low voltage cable selection

[vertigodj]

I'm planning on installing 6 low voltage down-lighters,each lamp is 20w.Supply of lamps will be coming from two 20-60 transformers-each transformer controling 3 lights.Cable i plan on using from transformer to each light (which in length is approx 2 metres max to furthest point) via a junction box is going to be 0.75mm 2 core.Is 0.75 ok

 

[RF Lighting]

Each set of three lights (60W) will use 5A at 12V. According to BS7671 0.75mm2 is capable of carrying 6A providing there are no derating factors to consider such as thermal insulation etc. I personally would advise you to fit 1.5mm2 cable as the 0.75mm2 you propose is very close to the design current, and should you decide in the future that you would like the room brighter it is much, much easier to uprate you transformer and lamps than to have to rewire the whole lot (also the difference in cost of the two cables is negliable). Also even on such a short run you may have problems with volt drop causing the last light an the circuit to be dimmer than the first one. (I've seen it done).

HTH

Rob

 

[vertigodj]

ok i follow that thx , if each light had its own dedicated cable directly going back to the transformer (transformer has provision for 3 cables on 12v side) would the same principle apply?

 

[RF Lighting]

With regards to the loading this should be ok as the highest wattage lamp avaliable for low voltage downlighters is 50W, this will use a current of just over 4A should you ever want to upgrade so the loading in the cable would not be a problem but I think the volt drop may still be a problem so i would install 1mm2 to be sure. If someone could clear this up feel free.

 

[ban-all-sheds]

0.75mm² drops 63mV/A/m.

So a 60W load over 2m will see a 0.62V drop, i.e. over 5%, i.e. too much.

With daisy-chained loads the calculations get harder, as not all of the cable will be carrying the full current.

I would:

1) Use 1mm² cable, or even 1.5mm² if it will fit.

2) Use 3 cables, i.e. 1 per lamp, and I'd make them the same length, even if they don't need to be.

 

[vertigodj]

ok thx for the info,much appreciated thx