What Size of Fuse at the Battery

[Fuss]

I want to reduce the risk of a dc fire on my boat and I have no fuses at the batteries.
But I don't want to create any additional problems like blown fuses if I had a motor problem for example and spent a while trying to start my engine.

I have a 700a 24v house battery running everything including anchor and bow thruster.
The cable looks to be about 90mm2 - AWG 000.
What size of fuse do I need at the battery?

I have a 12v 135 hp 6 cyl diesel requiring 2 x 800 cold cranking amp batteries in parallel to start.
The starter cable is 70mm2 - AWG 00.
These same batteries are used to start my 3cyl 7kw generator
The starter cable is 50mm2 - AWG 0.

What size of fuse do I need at each battery?

Thanks for any advice
John

[btrayfors]

The starter can draw up to about 500 amps, depending on the starter itself, the engine, the temperature, and the condition of the starting batteries.

In sizing a fuse for the starting circuit, it would be best to find the specs on the starter motor and, in addition, do a real-world test. Put a clamp-on ammeter on the starting cable when cranking -- best when cold cranking, as that will draw more current.

For the house bank, you want to do some load calculating -- what is the maximum current draw? Then, size the fuse about 25% higher than the anticipated maximum draw. In your case, that's very likely to be less than the rated ampacity of the 3/0 cable.

Bill

[djmarchand]

John:

ABYC standards do not require a fuse or circuit breaker between the starting battery and the starter and it is not a good idea in any case.

Your house battery definitely requires protection however. There are two ways of looking at how to size the fuse/breaker. One is to calculate the maximum load as Bill says and add a safety factor. Another way is to look at the ampacity of the wire from the battery to the first branch circuit breaker and size the breaker accordingly.

If you use the first method, you also have to satisfy the ampacity requirement anyway but that is almost always more current, particularly with 3/0 wire which can carry almost 400 amps.

David

[Fuss]

Originally Posted by btrayfors

The starter can draw up to about 500 amps, depending on the starter itself, the engine, the temperature, and the condition of the starting batteries.

In sizing a fuse for the starting circuit, it would be best to find the specs on the starter motor and, in addition, do a real-world test. Put a clamp-on ammeter on the starting cable when cranking -- best when cold cranking, as that will draw more current.

For the house bank, you want to do some load calculating -- what is the maximum current draw? Then, size the fuse about 25% higher than the anticipated maximum draw. In your case, that's very likely to be less than the rated ampacity of the 3/0 cable.

Bill

Thanks for the replies,
The fear I have when I size the fuse 25% more than the calculation is that maybe sometime later on a cold day or when everything is older and a mechanical problem puts an additional strain on everything, then the fuse blows. This could be at a very inconvenient time.
So I definitely do not want to have this fear.

On the other hand I want to protect as much as possible against fire risk.

How many amps would a 50mm2 - AWG 0 cable draw when shorted. I have not tried it but surely this would be extemely high.
So if I put say a 750 amp ANL fuse from bluesea systems near the positive posts of all 3 batteries then would this reduce the fire risk. The risk of a blown fuse for no good reason would be reduced and if there was a short in even the 50mm2 small wire then the fuse would blow.

This is the biggest fuse Bluesea systems have.

Is this thinking ok or is it flawed.

Thanks again for the replies so far.

[btrayfors]

John,

No, I think your desire to prevent a possible onboard fire is right on. And, by the way, meet ABYC recommendations re: the house bank. Yeah, you can size the fuse larger than any anticipated load -- so long as the fuse doesn't exceed the ampacity of the cable -- but 750A seems extreme. I'd much prefer calculating potential maximum loads and sizing up a bit from there.

RE: installing a CPD (fuse or breaker) in the starting circuit, here I don't agree completely with David, though I know what he's saying. It's true that ABYC recommendations do not require one. However, that leaves the possibility of a spontaneous fault in the solenoid or starter motor which could cause a fire. While rare, these do happen. It happened last year on a friend's 42' sloop while 100 miles offshore, causing a nearly disasterous engine-room fire.

On my own boat, several years ago I installed a large fuse in the starting circuit (400A). I have a Perkins 4-108...smaller than yours.

Originally, I had installed a 250A fuse but it blew one cold morning at anchor. I changed the fuse to 400A, no problems and I'm a bit happier.

Working on wiring systems on boats professionally, I tend to be a bit paranoid these days :-)

Bill

[GordMay]

Originally Posted by btrayfors

... you can size the fuse larger than any anticipated load -- so long as the fuse doesn't exceed the ampacity of the cable -- but 750A seems extreme. I'd much prefer calculating potential maximum loads and sizing up a bit from there...

A 3/0 Cu cable, with 105 deg. insulation, is rated at about 330 Amps, which is the maximum permitted fuse size.

The actual short circuit current will be an Ohm’s Law function of the battery’s internal resistance and that of the grounded wire.
An ideal battery (0 Internal Resistance), in a short circuit with 0 Ω resistance, would be able to supply an infinite amount of current. A real battery, on the other hand, could only supply a maximum of about 600 amps (12 volts / 0.02 Ω) to a short circuit of 0 resistance, due to its internal resistance.

