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Old 12 February 2013, 14:47   #1
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Wiring Question

When I was setting up my new RIB I wired in a 50 AMP fuse between the master switch and all the electronics.

The switch panel also has fuses for each devise according to the manufacturers recommendation.

All I'm running is a 40hp OB, plotter/fishfinder, VHF, Stereo, lights (LED deck lights x 3 + anchor light), Nav Lights and Bilge Pump.

Question I have is do I really need the 50 AMP fuse or is it overkill?
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Old 12 February 2013, 14:51   #2
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depends what size cable is
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Old 12 February 2013, 14:51   #3
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Your house working back from the appliance has something like 3-13A(socket), 30-65A(ring), 100A master in the fuse box, 200A at the meter.

I wouldn't want to not have any of them...
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Old 12 February 2013, 16:36   #4
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Originally Posted by falcon0310 View Post
depends what size cable is
Cables from the battery to main switch and to the 50amp fuse block are 8 B&S tinned (85Amp). Same sized cables run aft to the motor. Cables from fuse block to the switch & fuse panel are 6mm (31 Amp). Cables from the switches to the electrical items where cable was not supplied with the items are 4mm (15 Amp).

All the instruments have a fuse in their respective +ve leads. Also where they run off a common switch there is a integrated fuse where I have simply added the total AMPS of the items connected to work out a suitable fuse size.
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Old 12 February 2013, 16:50   #5
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Quote:
Originally Posted by WOPALX View Post

Question I have is do I really need the 50 AMP fuse or is it overkill?
On the basis your starter motor will need a good portion of this to fire your engine, and the alternator may push out a similar amount at peak load, then I'd say yes.
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Old 13 February 2013, 12:45   #6
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It is always well to remember that a fuse protects the device AND the wire.

So, to answer your question you need to give more info, some of which you've already given in other answers.

First, the starter motor will be the biggest draw on the battery. It is common to have this wired direct onto the battery in cable sufficient to carry the starter current. This will be high, higher than 50A I would guess. So I am assuming your engine is wired direct onto the battery. This will also take care of the alternator charging current, which will be a lot less than the starter current drawn. If it isn't just treat it as another big device.

Now the devices.

If you assume they are all on together just add up the individual fuses, that's the size you need on the master fuse (plus, say, 10%). You then need to ensure that the cable from that fuse back to the battery is sufficient to carry that current. You have already said this cable is rated at 85A, should be plenty (if starter isn't included).

The cable from the main fuse to the fuse board with the individual fuses needs to be the same size as the one to the battery because it may carry the same current.

The cable from each individual fuse to each device needs to be sufficient to carry the current of that fuse.

You should ensure that the main fuse is as close as possible to the battery. This is because the length of cable from the battery to the fuse is unprotected.

Don't forget the return (sometimes wrongly called the earth). All the cable size issues apply to the return cable too. It is often convenient to have a return block wired back to the battery in heavy cable.

Where possible I would always use tinned cable. All joints should be shrink wrapped. Where possible solder the joints. Keep the cable runs out of the bilges. Label everything. Buy plenty of cable ties. Its easy to get circuit breakers for 12v, far better than wired fuses. Spray occasionally with waterproofer, WD40 if nothing else.

If in doubt always use a fuse SMALLER than the rating for the cable.

I try to fit a master switch direct off the battery. This way I know that when this is turned off the battery is totally isolated. Good for piece of mind, but how many times have I been at home and then wondered if I did turn it off?... loads!

I am about to fit a solar trickle charger to my boat. I have done this in the past and it makes an amazing difference. I fitted a very cheap, low wattage (5 watts from memory) to my boat that sat on a mooring. I was able to fit the solar panel facing south. after fitting it I never had a problem with the domestic battery again (before fitting the solar charger it often went flat because we sat at anchor with fish finder and GPS running (hence no engine charge).

Of course the charger must be wired direct to the battery, not via the master switch.

Sorry, bit of brain dump, hope it helps.

Ian W
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Old 13 February 2013, 16:37   #7
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Thanks for the information below.

Seems the only thing I have not done is the starter for the motor does go via the 50 AMP fuse. However the starter draw & alternator input are both less than 50 AMP so works fine.

Basically the battery +ve goes via the main switch to the 50 AMP fuse block. Everything else is then taken from the other side of the fuse block.

I might look up the information on max AMP's for the alternator and starter to see if I can reduce the fuse from 50 AMP to 30 AMP as the cable going to the switch panel is only rated at 31 AMP.

Thanks again for your input, appreciated.

Quote:
Originally Posted by j.i.wilson View Post
It is always well to remember that a fuse protects the device AND the wire.

So, to answer your question you need to give more info, some of which you've already given in other answers.

First, the starter motor will be the biggest draw on the battery. It is common to have this wired direct onto the battery in cable sufficient to carry the starter current. This will be high, higher than 50A I would guess. So I am assuming your engine is wired direct onto the battery. This will also take care of the alternator charging current, which will be a lot less than the starter current drawn. If it isn't just treat it as another big device.

Now the devices.

If you assume they are all on together just add up the individual fuses, that's the size you need on the master fuse (plus, say, 10%). You then need to ensure that the cable from that fuse back to the battery is sufficient to carry that current. You have already said this cable is rated at 85A, should be plenty (if starter isn't included).

The cable from the main fuse to the fuse board with the individual fuses needs to be the same size as the one to the battery because it may carry the same current.

The cable from each individual fuse to each device needs to be sufficient to carry the current of that fuse.

You should ensure that the main fuse is as close as possible to the battery. This is because the length of cable from the battery to the fuse is unprotected.

Don't forget the return (sometimes wrongly called the earth). All the cable size issues apply to the return cable too. It is often convenient to have a return block wired back to the battery in heavy cable.

Where possible I would always use tinned cable. All joints should be shrink wrapped. Where possible solder the joints. Keep the cable runs out of the bilges. Label everything. Buy plenty of cable ties. Its easy to get circuit breakers for 12v, far better than wired fuses. Spray occasionally with waterproofer, WD40 if nothing else.

If in doubt always use a fuse SMALLER than the rating for the cable.

I try to fit a master switch direct off the battery. This way I know that when this is turned off the battery is totally isolated. Good for piece of mind, but how many times have I been at home and then wondered if I did turn it off?... loads!

I am about to fit a solar trickle charger to my boat. I have done this in the past and it makes an amazing difference. I fitted a very cheap, low wattage (5 watts from memory) to my boat that sat on a mooring. I was able to fit the solar panel facing south. after fitting it I never had a problem with the domestic battery again (before fitting the solar charger it often went flat because we sat at anchor with fish finder and GPS running (hence no engine charge).

Of course the charger must be wired direct to the battery, not via the master switch.

Sorry, bit of brain dump, hope it helps.

Ian W
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