What size fuses?

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richardabeattie

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I have a simple 12v system driving nav lights, cabin lights etc. No high current items. The domestic side fuse panel contains glass tube fuses which seem to be a bit random - most are rated at 240v and 10amps. Does that sound right?
 
So the fuse is about what you would get on a domestic Iron , Electric Heater Built in Oven .. So all I can say is I hope you have good cabling .. Nav Light is about 0.3 amps .. The idea of a fuse is it blows BEFORE the wiring .. So depends on what you have .. 3 amp car fuse will probably be ok .. /forums/images/graemlins/smile.gif
 
Yes the fuse rating is dictated both by the current the service draws so 1 to 2 amps for nav lights etc etc.The fuse must have a rating above this current.
It is also dictated by the max current rating of the thinest wire.

When a wire carries a very high current as in a fault (short circuit) it gets very hot and can start a fire if in contact with flamable material. (and gives off a lot of smoke) The size of the wire dictates the highest current it can safely carry. Te fuse rating must be less than this current. The fuse is a thin piece of wire in a fireproof container that will fuse (melt) early to open the circuit harmlessly.
The 250V rating is a max and is not significant to your 12v system.

Now most sensible people wire their boat with are relatively large wire size as it is more robust to resist physical damage and corrosion attack.Larger wire is also needed for minimising volt drop on long wire runs.(mast head light) If this is the case the wire should safely carry around 10 amps.

So 10 amp fuses may be OK 5amp might be safer perhaps 7.5 is more correct.

I have an old friend who was getting his mast head light working after having a new mast fitted.(old stay wire let go) The system was no more than a battery switch and light. He turned it on and waled away fro m the boat to try to see if the light was working (daytime) On turning to the boat there was smoke coming out of the cabin. A friend came with an extinguisher. The wiring ran down the side of the cabin and his new set of high tech sails were touching the wire. The sails were ruined.
It turned out the wire running up the mast was pinched under the mast step causing a short circuit hence high current. He didn't have a fuse in the circuit.

So you are right to be concerned about the fuse rating. Unfortunately the fuse hollders tend to suffer from corrosion and bad contact so watch that point. good luck olewill
 
Thanks

I realise that it's the amps that matter (and that at only 12V you need more amps to deliver a given wattage) but I thought 10amps was a bit too high and you agree. Now I will have to try to find lower amp fuses of the same physical size!
 
[ QUOTE ]
I will have to try to find lower amp fuses of the same physical size!

[/ QUOTE ] MAPLIN

Don't know where Sea Mist gets is 0.3 amps for a nav light from. An LED light maybe but a 10watt bulb will draw nearly 1 amp an a 25 watt (tricolour for example) just over 2 amps.

If you have several bulbs on the same circuit (2 side lights a stern light and a "steaming" light for example) all rated at 10watts you are looking at between 3 and 4 amps total . 4 x 25 watt bulbs would be over 8 amps.

Amps = watts divided by volts.

But don't forget some items, pumps for example, draw a much higher current on start up.
 
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