It’s a little more complex that , but a battery can provide a lot of current into a ground fault.

See ➥ http://www.battcon.com/PapersFinal20...rFINAL2003.pdf

And ➥ Battery Sizing Basics

[Maine Sail]

Originally Posted by GordMay

A 3/0 Cu cable, with 105 deg. insulation, is rated at about 330 Amps, which is the maximum permitted fuse size.

The actual short circuit current will be an Ohm’s Law function of the battery’s internal resistance and that of the grounded wire.
An ideal battery (0 Internal Resistance), in a short circuit with 0 Ω resistance, would be able to supply an infinite amount of current. A real battery, on the other hand, could only supply a maximum of about 600 amps (12 volts / 0.02 Ω) to a short circuit of 0 resistance, due to its internal resistance.

It’s a little more complex that , but a battery can provide a lot of current into a ground fault.

See ➥ http://www.battcon.com/PapersFinal20...rFINAL2003.pdf

And ➥ Battery Sizing Basics

I still prefer a fuse, if at all possible, or at the least full conduit for the starter circuit. I have been running start battery fuses for many years, usually ANL, and have yet to blow one but I size them with a DC ammeter with in-rush capability. Some engines just can't be fused due to the size of the starter load but on small sailboats I have never had a problem. I have however seen a number of problems arise from no battery fuses.

I have never understood why for non-motor conductors the OCPD ratio can be 150% of table IV, but for motors, only what is shown in table IV. If we are protecting against a dead short then 3/0 wire should theoretically handle 150% of table IV ..No? It can handle this if not connected to a motor load so why the rigid adherence to table IV when we are really trying to prevent a fire in the event of a dead short. Personally I would feel safer with a 450 amp ANL on 3/0 wire than no fuse at all because the starter draws 350 amps and table IV say you can only fuse to 327 amps. Isn't any fuse at the 150% table IV rating better than none if the load used can handle it? We know what happens with no fuse..

That would make protection of a 3/0 cable up to nearly 500 amps inside an engine room if you choose to ignore the "motor conductor" clause of E-11.

Perhaps one of you guys can expand on why ABYC does not want you to have a fuse on a motor at 150% but for everything else it is fine? If wire is protected to 150% of table IV to my way of thinking I'd rather have a fuse at 150% than none at all. I think I know why this is but I am curious to hear other explanations.

[Fuss]

Originally Posted by GordMay

A 3/0 Cu cable, with 105 deg. insulation, is rated at about 330 Amps, which is the maximum permitted fuse size.

It’s a little more complex that , but a battery can provide a lot of current into a ground fault.

I somehow find these battery fuses confusing. I know that my motor pulls more than 500 amps to start. And it does this through a 70mm2 -AWG 00 cable running over 5 meters.
If its true that an AWG 000 cable is rated at 330 amps which is also the maximum permitted fuse size then I am sure that on a cold day with some starting problems (air in the line for example) that a 330 amp fuse is going to blow.

Does this sound like a reasonable assumption?

So why then is a 330 amp rated fuse correct?

If as I said I installed a 750 amp fuse and then ran the AWG 00 cable from the positive to the negative terminal on my 24v 700 ah battery bank and closed my eyes, what would happen.?
Would the battery explode?
Would the cable catch on fire?
Would the 750 amp fuse blow?

Sorry for the further questions, but I just can't get my head around a 330 amp fuse being big enough.

I absolutely want to protect against fires but i don't want to create a new uncertainty of potential fuse blowings.

Thanks again for all the replies
John

[btrayfors]

Part of the explanation is that starter use is intermittent and of very short duration. It is typical to have large loads on smallish battery cable, larger than the ampacity ratings of the cable.

Since ABYC doesn't require a fuse, I'd say put in any size you want. However, do the inrush measurements first. How do you "know" your starter draws more than 500 amps?

Bill

[Fuss]

I think that its around 450a if the battery is full, but if its a bit flat I think I saw 550a on the link20 monitor. I have a special heavy duty shunt for the xantrex monitor.

[um saudade]

Think in terms of two different purposes of fusing a circuit: A ground fault, which means maximum current available from the supply until such time as the protection blows, the wires do a burn-through which opens the circuit, or in the case of a battery supply, the batteries run down.

Naturally you want to limit time of the ground fault condition and almost any fuse with a current rating less than the battery capability will do that.

BUT: To protect the circuit and the wire we have a different design consideration: Fuse to the size of the wire! Allowances are made for inrush current being high and in the case of continuous current (100% duty cycle) we must derate the wire ampacity simply because over time the heat builds up.

There is a lot of additional to calculate. Can’t always go by the current the higher temp wire is rated for either. This is because all wire is rated to a standard current for each wire size. But we can use the higher current/heat rated wire number in the calculations and derate from the higher number. So for example a 25 amp #12 wire would take some derating and still be usable for a standard 20 amp circuit where as a lower heat/lower ampacity # 12 would end up too small and a #10 wire would have to be used.

It takes a little study to get it right but it’s all in the code.

Um Saudade

[Boatwright]

Practical Considerations for the Cruising Sailboat:

I've done complete electrical rewire/updates on 2 older auxiliary sailboats - 30 and 35 feet.

