuse of say 30 amps to a 30 amp fuse block 6 or 12 positions. Then disconnecting your battery is super easy.

[fallguy1000]

Bluesea 5025 or 5026 are ideal. They also have a ground line.

The sizing of the wire fuse is based on load and distance, but in general, a #10 wire would give you 10' of run at 30amps max load.

A real professional approach is yellow ground wire #10 and red hot #10 off a mrbf fuse to the fuse block. Then the starter battery has only the starter wires and the mrbf fuse holder on the hot and the neg post has the starter neg and the #10 yellow. If you have a bms; that changes it a bit.

Also, remote battery switch(rbs) changes it a bit. The starter and loads is/are wired to the rbs.

[fallguy1000]

Here is a picture; although I'd not bother with the lead clamp. The red wire is on top to starter and the loads come off the fuse or the fuse goes to a fuse block.

[Jaysen]

I think the info you are looking for is determined by answering one question: what is on the circuit?

Once you know the draw, you size the wiring to not start a fire. Then you match a the fuze/breaker to avoid letting the wire catch fire. I highly recommend going up a size (or two) on feeds from battery and to secondary distribution. This gives you some expansion.

Hope that made sense.

[TomTom]

I think the info you are looking for is determined by answering one question: what is on the circuit?

Once you know the draw, you size the wiring to not start a fire. Then you match a the fuze/breaker to avoid letting the wire catch fire. I highly recommend going up a size (or two) on feeds from battery and to secondary distribution. This gives you some expansion.

Hope that made sense.

Thanks Jaysen - my main question was whether the fuse must take into account starting the engine. I think Fallguy has answered that by saying this is (never?) fused?

That’s sort of where I was leaning - based on the fact that I’ve never seen a fuse on factory boat cables; but I have a friend who was suggesting otherwise?!

[TomTom]

Starters are not fused.

What protects against a short on the starter cables? Or is it just assumed that you must make sure that this would never ever happen?

[joe2700]

Starters are not fused.

What protects against a short on the starter cables? Or is it just assumed that you must make sure that this would never ever happen?

It's certainly common to not fuse them when the battery is sitting right next to the motor, I've never seen it fused on the battery side. That said with my starting battery up in the console I wasn't comfortable having that long run unfused. I used a 200A MRBF fuse on each battery for the connection to the battery switch(and therefor motor), and a 40A fuse for the connection to the ACR by putting a double MRBF fuse block on each battery positive. Higher than the starter would ever draw if functioning, but protects from a dead short in 4AWG wire coming right off the battery which would be catastrophic.

[Jaysen]

I just looked at a couple production center consoles with starting bats in console/under leaning post. All had 200hp (or larger) motors. All used 200a BREAKERS.

I checked a couple sailboats because … I can. All the “new” boats had breakers. All the old boats were directly wired. All were diesel. Perkins, westerbeak, yanmar and the Mercedes block that never wants to run.

If it were me. Breaker sized to prevent burn of wiring. Get the “slow” trip type. I think that’s what FG1k mentions.

[OrangeQuest]

I need some advice on what size fuse to put on the main wire coming off the battery.

Do I size this based on the engine starting amperage?

Or is this normally dealt with by the engine fuse and I don’t need to worry about a fuse on the battery cable?

In which case do I just add up the total amperage of everything that will be running on the boat at one time and use this to determine the fuse size?

If you have one main wire that supplies power to everything but the starter you would want that fused as close as possible to the battery, then a fuse panel with smaller fuses for all the circuits connected to it. A main ground wire that runs to a ground bus that all the circuits, each with their own ground runs to.
Then off the main fuse panel you could have sub-circuits that have their own smaller fuses matching their demand. Let's say you have an accessory circuit fuse (20 amps) on the main fuse panel. Branching off that you have a stereo (10amps), gunnel lights (5 amps) and well lights (5 amps). The acc fuse at the main panel is fused to handle the current everything it supplies at once but each of them is limited by their own fuses to what they need. You cannot protect any of these circuits with the 20-amp fuse, they would all burn up before the 20-amp fuse blows.

I hope that makes sense.