Excuse my ignorance - I've never run DC cable before!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?
Also, what _type_ of cable? What core colours for after transformer?
(On site guide allows almost anything for ELV).

TIA.

David.

"David Robinson" wrote in message
...
Excuse my ignorance - I've never run DC cable before!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?
Also, what _type_ of cable? What core colours for after transformer?
(On site guide allows almost anything for ELV).

TIA.

David.

I'll give you a starter for 10. It is not DC.

But on the plus side (pun intended) you are more than welcome to give more
info about your suggested setup to get the correct answer:-)

Cheers

Adam

On Wed, 23 Jun 2010 09:36:50 -0700 (PDT), David Robinson wrote:

Excuse my ignorance - I've never run DC cable before!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?
Also, what _type_ of cable? What core colours for after transformer?
(On site guide allows almost anything for ELV).

One transformer per light. The cost is minimal. If you group several lights
on a 'large' transformer and that goes fsulty (it does happen!) you've lost
several lights until you get a replacement.

I have severa lv lights, in the kitchen and also scattered around the
outside of the bungalow, set up in the soffits. Each one has its own
transformer, typically 50-70w for a 35 w bulb. Not had any problems at all.

--
The Wanderer

Inertia keeps me going!

"David Robinson" wrote in message
...
Excuse my ignorance - I've never run DC cable before!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?
Also, what _type_ of cable? What core colours for after transformer?
(On site guide allows almost anything for ELV).

TIA.

David.

It's the transformer that limits you: our kitchen ceiling has two rows of 3
with two 60W transformers. That was the electrician being unnecessarily
stingy, as it is quite difficult to tell the lamps apart, and when a 30W one
got into one of the holders, the set cycled on and off with the overheat
(presumed) cut out, and I had the floor above up, before I realised...

Thus, when I did the bathroom I allowed for some particularly light obsessed
individual being blind enough to put in a 50W lamp, and so allowed one 100W
transformer for each pair - though still only need 20W for most purposes,
but one brighter one (splash proof) over the shower to help wake me up!

Incidentally, we have a couple of round in-cabinet lights as well. Ours are
'brassed' and have frosted glass and take little 10W lamps. Make sure you
'try before you buy' cos ours are rubbish: the 'metal' being so fragile that
it distorts and soon cracks the glass when you try to unscrew it to change
the 'bulb'. You end up with a handful of broken glass.

S

"Spamlet" wrote in message
...

"David Robinson" wrote in message
...
Excuse my ignorance - I've never run DC cable before!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?
Also, what _type_ of cable? What core colours for after transformer?
(On site guide allows almost anything for ELV).

TIA.

David.

It's the transformer that limits you: our kitchen ceiling has two rows of
3 with two 60W transformers. That was the electrician being unnecessarily
stingy, as it is quite difficult to tell the lamps apart, and when a 30W
one got into one of the holders, the set cycled on and off with the
overheat (presumed) cut out, and I had the floor above up, before I
realised...

Thus, when I did the bathroom I allowed for some particularly light
obsessed individual being blind enough to put in a 50W lamp, and so
allowed one 100W transformer for each pair - though still only need 20W
for most purposes, but one brighter one (splash proof) over the shower to
help wake me up!

Incidentally, we have a couple of round in-cabinet lights as well. Ours
are 'brassed' and have frosted glass and take little 10W lamps. Make sure
you 'try before you buy' cos ours are rubbish: the 'metal' being so
fragile that it distorts and soon cracks the glass when you try to unscrew
it to change the 'bulb'. You end up with a handful of broken glass.

S
ps. 'TW' above may be being a bit over cautious in allowing only one lamp
per tf : so long as we've kept within the wattage limits (which means
leaving a label somewhere to tell any user the maximum you've allowed for)
we've had no failures at all in quite a few years now (sshhh!).

S

On 23/06/10 17:36, David Robinson wrote:
Excuse my ignorance - I've never run DC cable before!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

That's probably about right then.



--
Bernard Peek

On 23 June, 20:17, "Spamlet" wrote:
"Spamlet" wrote in message

...



"David Robinson" wrote in message
....
Excuse my ignorance - I've never run DC cable before!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?
Also, what _type_ of cable? What core colours for after transformer?
(On site guide allows almost anything for ELV).

TIA.

David.

It's the transformer that limits you: *our kitchen ceiling has two rows of
3 with two 60W transformers. *That was the electrician being unnecessarily
stingy, as it is quite difficult to tell the lamps apart, and when a 30W
one got into one of the holders, the set cycled on and off with the
overheat (presumed) cut out, and I had the floor above up, before I
realised...

Thus, when I did the bathroom I allowed for some particularly light
obsessed individual being blind enough to put in a 50W lamp, and so
allowed one 100W transformer for each pair - though still only need 20W
for most purposes, but one brighter one (splash proof) over the shower to
help wake me up!

Incidentally, we have a couple of round in-cabinet lights as well. *Ours
are 'brassed' and have frosted glass and take little 10W lamps. *Make sure
you 'try before you buy' cos ours are rubbish: the 'metal' being so
fragile that it distorts and soon cracks the glass when you try to unscrew
it to change the 'bulb'. *You end up with a handful of broken glass.

