12v lighting wire size

[Barney78]

They run in parallel

 

[breezer]

sorry i do not follow, you have made a statment of fact, but no question. so what is your question?

 

[Barney78]

Sorry I was replying to the other fella who asked whether they ran in parallel off of one cable or whether there was a cable running to each light.

The setup with the cable in the kit is one cable with light connectors every 70 centimetres.

 

[breezer]

ok

 

[Barney78]

OK I think i've worked this out. The last metre of the 5 only has 20W of load on it, the 3-4 metre length has 40W and so on. This means that only the first metre of the cable from the transformer to the first light has 100W of load on, meaning that 1mm or 1.5mm would be ok.

Does this make sense?

 

[ban-all-sheds]

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

1.5mm² T/E has a V.D. of 29mV/A/m, so if your target is 4% volt drop, 1.5mm² will carry 10A for all of 1.66m

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?

Yes - it's not a very good product, but it is not dangerous, which is how they can get away with selling it - the cable can easily carry the current required in complete safety and with a good margin. But why did you think that for some reason nobody would ever sell shoddy electrical items? Why should they be the only products immune from poor design and inadequate performance?

With the results from the cable calculator you are confusing the cable size required to safely carry a given current with the size needed to carry it a given distance without excessive volt drop.

As described, with 1mm² cable, and a 20W load every 0.7m, the voltage present at each of the lamps will be:

11.80
11.59
11.43
11.31
11.23

This means that the last lamp is receiving just under 94% of the nominal 12V. It's probably unlikely that you would notice that, but a 6.4% volt drop is already poor.

If you were to extend the cable by adding 2.5m of 1mm² cable before the first lamp, those voltages would be

11.12
10.93
10.78
10.67
10.59

i.e. the volt drop at the final lamp is 11.75% - far too much.

I don't know where, and by how much you need to lengthen the cable, but your best bet would be to use a short thick piece to a JB, and then individual 1mm² cables, of the same length even if they don't need to be, to each lamp.

Oh - and if you care, once you've modified this kit, it is no longer a CE marked pre-assembled ELV kit, and therefore it is a "special installation" and therefore the law says that you have to notify LABC in advance...

 

[Barney78]

Thank you, that clears things up. I'm beginning to think that my best bet is to buy 7 more transformers and run one light off each, at least that way i get 9 lights all the same brightness.

 

[ban-all-sheds]

One "transformer" per light is usually the best approach, but be careful given the small load of each of your lights - some cheap power supplies need a minimum load to work properly.

But as long as you keep the length of cable the same to each light, they will all be the same brightness, and as long as you use cables of the correct size, the volt drop will be OK.

Calculating volt drop is just simple Ohm's Law, and you'll find the figures for cable resistance in the On-Site Guide. If you don't have a copy of the OSG, you should buy one - nobody should fiddle with wiring if they don't have a copy...