12v wiring

[AmandaMc]

Hi, I have just bought some 2 core cable - 6amp, colours are brown and blue, they didn't have red and black in B&Q, I know I have the right amps as I have already calculated this, is there any issues with using brown for poss and blue for negative?

[Paul C]

Yeah but no but yeah but no but year....it depends how strict/picky/correct/anal you want to be about the wiring on your boat:

 

1. There are no "rules", you could choose any colour you like, if you wanted to

2. It would make sense to use red for +12V wires and black for 0V wires

3. It would make sense to not use red for everything which is +12V

4. It would make sense to never use blue, brown or yellow/green for 12V wiring, since these are the colour codes used in 230V AC wiring

5. It would make sense to follow a convention of some kind - there is at least 1 convention for "car" wiring colours, although it doesn't cover items like fridge, toilet etc obviously!!!!

[ditchcrawler]

Hi, I have just bought some 2 core cable - 6amp, colours are brown and blue, they didn't have red and black in B&Q, I know I have the right amps as I have already calculated this, is there any issues with using brown for poss and blue for negative?

So what are you planning to use it for?

[Tiggs]

It's nie to be able to wire a boat how you want to, rather than have to comply with a whole load of regulations like house wiring.

 

Personally I would have used red and black for 12V wiring though, because it means that someone else in the future would see it as that.

 

Car parts shops supply it.

[Keeping Up]

Are you using 6 amp cable because you know that you won't be putting more than 6 amps through it, so you know it won't overheat? Or are you using this cable because you know it's cross sectional area and you have calculated the voltage drop along its full length at the expected current loading? If it's the former you may find that there's not a lot left of your original 12v by the time it gets through to the other end.

[Paul C]

Are you using 6 amp cable because you know that you won't be putting more than 6 amps through it, so you know it won't overheat? Or are you using this cable because you know it's cross sectional area and you have calculated the voltage drop along its full length at the expected current loading? If it's the former you may find that there's not a lot left of your original 12v by the time it gets through to the other end.

 

There's an article/useful webpage somewhere, about choosing the correct wire gauge. There's 4 factors, not just "can the cable take the amps without catching fire".

 

http://www.smartgauge.co.uk/cable_type.html

[Alan de Enfield]

Just as an example of what 'keeping up' is referring to :

 

My 12v fridge is wired up with "11 amp cable", in theory it draws 45 watts (4 amps) so no problem.

 

The fridge is just over 2 metres away from the main busbars.

 

The fridge will not work because the cable is so small that the volt-drop, means that the fridge is only getting 11.5 volts.

 

So -

question 1 - what makes you think that 6 amp (0.75mm²) cable is big enough for wiring your boat ?

question 2 - do you understand 'volt drop' and how it is affected by cable size ?

[AmandaMc]

Hi all, thanks for the feedback, the amps on my max view tv signal booster is 50mA so very low, the distance is 10m, basically 1 end of my boat to the other, I need to add some cable to an existing piece of redundant 12 v wiring as my max view booster is struggling with the voltage drop when wired in to my lights, so I want to have a dedicated feed, the existing feed is wired into my victron inverter so has the protection of that system, I want to make sure that the two different size wires are compatible and it won't cause any over heating, my existing 12v wires have a higher amp rating than the one I have bought and the wire on my max view is about 3 amp rating, very thin wires, it is supported by a F2AL 250v fuse which is part of the power lead to the max view booster

[Gareth E]

Slightly off topic but slightly on: I recently installed a new battery charger and wiring. I'm not 100% certain that the wiring is ideal. It's rated at 30 amps but the wire is around 2 1/2 metres long, 20a battery charger. Would I be right in thinking that if I set the charger at a certain voltage, then measured the voltage at the batteries while the charger is doing its thing, any differences between these figures would be the voltage drop? And: if there was a difference, could I simply increase the voltage on the charger until the voltage at the batteries is at the optimum level?

[tonyt40]

You need to make sure that all the wiring on that circuit is installed to carry the rating of the fuse or circuit breaker protecting it at the distribution end. The max view has a 2 amp fuse protecting it's own lead. A fuse or circuit breaker always protects the cable or equipment down stream of it so if you have a 10a fuse all the wiring needs to be rated at least 10a. If not, under overload conditions a smaller cable could catch fire before the fuse or circuit breaker blows. The size can be reduced following another fuse or circuit breaker down stream to a smaller size but using the same logic.

If you are extending a circuit always use the same size cable. The other consideration is voltage drop if you are extending.

