cable sizes with 12v supply

[simon1]

been asked to wire some garden lights but customer unsure if to have 12v or 240v.at present there is a 1.5 3c swa available to use that is supplied from c.c.u. if a 12v system is wanted is there any reason not to use this cable to carry supply from 12v transformer, and also is it ok to connect the 1.5mm to any wire that may be preconnected to the lights if that is of a smaller c.s.a. any comments would be gratefully received........... thanks simon

 

[eskimo39]

I don't think 1.5mm would be enough unless the lights are reasonably close to the CU. I recently installed some 12v lights next to an indoor swimming pool over a distance of about 30 meters and was advised by the lighting supplier to use 6mm armoured due to large volt drop in low voltage installations.

As long as the circuit is on RCD, you can get some really good quality 230v lights which would easily be powered by the 1.5mm swa already in place. Hope this helps

 

[Guest123]

Agreed VD is key here. As is the need for RCD protection.

 

[simon1]

r.c.d will be there as i am upgrading c c u as total length of light run is about 35m i think mains voltage is the way to go. thanks for input fella`s .

 

[Guest123]

Is that length of run from the CU to the last light, or just from the transformer to the last light????

 

[simon1]

from transformer

 

[Guest123]

A circuit with an Ib of 3A, at 12V over 35M would need a 16mm SWA to combat the VD.

 

[simon1]

does that explain why preconnected 12v lighting systems always come on short runs
e.g under cupboard and plinth lights

 

[Guest123]

Yep.

Best to wire lights like this in parallell hence some larger transformers come with a number of load output terminals.

 

[eskimo39]

Could well do, in a basic way of thinking the lower the voltage the less strength it has to push down a cable so you have to make the resistance in the cable lower. This means longer runs need larger CSA.

On a 35m run 230v on a 1.5mm cable at 30 degrees will lose about 4 volts on a 4amp load, so there would be no problems running the load. However a lot of 12v lamps etc will not run if it drops below 11.3v so a drop of 1 volt could cause problems.

 

[andyb]

A circuit with an Ib of 3A, at 12V over 35M would need a 16mm SWA to combat the VD.

That's true Lenny, also you could only have 1 x 30w lamp. If for example you wanted 6 x 50w lamps then you would be drawing 25 amps. Then you've got a problem.