S

ps. *'TW' above may be being a bit over cautious in allowing only one lamp
per tf : so long as we've kept within the wattage limits (which means
leaving a label somewhere to tell any user the maximum you've allowed for)
we've had no failures at all in quite a few years now (sshhh!).

S

Wound trafos need to be close to their loading at all times, but are
usually only used on suspended wire systems nowadays,electronic trafos
don`t like the long lines and can act like an aerial.

Its voltage drop in the cable thats the catch, any more than 0.5V is
very noticeable and means your cables are getting warm. Hence thick
cable at any run length and putting a trafo per light usually easier.

Cheers
Adam

On Jun 24, 10:27 am, John Rumm wrote:
On 23/06/2010 17:36, David Robinson wrote:

Excuse my ignorance - I've never run DC cable before!

and you are not about to either - these LV lights are AC ;-)

I am running DC cable! I'm running 30 LEDs that seem to be taking 70
milliamps in total at 12 volts. I could use telephone cables.

In article
,
Matty F wrote:
On Jun 24, 10:27 am, John Rumm wrote:
On 23/06/2010 17:36, David Robinson wrote:

Excuse my ignorance - I've never run DC cable before!

and you are not about to either - these LV lights are AC ;-)

I am running DC cable! I'm running 30 LEDs that seem to be taking 70
milliamps in total at 12 volts. I could use telephone cables.

But what do you used for lighting?

--
*Do infants enjoy infancy as much as adults enjoy adultery?

Dave Plowman London SW
To e-mail, change noise into sound.

"John Rumm" wrote in message
o.uk...


Yup, that's a slightly different proposition from halogen LV systems that
can suck 10s of amps at 12V!

The last time I fitted LV lamps I just used a cheap transformer for each
lamp and ran mains.
They are £3 and it means that a transformer fault doesn't leave you in the
dark.

On 23 June, 23:27, John Rumm wrote:
On 23/06/2010 17:36, David Robinson wrote:

Excuse my ignorance - I've never run DC cable before!

and you are not about to either - these LV lights are AC ;-)

My physics teacher would be ashamed of me!

Though I remember using P = I^2 * R to calculate the power dissipated
in cables as heat - i.e. higher voltage = more efficient and lower
voltage = huge cables or lots of electrical energy turned into heat on
the way!

I'm installing some of these...
http://www.tlc-direct.co.uk/Products/QLCD10C.html

Various ways of grouping them are possible, using various transformers
(e.g. one 0-250W, a few 0-50W and 0-100W).

I've done some quick calcs of the required cable size, and it seems
stupidly large.

Indeed *- if you run several lights from one cable the current can be
high, and they are very sensitive to voltage drop (in the sense that the
difference will be visible)

What size cable do people use in practice? How do people usually group
the lights? Big groups / small groups? Daisy chained cable or star
wired?

Personally I usually star wire them in groups. Typically in 1.0mm
T&E[1], keeping each cable in the 1 - 3m sort of length range.

With six lights I would probably go for two 150W transformers. If
possible positioning the thing such that it can be extracted later if
required.

I assume this means the connector block between the transformer and
the wires needs to be very hefty?

I found this on-line calculator that seems useful (though from what
you've said 1% voltage drop at 12V is already far too much)...

http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

I'm putting lights on top of (and underneath) kitchen cupboards. This
means they're physically grouped (2 left of window, 5 right of window,
9 across the back wall), and with transformers on top of the cupboards
they'll be easily accessible, with suitable wires run down in the wall
to the lights below.

I was going to run one wire down per run of units, but I see now it
makes more sense to run one wire down per light! (Even one per pair of
lights would be pushing for 1.5mm2 according to that calculator?!).
Not quite so neat, but normal people don't peer under the bottom of
cupboards anyway.

Cheers,
David.

On 24 June, 14:34, John Rumm wrote:

[snip]

Thanks for all the useful advice John (and others).

on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV total,
or around 1%. So that would be fine.

With lamps that could be up to 50W, then one cable per lamp would be ok
for shortish distances. If you need longer runs (say 5m or more) then
you could go up to 1.5mm^2.

I don't have any 1mm twin and earth at present. I wonder if I can re-
use all the old red+black 1mm I'm ripping out when re-wring? ELV
doesn't have to pass part-P, does it? I can sheath it brown and blue
if they're that bothered.

That's given me an idea...

Cheers,
David,

On Jun 24, 6:33*pm, David Robinson
wrote:
On 24 June, 14:34, John Rumm wrote:

[snip]

Thanks for all the useful advice John (and others).

on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV total,
or around 1%. So that would be fine.

With lamps that could be up to 50W, then one cable per lamp would be ok
for shortish distances. If you need longer runs (say 5m or more) then
you could go up to 1.5mm^2.

I don't have any 1mm twin and earth at present. I wonder if I can re-
use all the old red+black 1mm I'm ripping out when re-wring? ELV
doesn't have to pass part-P, does it? I can sheath it brown and blue
if they're that bothered.

