How to find out which wire is positive

[hunter323]

My yachts wiring is a mess!!!! Not me but the previous owners mind you I haven't much idea. I have a couple of cables which used to serve an electric water pump. I would like to utilise that connection but need to find out which wire is positive and which is negative. How can I do that please

 

[macd]

Got a multimeter? Set to volts range. Put red probe to one wire, black to the other. If it reads +12V (ish) the red is on the positive. If it reads -12V, black is on the positive.

 

[nigelmercier]

And if you don't have a multimeter, buy one. No boat should be without one*.

This one looks good value: http://www.amazon.co.uk/exec/obidos/ASIN/B003NEGZLS/dolcetto-21

* OK, perhaps one isn't enough. I've got five on my boat

 

[macd]

Would that be a pentamultimeter or a multipentameter?
Resistance is useless.

 

[Rigger Mortice]

Get a bulb and a longish piece of wire. Connect wire to a known earth connection. Hold other end of wire onto bulb.
Attach one of the unknown wires to bulb. If it lights up then you've found the +ve. If it doesn't light try other wire.
If neither lights the bulb then check that everything is switched on and try again.

It's easier to get a multimeter though!

 

[MASH]

If you don't know how to identify +ve from -ve is it a good idea to be playing with electrics at all?

 

[macd]

If you don't know how to identify +ve from -ve is it a good idea to be playing with electrics at all?

What a very negative attitude. Or maybe positive. Potentially.

 

[MASH]

I'm not suggesting he doesn't have the capacitance to learn, just a reluctance to see his ohm ionised through a shortage of skill.

 

[Vara]

I prefer an analogue multimeter, always works even if the battery runs out, I always leave the digital jobbie on and it's NFU when I need it.

Shouldnt cost more than a fiver.

Oh and if the needle goes the wrong way, black is positive.

 

[Lakesailor]

I like the yellow ones. Mine is yellow and I like it. It has the needle, so I don't have to.

 

[Slowtack]

You will know by whether it is pumping the water out of the boat or into the boat. Sorry, must be the ISCE syndrome (incipient solstice cranial effect).

 

[Andrew G]

To the OP. I think you got some good answers to a legitimate question, in amongst the lads' humour. They didn't mean to PIC on you. A

 

[rib]

lick the wires,your find em

 

[VicS]

My yachts wiring is a mess!!!! Not me but the previous owners mind you I haven't much idea. I have a couple of cables which used to serve an electric water pump. I would like to utilise that connection but need to find out which wire is positive and which is negative. How can I do that please

without a multimeter:

1./ soak a strip of red litmus paper in salt solution. Hold the two wires on it. The litmus will turn blue around the negative wire and slowly be bleached around the positive


2./ using copper wire ( not tinned) stick the two wires into a cut potato. You will get a green coloration around the positive wire.

 

[Paddingtonbear]

All very funny and helpful but in all seriousness how do you learn about electrickery? I would dearly love to understand it but book learning tends not to be that helpful. My friend the Moomin just has to look at something and it is repaired or in a serious case pass his hand over the problem. How do you learn that trick?

 

[VicS]

All very funny and helpful but in all seriousness how do you learn about electrickery? I would dearly love to understand it but book learning tends not to be that helpful. My friend the Moomin just has to look at something and it is repaired or in a serious case pass his hand over the problem. How do you learn that trick?

There is some good instructional stuff on basic electricity in the "Boat electrical notes" at http://www.tb-training.co.uk/

 

[MASH]

Vic, your 2./ above is a brilliant tip! I'll remember that one (my boat doesn't usually run to litmus paper)

 

[Stemar]

All very funny and helpful but in all seriousness how do you learn about electrickery? I would dearly love to understand it but book learning tends not to be that helpful. My friend the Moomin just has to look at something and it is repaired or in a serious case pass his hand over the problem. How do you learn that trick?

A trick is what it is. To quote Aurther C Clarke (I think), "Any sufficiently advanced technology is indistinguishable from magic." It's the same with skills. Once you've got a basic understanding of how electricity works, and VicS has given a good answer on where to start getting that, applying that knowledge to simple problems develops the skill. More complicated problems will still have you scratching your head, but what's complicated today, like which is the +ve wire, will be easy tomorrow. Then, one day, you'll give someone a hand with a little problem and they'll be wondering, how did he do that?

You'll make mistakes, but you're very unlikely to burn the boat down around your ears. The people who do that are the ones who think they know it, but don't. A little humility is the DIY equivalent of a condom on a one night stand - it protects against expensive and/or unhealthy consequences.

 

[KellysEye]

The positive wire is brown and the negative blue, green and yellow is earth. If it is old wiring red will be positive and black negative.

 

[pmagowan]

My boat wiring was also a mess and a mix of B&Q stuff and random cables. My engine once failed due to invisible corrosion of the heavy red wire to the starter motor from the battery. I got Nigel Calders book on boat mechanicals and electrics and with help from those on here, VicS et al, I completely rewired it. I would not say that I am knowlegeable yet but I certainly know all about my own systems and would have a good idea how to go about it if there was a problem. I always have a multimeter on board but have just about relegated the electrics toolbox to shore due to lack of space and the fact that the electrics are so good (if I do say so myself, famous last words) that the normal tool box should suffice for repairs.

In the end I found it much better completely rewiring as I then knew what was on board and could be sure of the quality. All my wires are tinned with fire quenching oil and chemical resistant sleaves. They are all joined with glued crimps that are then covered by shrinkwrap glued sleaves for a waterproof seal. All the wiring is above the size required for my LED lights so that if someone replaces with an incandescent bulb I will have no problems. I was also able to make a nice neat electrical panel with all the appropriate circuit breakers and a battery monitor.

It cost a bit and took a bit of time but then it is good fun fiddling with boats. I used merlin for most of my supplies. Good luck