I want to wire up some 12V downlighters to the transformer. I cannot find out what the correct wire is to use for this. The easiest thing to use would be 1mm T&G. I'm thinking:
35W bulb at 12Volts = 2.91 amps. 3 bulbs on the circuit, so call it 9 amps. 1mm T&G clipped direct (ref method 1) in the roof void allows 15A. So that's OK.Have I got this right, or is there a special cable that I should use for this?
R. Thanks in advance.
Sort-of. It won't set the house on fire. But, remember that the voltage drop for a 12V circuit is the same for a
240V circuit. But, for the same guage wire, and wattage load, there will be 20 times the voltage drop at 12V (20 times the current), which has 20 times the effect, or in other words, you can use about 1/400th length of cable you can at mains voltage, and get an acceptable drop.How far?
Yes so far as current carrying capacity, but that is not the end of the storey - you need to also consider voltage drop. Both need to be calculated and a cable chosen which satisfies both criteria. Voltage drop will be much more critical at 12v.
Yes. I use 2.5mm² for a 6m run feeding 2x20W lamps (upgradable to 3x20W). Also using 2.5mm² for a 2m run feeding 10x10W lamps. Both these work fine.
I'd use the appropriately rated Auto wire (ie the stuff used for wiring cars)
sponix
NO!! A voltage drop of a few volts when using 240v mains doesn't matter too much - but the *same* drop in a 12v circuit is catastrophic! Use at least
2.5 sq.mm cable.
Locate the transformer nearer to the light fittings! You will find it a lot easier just to extend the mains wire rather than the 12v side.
Thanks for all the replies. I had, indeed, ommited to calculate the voltage drop. The lamps can be physically about 1 metre from the transformer.
I had planned to wire them in series, so the last lamp would be about 3 metres from the transformer - a drop of about 1.2 volt on 1mm T&E (44mV/A/m), but only 0.48 volts with 2.5 T&E (18 mV/A/m).
But if I wire each lamp with it's own 1mm T&E cable (radial) then the drop will only be about 0.13 Volts. This seems the best course.
(all using table 6D2 from the IEE on-site guide appx-6)
If I got it right, of course.
R.
Be warned though that many electronic transformers have limited screw terminal capacity ... I used a load of ones from TLC (Dolye & Pratt mnfctr) great units, never had one fail ... but if you are running more than one cable from them ... i.e. 2 sets pf lights ... it is VERY hard to physically get 2 x 1.5mm2 conductors in the terminals. Plus because terminals are very close to cable restraint it is again hard to get cables to be neat enough to clamp. It's not impossible - but it is VERY fiddly.
This is partly due to poor plastic moulding that partly obscures the terminal holes - reported to Mnfctr, who advise they will modify fitting of terminal block.
Rick
On Wed, 18 Jan 2006 21:25:27 +0000,it is alleged that Richard A Downing spake thusly in uk.d-i-y:
On Wed, 18 Jan 2006 21:25:27 +0000,it is alleged that Richard A Downing spake thusly in uk.d-i-y:
[snip]
A suggestion: 2.5 is going to be much better (0.13 volts is still > 1% volt drop at 12v), and make all lengths of cable the same, that way there won't be a variation in brightness.
When I did mine, I used a junction box in a central location with individual short 1.5mm^2 cables from that to each lamp. The feed from the transformer to the junction box was 2.5mm^2
You could use 3 core, and join 2 cores at the furthest lamp, and use the feed back from the furthest lamp together with the shortest for each lamp. This would equalise the voltage drop to all lamps, so they would all be the same brightness.
This seems like a good plan. I'm concerned that the terminal block in the transformer may not have enough physical capacity for 3x2.5mm solid wires. A very short length of 2.5mm (say 30mm) would have minimal voltage drop (~0.016V), and yet get me to a heavy-duty junction box. Then the individual lamp wiring can be as big as necessary.
The Lamps and Transformers should arrive today or tomorrow and then I can see the size of the terminals.
R.
Now arrived. The transformer's documentation says the the output (lamp) cables should: "Use cable type H03VV-F or type as specified in the relevant wiring/building regulations or national standards". The instructions say that an earth is not required and should be less that
1m long - this is OK for me.I think that H03VV-F is standard PVC Flex (as for a table lamp), but this thread has suggested that 2.5 T&E is a better cable for 12V in view of the voltage drop. I have checked the IEE On-Site guide, but do not have a copy of the full regulations. Can anyone confirm that 2.5 T&E will be acceptable under the regs? Certainly, the current and voltage drops would seem to be withing spec.
I'm sorry to niggle on at this, but I'm determined to do this right.
R.
Richard A Downing said the following on 19/01/2006 11:15:
"Standard" PVC Flex would be H05VV-F (rated at 500V not 300V).
You can safely use any type of electrical flex for the output side of your transformer, provided the insulation doesn't get too hot. 2.5 T&E would be OK as would 2.5 PVC flex.
I did much the same except with 1.0mm^2 radials to each lamp. Since each fitting was only going to be 35W the voltage drop was under 2% for that load and the relatively short cable lengths I was using.
My solutiin for 3x 50W was a really fat bit of cooker cable, to a junction box near the lamps, and then normal mains T&E for teh lasty 1.5 meters! :-)
Even so at 12A that cooker cable drops a bit.. you may find that someth8ing like 'cooking'grade* car loudspeaker cable - its multistrand and usually silcones nsulated - is able to carry the current better with less voltage drop.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.