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Three Brown wires

I recently removed a 12v fluorescent light from the wall in my boat and didn't note the connections. When I came to look at the wires, there were three brown ones. How can I deduce which is positive and negative so I can put in a new light. Yours with hope, RF

Asked by: Robert Fresson  | 12.26pm, Tuesday 3 March


WW says:

The best way to identify your wires is with a multi-meter and care.
If you identify each wire with tape and number them 1, 2 & 3; then, after ensuring that they will not touch each other or any metalwork, replace the fuse to energise the circuit.
Measure the voltage between each pair of wires (1 and 2, 1 and 3, 2 and 3) noting down the results.
The 3 wires are probably supply (+12V) and return (0V or earth) for the light, plus another cable going to another light. It was probably connected to one of the other wires, the difficulty is to know whether it is a supply or return cable.
One voltage reading should stand out as being the obvious supply pair, the positive being the one connected to the red multi-meter lead with the reading being about +12V, and the negative connected to the black lead.
One wire will possibly show nothing, which probably means it is the supply wire to another light, and should be connected with the +12V wire. The other alternative is that it is an earth cable so a bit more investigation might be required.
When all the cables have been identified, mark them (masking tape and pen works well)!
If you are unsure which wire is which; reply with the voltage readings and I will identify them for you.

Rupert Smedley  | 6.33PM, Tuesday 3 March

Glad to be of help, it is always nice to have feedback!

Rupert Smedley  | 11.45AM, Monday 9 March


Readers say:

Thanks very much for this informative walk-through! I have now fitted the light and it works like a light should! I also now own a multimeter and have gone around testing out all the other 12v appliances! Thanks!

Robert Fresson  | 10.55AM, Saturday 7 March

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