30 footer has a Yanmar 2GM diesel
35 footer has a Volvo MD11 diesel

No anchor winch or bow thrusters.

All wire is marine rated tin plated with quality heat shrink protected crimped/swaged connectors and lugs.

All lighting converted to LED - with all lights on the load is ~3 amps.
Circuits are divided and fused at the central switch panel.
Maximum fuse size is 10 amps for the 12v auxiliary (cigarette lighter style plugs used for laptop power),
All branch circuit wire is #14, with fuses sized at ~150% of the connected load. For example: the 3 amp lighting circuit is conservatively fused at 5 amps - well under the ampacity of #14 wire.
The main house bus is fed by #6 wire fused at the battery with 50 amp MRBF fuses. The heavy copper main bus is also protected at the bus by a 50 amp ANL fuse.

The starter is fed by a #2 cable protected at the battery by a 150 amp MRBF fuse.
Contrary to popular belief ANL, Class T or marine rated battery fuses (MRBF's) do not blow or trip at face value unless the duration is long enough. These fuses have a 450 amp/1 second and 300 amp/5 seconds rating - more than enough to handle starter inrush for this size engine, even in cold weather.

We have never had a fuse blow. Both engines are in good shape, and even without glo-plugs start well in cooler weather. If you have to crank for more than 5 or 10 seconds you are abusing your starter no matter what the fuse size - time for a tune up.

[wildernessdave]

This is a really interesting thread. My wife and I recently purchased a (new to us) Vancouver 27ft (1984) and it has been recommended that we add a "main" breaker.

The electronics are very simple. No AC power, just DC for lights, depth sounder, and other small electronics, manual windlass so nothing that draws a lot of power. Batteries are 2 x 100 amp hour 12 volt batteries in parallel.

The engine is a 1984 Bukh 20 hp.

I am trying to figure out a good fuse or breaker to protect this system.

It seems like many people suggest protecting the starting wire as well as the "main" wire leading from the battery bank to the house load.

The batteries and the engine are not far apart (probably 5 feet). Cables are pretty thick, but I have not measured them to determine what size wire is used to connect the starter or the house panel.

Would adding a 50 amp fuse or breaker between the house breaker panel and the batteries be good? Should this be within a few feet of the batteries?

Then add a fuse or breaker in the engine compartment that has a higher amp rating?

Thoughts on fuses versus breakers in this situation?

Any thoughts on how much the starter will draw and what size or type of fuse/breaker? I was hoping to install the breaker/fuse for the engine before launching the boat, but in that instance I would not want to start the engine and measure amperage during starting because the engine is winterized.

Thanks for any information you can provide.

Dave

[thomm225]

Originally Posted by wildernessdave

This is a really interesting thread. My wife and I recently purchased a (new to us) Vancouver 27ft (1984) and it has been recommended that we add a "main" breaker.

The electronics are very simple. No AC power, just DC for lights, depth sounder, and other small electronics, manual windlass so nothing that draws a lot of power. Batteries are 2 x 100 amp hour 12 volt batteries in parallel.

The engine is a 1984 Bukh 20 hp.

I am trying to figure out a good fuse or breaker to protect this system.

It seems like many people suggest protecting the starting wire as well as the "main" wire leading from the battery bank to the house load.

The batteries and the engine are not far apart (probably 5 feet). Cables are pretty thick, but I have not measured them to determine what size wire is used to connect the starter or the house panel.

Would adding a 50 amp fuse or breaker between the house breaker panel and the batteries be good? Should this be within a few feet of the batteries?

Then add a fuse or breaker in the engine compartment that has a higher amp rating?

Thoughts on fuses versus breakers in this situation?

Any thoughts on how much the starter will draw and what size or type of fuse/breaker? I was hoping to install the breaker/fuse for the engine before launching the boat, but in that instance I would not want to start the engine and measure amperage during starting because the engine is winterized.

Thanks for any information you can provide.

Dave

My boat is of a similar setup as yours with two 12's in parallel but I also have a 400 watt inverter and a 1500 watt inverter (not hooked up yet)

I was planning to do something like you have in mind and do it at a very low cost but it was explained to me to do it the right way and with something like Blue Sea Terminal Blocks and marine rated battery fuses (MRBF's)

That is what I did and I ordered 175 amp fuses due to the 1500 watt inverter plus......I have no starter battery

The items arrived yesterday and are of a very high quality.......the terminal blocks connect directly to your battery posts

https://powerwerx.com/mrbf-terminal-...FYlrfgodBQQIFw

https://www.westmarine.com/buy/blue-...09_273_007_517

[CarlF]

The ABYC should update the starter circuit rule to either require a fuse or armored conduit. An unfused circuit might have been an acceptable boat builder cost saving 30 years ago on a 70hp outboard runabout with a single Group 24 flooded battery - but not today with large banks of AGM's or even Lithium Ion batteries. Boat fires can kill people.

While they are at it, they could update the wire color chart. How many boats today have all their cabin lights wired with dark blue wires?

https://newcontent.westmarine.com/co...ech-Spec-2.jpg