Fuse ratings

pjbharrison 3:02pm, 3 June 2009
Hi all
I'm working on installing electrics in my A24. I'm using 5, 12v 17Amp/hr batteries from computer UPS's.
I will be running lights and a bilge pump for the moment. I will also be installing a 3 port 12v cigarette lighter type socket unit with a battery condition indicator. This will be used for topping up handheld radio and/or GPS unit. In time I will be fitting a fixed radio.
What rate fuse should I fit on the battery and what rate fuse should be fitted on the switch panel for each of the units? The lights are a combined unit with two 10W bulbs. The anchor light is also a 10w bulb.
Just read that the pump needs a 5Amp fuse.
Skykomish E29 9 years ago
PJ, you can work out your fuses using the formula of watts/ volts = amps

so 25 watt bulb run on 12 volt system = 25/12 = 2.08 amps.
(from "This Old Boat" by Don Casey)
pjbharrison 9 years ago
Thanks Malcolm
Should have remembered that from school!
blueachilles 9 years ago
Sorry if i am hijacking this topic, but I have just bought a 12v charger for a new mobile phone, and intend to get one for the boat as well because the switch panel (with built in fuses, bought fairly cheaply from Force 4, PJB) on Blue includes a cigarette lighter socket.

We have two leisure batteries, which normally just about last us all season; we only run VHF, log/depth, and lights. We now have a charger and shore power lead so we can top up the battery if we spend a night in a marina.

My question is - if we (a) charge a phone just off the battery, or (b) make a call with the phone plugged in, just on the battery, is it a huge drain on the battery?
pjbharrison 9 years ago
That was my next question!
What rate fuse should be on the battery between it and the switch panel?
santalars 9 years ago
Malcom is right with his formula.
However, you should consider that various instruments consume higher power short term when switching them on. Particularly motors.
Therefore you should have an allowance (approx 50 - 100%) for this.

For the mobile phone charger I would have to be very vague, but it will not be more than 300mA. These low draws can easily be measured with a simple multimeter in line. It will give you a more accurate figure.

The fuse rate between the battery and the switch board will basically be all possible consumers at one time added up. However, as you don't want to have a fuse (particulrly the main fuse) blown on many occasions it should be noted that the fuses are there to prevent damage (fire) to/on connecting cables and therefore a usefull rating would be related to the wire size, the max amps and length. Hence the fuse rating should be below the wire rating to provide reliability and safety.

The wire rating can be figured out with the calculators below:
pjbharrison 9 years ago
Thanks Lars
rothwell_neil 9 years ago
Most 12V cigarette circuits are fused at 10Amps. This is the maximum recommended for a cigarette circuit. However my inverter runs at up to 150W off the cigarette socket which is up to 12Amps and then has to be run through direct connection to the battery terminals for over 150W. My PC takes 60W and therefore 5 amps, the phone charger for nokia 7W or 0.6Amps.
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