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What are the problems involved in extending an existing 12v lighting circuit?

The existing centre lights in my 57ft narrow boat are inadequate for reading/food preparation and I would like to fit perhaps up to six spotlights in the galley and main cabin area on the cabin walls.
I was thinking of breaking into the existing wiring system through the centre lights and running wires across the under side of the roof and down the cabin sides using brown plastic mini-trunking (as an alternative to trying to get under the roof and cabin wall linings).
Do you have any advice as to whether this is practical (and safe) and how best the spur system should be wired into the existing circuit?
It occurs to me that many people probably want to add additional lights and this might be a good topic for an article in WW.

Asked by: John Burt  | 1.26pm, Sunday 21 February

WW says:

Thank you for the suggestion for an article- consider that being worked on!
It could be practical to add the lights, but it depends on several factors:
Firstly, what will be the current draw of the lights that you are going to fit? This affects the next point, which is;
What is the thickness of the cable that supplies the existing lights?
In the Narrowboat Builders book and Inland Boat Owners book, there are tables of cable thicknesses against length. If you have too thin a wire, compared to the current you expect it to carry, the wire will become hot, but, more importantly, the voltage will "drop" which can reduce light output, or stop some bulbs from working.
A way round this, might be to use LED bulbs (but ensure they are voltage regulated, so can cope with 10V to 15V), which give lots of light for low current draw.
Once you are satisfied that the cabling is OK, then it is simply a matter of cutting the wire and adding a junction into the positive and negative.

Mark Langley  | 1.38PM, Sunday 21 February

I should add that the trunking is perfectly acceptable, however, just consder the aesthics!

Mark Langley  | 1.39PM, Sunday 21 February

Readers say:

Many thanks and I am going to buy the book.
The mini trunking is not dissimilar in appearance to the beading which already runs across the ceiling and sides of our boat.
I don't think it is practical to try to run the wire behind the linings because (presumably) there will be insulation underneath.
What is the alternative - to run the cable under the gunnel?

John Burt  | 3.37PM, Sunday 21 February

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