fuses

[jonnyboy1958]

I am installing a 12v circuit in my van and wondered what size fuses to use on 1. fridge 2.led lighting (four downlighters) 3. water pump for tap. are these usually standard sizes or should I look to the manufacturer for advice?

[Beaker]

find out what your appliances are rated at, and use the formula I = P/V

So for example my water pump is rated at 25 Watts, so I (current) = 25 / 12 = just over 2 Amps continuous rating. You need to allow for voltage drop over the cable length, and current surge when you turn on and off (nuisance blows), so double that 2 Amps to 4 and a bit Amps. 5 Amp fuses are dead common, so it's fine to use one of these.

LED lights, you can use 1Amp fuses, probably 15 Amp for a fridge - but do check the rating on your fridge and do the maths to be on the safe side.

[jonnyboy1958]

Thank you for your reply it is most useful.

[hectors-tranny]

One other thing. Remember what fuses are for - to be the first thing to blow if there's a short. Hence your cable rating in Amps should be at least twice the fuse rating protecting the circuit.

I use 2A fuses for just about everything except fridge which is 10A and water pump 6A.

HT.

[jonnyboy1958]

Thanks Ht, I will bear that in mind.

[Beaker]

Yeah, HT is correct. You probably want to use 2.0mm wire for most of your wiring, which is normally rated around 21 Amps. That's total overkill for lights, fans, waterpumps etc. but it has plenty of overcurrent leeway in the event of a fault. It's also MUCH easier to work with than thinner 1.0mm or 1.5mm wire. Stripping that stuff can be tricky as you can all too easily cut right through it instead.

For a fridge you probably want something like 4mm or 6mm wire which is good for around 40 and 50 Amps respectively. Again, look at the Wattage/Ampage of your fridge and do the maths to check.

[jonnyboy1958]

Thanks Beaker

[hectors-tranny]

Yes Beaker's spot on. Better overkill for cables than anything else, then you can be sure your fuses will blow first even if you put in higher rated ones than strictly necessary. I basically used the same sizes as Beaker.

Higher rated cable will also reduce voltage drop over the length of the cable as less resistance. There's a useful tool here, which you can use to calculate voltage drop for any rating of cable and run length.

http://www.antares.co.uk/design-tools.html

[travelling man]

With regard to your fridge,try to keep wire runs as short as possible to avoid voltage drop,the longer the run,the heavier the wire needs to be.
Also,make sure your fridge is switched with a suitably rated switch.

[jonnyboy1958]

Thank you for your reply, I am away in it tomorrow but will use cooler box and tent ehu. will hunt around for some decent switches on return.