Power wire

[oldguy49]

I posted somewhere else that I did not know enough about electricity to even be dangerous so I am asking for your help. I am planning on adding a couple of 12 volt and USB outlets to my trailer. The major problem I have is I do not know how to use a multi meter to help me determine which one of the leads to the light I would like to tap into is the power lead.

If anyone can help me out with this it would be greatly appreciated. I would think that the one thing to keep in mind is that there is no grounding point any where close to the light I want to tap into. I am not sure if this presents a problem. I have never had the need for a multi meter since everything I have ever added to a 12 volt system was just pieced together and as a kid not something I really worried about.

Well, enough rambling. If anyone can offer some guidance it would be greatly appreciated.

 

[RandyB]

You can buy a 12 volt test light. A multimeter is not necessary. They look similar to this:

You connect the clamp that is at the end of the wire to a good, known ground (-12volt DC). The pointy end of the test light is a probe. You poke it onto a connection and if the light in the handle of the test light actually lights up, you've found +12volts DC.

If you are wishing to tap into a light switch, the most simple is a switch with 2 wires. One is the +12volt DC and the other is the switched wire going to the device. If the switch is "on", both wires will illuminate the test light. In this case, turn the switch off and only one should be lit up. That's your one.

Most any auto parts store, Walmart, Meijer and maybe some big box home stores should have a light like this.

 

[TJ&LadyDi]

You will need some kind of a ground to complete the 12V circuit. As for using the multimeter to determine the "hot" wire, set it on DC voltage and find a ground point somewhere reachable. With the probe on the negative (black) lead, make contact with the ground. Us the positive (red) probe, touch uninsulated wire termination points until you get a reading of something over 12V. That would be a "hot" or "positive" wire.

An easier way is to use a contactless voltage detector like this one, or similar...

https://www.amazon.com/dp/B004FXJOQO/?tag=rvf01-20

It will simply show the presence or absence of power.

TJ

 

[RandyB]

Jim, I think the Klein voltage tester you've referenced is AC voltage only?

 

[TJ&LadyDi]

Jim, I think the Klein voltage tester you've referenced is AC voltage only?

Oops! My bad! Sorry.

TJ

 

[Kevin D Pem]

Things have changed with time but I have found house wiring to be much like s&b house wiring, in that a ground is run as well as power. I would guess that is because the frame is not a convenient ground. I suggest pulling down a fixture and see what colors are used for ground and +12v. In my rig white is + and red is - at least in the lighting circuit. using an LED light with wires hooked to it is a good test instrument as it will only light if the power lead and ground lead are connected the right way. I use a sewing pin to pierce the wire on the led and the wire I want to test.

On another note I don't like cigarette style 12v plugs, I find them unreliable. do spend the money for really good ones.

 

[cpeterso87]

My experience prior to RVs was that +12V tended to be red, and Ground tended to be black. In my RV now, colors are all over the map and tend to be unique to whatever the application is - this makes sense in that it could make it easier back at the power distribution block, but man it can be a headache for us DIYers.

And don't get me started on the sound system wiring in my unit - geez, it's a good thing I'm not color blind.

 

[oldguy49]

I took a look at one of the lights that I want to tao into and found that it has a black wire and a white wire. I think I can determine which is which by mmaking sure there is power running to the lights(have battery installed) and then checking with the volt meter. If i get it right it will read 12.xx and if it is backwards it will read -12.xx.

I will try it later this week to see what I get and report back. Thanks for all the ideas, I just want to figure this out and get the plugs installed.

 

[oldguy49]

Just wanted to let every one know that I was able to determine which wire was which in the location that I wanted to use. I used my multi meter as stated in my previous post and it worked as hoped.

The only problem I ran into was that every place/wire that I checked, the wires were different colors. Just a word to the wise, Do not expect all of the wires to follow a normal pattern of color coding and check before tapping into them as the mfg. seemed to hace used which ever color of wire was closest to him at the time.