Edited September 17, 2016 by tonyt40

[Paul C]

Slightly off topic but slightly on: I recently installed a new battery charger and wiring. I'm not 100% certain that the wiring is ideal. It's rated at 30 amps but the wire is around 2 1/2 metres long, 20a battery charger. Would I be right in thinking that if I set the charger at a certain voltage, then measured the voltage at the batteries while the charger is doing its thing, any differences between these figures would be the voltage drop? And: if there was a difference, could I simply increase the voltage on the charger until the voltage at the batteries is at the optimum level?

 

Look at the link I posted previously. For now (for simplicity) assume that the "voltage drop" requirement trumps everything else, so forget about "rated at 30 amps" and look at/judge/purchase wires in accordance with an acceptable voltage drop. You'll notice that its a linear relationship - if the wiring is twice as long, it needs to be twice as thick.

 

No, you can't simply increase the voltage to compensate for the voltage drop because you also need to remember that the current will vary. So the voltage drop along the wire varies too. What kind of charger is it (as in make and model)? It might have a battery sense wire connection which is for exactly this purpose - to sense the true battery voltage, not the battery voltage - voltage drop along the wires it uses to charge the battery.

[AmandaMc]

Hi Tony, thanks for the advice, I'll take the 6 amp cable back to B&Q and get the same size cable and colours as the existing feed, the voltage drop should be ok with the bigger size, and luckily the max view is low amps so I shouldn't get an issue once I've used the correct size wires, ill b able to watch some telly tomorrow night ⚓️?

[ditchcrawler]

I would also look into why you have low voltage on your lighting circuits.

[Murflynn]

After you've done your voltage drop calculations for each circuit, I suggest you buy the cable you need on ebay or Vehicle Wiring Products - you can specify exactly what you want in the colours you want. You'll also need the proper terminations (different for different size cables) and a quality crimping tool.

 

You won't find a good range of cables in B&Q or the local electrical store.

 

Be warned - you'll be surprised how expensive 12v cables can be.

[AmandaMc]

Thanks everyone, food for thought on my voltage drop with my lights, I wired in the max view on the very last led light in the circuit so I am assuming that was the issue, everything I buy for my boat seems to cost a fortune, that boating life ⚓️ ill check out some prices online before i purchase any 12v cable

[NMEA]

6a cable is only 0.75mm2, pretty useless for anything other than LED lamps and other very small loads. Blue and brown are inappropriate for extra low voltage DC and are in fact not permitted under ISO and NMEA standards if low voltage AC is also present on the vessel.

[Arthur Brown]

Learn about cable sizing by current capacity, and by volts dropped under load. Learn the useful skill of labelling things so that it's obvious what it does later when you are trying to fault find in the dark.

 

Eventually you will need a full wiring diagram especially when you have to post a "why doesn't my ... work?" post.

[AmandaMc]

Hi NMEA

I am taking the cable back to B&Q tomorrow and getting proper 12v cables, the same size I have coming from the inverter, this should sort out any voltage drop and stop any risk of over heating, it will also stop any confusion if anyone else does any electrics on the boat

[WotEver]

 

Look at the link I posted previously. For now (for simplicity) assume that the "voltage drop" requirement trumps everything else...

I can't think of any example on a 12V (or 24V) system where that wouldn't be the case.

 

Tony

[AmandaMc]

Hi Arthur, these are all good ideas, luckily I know my boat inside out as I have had it from a shell and done most of the work, obviously not gas or inverter but most other things, if I ever sold it then I would do a wiring diagram for the new owner but for now it's not needed

[Paul C]

I can't think of any example on a 12V (or 24V) system where that wouldn't be the case.

 

Tony

 

The only ones I can think of are the obvious ones - like a voltmeter or a battery sense wire or shunt connection wires or something like that.

[WotEver]

 

The only ones I can think of are the obvious ones - like a voltmeter or a battery sense wire or shunt connection wires or something like that.

Yes - signal cables that draw zero current (for practical purposes), of course you're right

[by'eck]

Hi, I have just bought some 2 core cable - 6amp, colours are brown and blue, they didn't have red and black in B&Q, I know I have the right amps as I have already calculated this, is there any issues with using brown for poss and blue for negative?

 

Electricity 's colour blind, but the does prefer to travel down some metals in preference to others. Not exclusively metal though, it will reluctantly travel through damp wood and can even be persuaded to jump through air if the voltage is high enough. It tends to make quite a fuss when doing this though

[The Bagdad Boatman (waits)]

12v needs multistrand cable not single core. to mesure voltage drop mesure at the supply end then mesure at the far end.

[Murflynn]

 

Electricity 's colour blind, but the does prefer to travel down some metals in preference to others. Not exclusively metal though, it will reluctantly travel through damp wood and can even be persuaded to jump through air if the voltage is high enough. It tends to make quite a fuss when doing this though

but it doesn't like fresh water much.

 

switch on the headlights of a car and drive it into the swimming pool.

 

surprisingly the 12V system will not short out - the lights will stay on for ages.