12v lighting wire size

[Barney78]

I bought some 12v 20W mini recessed spots which came in packs of 5 with one transformer and an already made up cable which is 1mm2 twin. I cant use the cable that was included because it is too short.

If I make up my own cables what diameter cable do I need to use because the total run to the furthest light from the transformer will be about 6 metres. The cable supplied was 3.5 metres

Or would I be better off doing most of the wiring from a junction box off the lighting circuit and having more transformers?

Cheers.

 

[breezer]

1mm twin and earth will be fine (obviously you wont use the earth)

 

[Barney78]

How come when i put the figures in to this cable size calculator it comes up with 6mm2?

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

 

[breezer]

How come when i put the figures in to this cable size calculator it comes up with 6mm2?

http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html[/QUOTE]

because you put in the wrong figures

think about it, if it already has 1mm cable, and you are in effect only doubling the length why would it go from 1mm to 6mm?

a 20watt lamp @ 12v only draws just over 1 1/2 amps now if the cable length was 6km that i could understand (mind you that would be too small)

 

[Barney78]

Well thats put my mind at rest seeing as i've already done half the wiring! I dont understand where i've gone wrong with the calculation though. The load is 100W if there are 5 20W lights attached right?

Cheers very much for the help though

 

[breezer]

Well thats put my mind at rest seeing as i've already done half the wiring! I dont understand where i've gone wrong with the calculation though. The load is 100W if there are 5 20W lights attached right?

Cheers very much for the help though

that is correct.

i can not see how you got it wrong, i just tried it again, and can not get it to say 6mm cable

 

[Barney78]

I put in:

Twin and Earth clipped to wall

Load 100 Watts

Voltage 12

Length of Cabing 6 Metres

It says 6mm

Please tell me i'm not going mad!

 

[Steve]

if the voltage is 12, not 240, the current is much much higher (20 times higher in fact), therefore you would need 6mm cable on the low voltage side of the tranny, but you are on the high voltage side. put in 240 volts, not 12.

EDIT: i have read entire post - sorry for misunderstanding but you can see my reasoning.

120 watts = 10 amps on the low volt side. maybe 1.5mm cable is called for here, but not 6mm

 

[Barney78]

no, thats the whole point, all of this cabling, including the3.5m wire that came in the kit goes on the 12v output side of the transformer.

EDIT: I've just read your edit.

 

[breezer]

i forgot to put in cable length

but i also assumed (maybe wrongly) that each cable / light was to be extended individually, if the whole lot was to be done on on the secondary side then yes it would need 6mm

 

[Barney78]

So surely there is something seriously wrong with them selling me a kit which has 1mm cable in it when if you put the figures in to the calculator it comes up with 4mm?

 

[breezer]

no.

the tlc "calculator" does state

Caution - for Guidance ONLY

NO guarantee as to the accuracy of these values is given.
And they should be checked against some other source.

 

[Barney78]

So will I be ok with 1mm cable then?

 

[Spark123]

Do the cables from the lights parallel up and go into one cable and then back to the transformer or do they go to the transformer individually?

 

[breezer]

if you extend it to each light then yes, as i said each lamp only draws just over 1 1/2 amps

the problem if you like is the length of cable run / v voltage drop

the longer the cable the bigger it has to be to allow for voltage drop, but if you run a single 1mm cable to each light you will be fine