Fuse size at battery to protect against fire

[fuss]

Sorry for this post if the answer is obvious but its not to me.....

I would like to protect against a fire from the battery main cables. But I don't want to create a situation where the fuse occasionally blows for other reasons as this could for example then blow my alternator diodes, so I want to be absolutely certain that the fuse will not blow under any general usage circumstances but would protect against the situation where the wire became so hot that there was a danger of it catching on fire.

I think the biggest slow blow fuse is 750a and I would have imagined that this would always blow before a large cale (50-90mm) caused a fire ???

There has been some other pbo posts where the recommended fuse for the wire size has been suggested. Here they are...

cable size fuse
35mm2 264A
AWG 1 305A
50mm2 345A
AWG 0 363A
AWG 00 432A
70mm2 443A
AWG 000 514A
90mm2 535A

Do these sizes sound right for what I wrote above?

Thanks for any comments
John

 

[charles_reed]

You may find a deafening silence to your request.

I did fit a 50amp thermal interruptor on my service circuits, but any fuse that is sufficient to ensure cranking power is unlikely to blow when needed.

 

[VicS]

You my find the table HERE helpful.

Of the two columns for current take one as being the max current that the wire can take when freely ventilated, ie not bundled with other wires or run through a confined space. Applicable perhaps to the battery to bus bar cables and the starter motor cables.
The other when it is bundled with other wires etc. Applicable to everything else.

Make of it what you will.

AWG to mm² HERE if you need it

 

[Marsupial]

I would agree with Charles, the problem with trying to fuse a motor is that its not easy to get a fuse a blow reliably at high amperages, you could try a breaker but it would be big. also the circumstances under which the cable will burn are often within its safe working range. In other words a fault develops that causes a high resistance and things get hotter than they should but in a localised area (like terminals/connectoins). The battery cable fire that sticks in my memory was on a truck that kept starting up on its own, it started itsself during the lunchbreak, the battery cables went round the fuel tank, they and the loom around them were ablaze as were most of the plastic bits in the engine compartment. The starter motor was now a generator and smoking. The truck was parked next to the waste oil tank and other vehicles were in the workshop. I put the fire out and the company I worked for at the time deducted the cost of fire extingusher from my pay! nice people.

The common wire fire on boats concerns undersized "charging cables" and in these cases its easy to get a wire ablaze from the splitter diode or battery switch to the bow thruster battery. The solution that is now accepted practise is to size the charging wire as if it was the main load carrying wire so if the bow thruster battery develps a fault the motor CAN get its amps "fix" from the main bank through a correctly sized conductor without it busting into flames.

Another route for overload and fire is the cable that connects any remote battery to a volt meter, on my boat (Jeanneau) they have done their homework and this little wire has a little inline fuse on it.

Hope this lot helps.

 

[pvb]

The views expressed about the reliability of fuses are incorrect. The fuse you should use to protect your battery cables is the Mega Fuse, made by Littelfuse. It has been engineered for predictable slow-blow characteristics, and works extremely well. The fuse sizes you listed in your post are appropriate.

I can vouch for its reliability. I use 250A Mega Fuses on my start battery (110Ah) and domestic battery bank (660Ah). Once, when replacing my second alternator, I foolishly created a short circuit on the domestic bank. The Mega Fuse blew instantly.

Mega Fuses need to be mounted as close to the battery as possible and in a special holder; I use the red holders made by Blue Sea Systems.

You can read a detailed technical brief on the Mega Fuse here, and the data sheet here has a graph showing the slow-blow characteristic. (Usual disclaimer, just a satisfied customer.)

 

[Marsupial]

The views expressed about the reliability of fuses are incorrect. The fuse you should use to protect your battery cables is the Mega Fuse, made by Littelfuse. It has been engineered for predictable slow-blow characteristics, and works extremely well. The fuse sizes you listed in your post are appropriate.

I can vouch for its reliability. I use 250A Mega Fuses on my start battery (110Ah) and domestic battery bank (660Ah). Once, when replacing my second alternator, I foolishly created a short circuit on the domestic bank. The Mega Fuse blew instantly.

Mega Fuses need to be mounted as close to the battery as possible and in a special holder; I use the red holders made by Blue Sea Systems.

You can read a detailed technical brief on the Mega Fuse here, and the data sheet here has a graph showing the slow-blow characteristic. (Usual disclaimer, just a satisfied customer.)

Please explain to me why my bow thruster which draws 365A is specified with a 250A fuse?

 

[pvb]

Please explain to me why my bow thruster which draws 365A is specified with a 250A fuse?

What make and model?

 

[pvb]

Please explain to me why my bow thruster which draws 365A is specified with a 250A fuse?

OK, doesn't matter which thruster you've got, let's consider a typical popular model like the Vetus 55kgf thruster, which uses 375A at 12v. Vetus recommend a 250A slow blow fuse for this thruster.

The characteristic of a slow blow fuse like the Mega Fuse is that it takes quite a long time to blow when only slightly overloaded, but will blow very quickly when significantly overloaded. For example, typical average values for a 250A Mega Fuse would suggest that it will cope for hours and hours with 100% load (ie 250A), but at 200% load (500A) it will blow in no more than 15 seconds. At 600% load (1500A - typical when there's a short circuit) it will blow in less than a second.

With a 375A load when the thruster's running, the 250A Mega Fuse will last for several minutes - a very long time in thruster terms. Remember too that Vetus say that their 55kgf thruster can be used for no more than 4 minutes per hour. So the 250A Mega Fuse is a good compromise between permitting adequate thruster operation and maximising protection for the thruster cables.

Your earlier post suggested you may have a dedicated thruster battery, and you're right to stress the risk that high currents can be seen in the charging cable during thruster operation, and so the cable should be meaty enough to cope. In my mind, this virtually negates the point of installing a dedicated battery; one might as well just run hefty cables from the main battery. However, another point to stress is that, with a remote thruster battery, the charger cable to the battery must be fused - what fuses do you use?