Wire for 12V lights

[Richard A Downing]

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.

 

[Harry Bloomfield]

Richard A Downing wrote :

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?

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.

 

[Andrew Gabriel]

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.

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.

 

[sponix]

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.

I'd use the appropriately rated Auto wire (ie the stuff used for
wiring cars)

sponix

 

[Set Square]

In an earlier contribution to this discussion,

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.

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.

 

[Richard A Downing]

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.

 

[Rick Hughes]

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.

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

 

[Set Square]

In an earlier contribution to this discussion,

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.

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

 

[Guest]

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.

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.

 

[Richard A Downing]

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

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.

 

[Richard A Downing]

On Thu, 19 Jan 2006 08:47:43 +0000

The Lamps and Transformers should arrive today or tomorrow and then I
can see the size of the terminals.

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.

 

[sPoNiX]

I think that H03VV-F is standard PVC Flex (as for a table lamp),

http://elandcables-specs.co.uk:8080/eland/details?CableTypeID=107

 

[Rumble]

Richard A Downing said the following on 19/01/2006 11:15:

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.

"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.

 

[John Rumm]

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

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.

 

[Andy Wade]

http://elandcables-specs.co.uk:8080/eland/details?CableTypeID=107

FYI there's a full list of harmonisation <HAR> codes here:
http://www.ove.at/puz/int/HAR_PD7_May_2004.pdf

 

[The Natural Philosopher]

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

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.

* By cooking grade I mean the stuff the pros use not the stuff that is sold
to idiots - gold plated litze wire guranteed to transmit at microwave
frequencies etc etc. You can also get the cable that is used for incoming
mains tails - that is seriously good fat stuff, as is the sort of car
battery cable that you can buy at motor factors.