Main Fuse size ?.

[graham]

Can anyone suggest a sensible size fuse for the main DC supply from my 85ah battery to the switch/fuse panel?

The boat has Usual Nav lights GPS,VHF, Sounder and log, 2 cabin lights and nothing else.

We try not to have too much running due to the limited charging we get from the outboard.

 

[cindersailor]

This depends entirely on the capacity of the cable you are protecting. In other words what is the rated current for the cable from your battery to the switch panel? From there cables to individual items are presumably protected by their own fuses/circuit breakers. If, as is common practice, your switch panel supply comes from your engine starter solonoid +ve terminal then the fuse should be inserted there. If you google something like "cable current capacity" you should be able to find a site with the necessary data. Generally the fuse should be rated a few amps less than the recommended current capacity of the cable it is protecting.

 

[graham]

Theres no engine starter,its a 4hp outboard with pull start.It has a 6amp charging coil that tops up the battery. Theres presently no fuse on the charging circuit so will need to fit one there also.

Dont know the capacity of the cables from the battery as they are existing cables I have shortened as I have moved the battery closer to the switch panel.

 

[cindersailor]

Sorry, should have noted in your original post that you have an outboard. You will need to somehow measure the cable to determine its spec. or replace it with one you know. Conversions from cable diameter to cross-sectional area (the usual cable spec) and to AWG can be found at http://www.sengpielaudio.com/calculator-cross-section.htm Remember that the measurements are of the copper conductor not the full cable including the insulator.

BlueSea have a useful calculator to determine the cable size needed for a given system at http://beta.circuitwizard.bluesea.com/#

 

[lw395]

Agree with post about cable size, but as a rough guide, if the cabling is reasonable sized, say >4mmsq or 2mm copper dia (roughly), a 25A fuse will blow ages before your wiring melts if a short arises. You can then put a smaller fuse in each circuit, so that if you short a light or something, only that circuit goes off. Make sure the fuse is not too near the battery, as the spark from a blowing fuse doesn't mix with a gassing battery!

 

[VicS]

Cindersailor is quite correct in saying that the fuse should be slightly less than the rating of the battery cables.

BUT

You should work out the maximum load in amps that you are likely to draw.

You could have port, stbd, stern and steaming lights all on at 10watts each call that nearly 4 amps

The VHF draws several amps when transmitting, say 6 amps

The cabin lights. I dont know what type you have but they could be anything from 5W to 25W each. Say 2 at 10w and call that another 2 amps

Ignore the sounder as it is quite a small load and I have rounded everything else up. similarly the GPS unless you have some fancy plotter.

That makes 12 amps. If you have more powerful nav lights or cabin lights you will have to allow for those.

Otherwise

That means a 15 amp fuse and 20 amp cable. There will be no harm done if you increase the sizes of both. In fact it makes sense to allow for any other equipment you may add, at least in cable size if not for now in fuse size, for example an autopilot, one of those 5 million candle power torches to use as a steamer scarer or a fancy HiFi system. (Perhaps you wont be using the VHF at the same time as the HiFi though)

BTW the VHF is best fed directly, via a fused connection, from the battery. It helps to reduce interference fom the engine and minimises volts drop when transmitting.

 

[Boathook]

I agree that the main fuse should be no bigger than the cable capacity, and possibly nearer the likely total load if that is smaller than the cable capacity.
As clear as mud ............

 

[graham]

Thanks for the replies everyone.

The old electrical system had a 16 AMP fuse I will probably stick with that as it has been OK since 1980 and very little has been added.

The VHF is on a seperate circuit with an 8 amp fuze plus a smaller inline fuse close to the radio.

Cheers People