That's given me an idea...

Cheers,
David,


Doesnt matter what colour it is, the conductors arent live & neutral
so no reason to sheath them in L&N colours.

12v 20w = 1.66A per lamp. Any cable larger than bell wire can handle
that


NT

David Robinson
wibbled on Thursday 24 June 2010 18:33

On 24 June, 14:34, John Rumm wrote:

[snip]

Thanks for all the useful advice John (and others).

on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV total,
or around 1%. So that would be fine.

With lamps that could be up to 50W, then one cable per lamp would be ok
for shortish distances. If you need longer runs (say 5m or more) then
you could go up to 1.5mm^2.

I don't have any 1mm twin and earth at present. I wonder if I can re-
use all the old red+black 1mm I'm ripping out when re-wring?

Line/+ve - Brown,black,red,orange,yellow,violet,grey,white,pi nk or turquoise
Neutral/-ve - Blue

if you want to be pedantic (App 11, Onsite Guide).

But, it really doesn't matter. Persoanlly, I think it's more useful to write
on the cable with "Sharpie" at regular intervals "SELV, 12V" or whatever, to
avoid confusion...

as to cable, T+E (I'd go for 1.5mm2 if you have a 250W transformer as the
current carrying capacity is closer to the potential short circuit current
(unless the PSU has a short circuit "knee" curve.

But many would use flex for some/all of the SELV, particularly the final
drops. Heat resistant is advisable if the fittings might get hot. I used
silicone cable, with the proviso it's not as tough as PVC but my
installation positions are well guarded.

SELV doesn't have much of a shock risk, but the fire risk is still very real
- so consider it in those terms.

ELV
doesn't have to pass part-P, does it? I can sheath it brown and blue
if they're that bothered.

Technically yes or at least under some conditions like special locations)
(which might change).

That's given me an idea...

Cheers,
David,

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

On 24 June, 21:04, NT wrote:
On Jun 24, 6:33*pm, David Robinson
wrote:



On 24 June, 14:34, John Rumm wrote:

[snip]

Thanks for all the useful advice John (and others).

on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV total,
or around 1%. So that would be fine.

With lamps that could be up to 50W, then one cable per lamp would be ok
for shortish distances. If you need longer runs (say 5m or more) then
you could go up to 1.5mm^2.

I don't have any 1mm twin and earth at present. I wonder if I can re-
use all the old red+black 1mm I'm ripping out when re-wring? ELV
doesn't have to pass part-P, does it? I can sheath it brown and blue
if they're that bothered.

That's given me an idea...

Cheers,
David,

Doesnt matter what colour it is, the conductors arent live & neutral
so no reason to sheath them in L&N colours.

12v 20w = 1.66A per lamp. Any cable larger than bell wire can handle
that

Indeed, until voltage drop bites and energy lost goes as heat.

Low voltage high current can melt things nicely...

Cheers
Adam




NT

On Jun 24, 11:55*pm, Adam Aglionby wrote:
On 24 June, 21:04, NT wrote:



On Jun 24, 6:33*pm, David Robinson
wrote:

On 24 June, 14:34, John Rumm wrote:

[snip]

Thanks for all the useful advice John (and others).

on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV total,
or around 1%. So that would be fine.

With lamps that could be up to 50W, then one cable per lamp would be ok
for shortish distances. If you need longer runs (say 5m or more) then
you could go up to 1.5mm^2.

I don't have any 1mm twin and earth at present. I wonder if I can re-
use all the old red+black 1mm I'm ripping out when re-wring? ELV
doesn't have to pass part-P, does it? I can sheath it brown and blue
if they're that bothered.

That's given me an idea...

Cheers,
David,

Doesnt matter what colour it is, the conductors arent live & neutral
so no reason to sheath them in L&N colours.

12v 20w = 1.66A per lamp. Any cable larger than bell wire can handle
that

Indeed, until voltage drop bites and energy lost goes as heat.

Not really an issue over 1-2 metres of wire.


Low voltage high current can melt things nicely...

1.7A isnt much, its when you've got 6x 50w lamps on one cable over
several metres that those factors kick in.

There's a related effect sometimes mistaken for Vdrop too, that
happens on ballasts that use very high frequency.


NT

On 24 June, 23:44, Tim Watts wrote:
David Robinson
* wibbled on Thursday 24 June 2010 18:33

On 24 June, 14:34, John Rumm wrote:

[snip]

Thanks for all the useful advice John (and others).

on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV total,
or around 1%. So that would be fine.

With lamps that could be up to 50W, then one cable per lamp would be ok
for shortish distances. If you need longer runs (say 5m or more) then
you could go up to 1.5mm^2.

I don't have any 1mm twin and earth at present. I wonder if I can re-
use all the old red+black 1mm I'm ripping out when re-wring?

Line/+ve - Brown,black,red,orange,yellow,violet,grey,white,pi nk or turquoise
Neutral/-ve - Blue

if you want to be pedantic (App 11, Onsite Guide).

Thanks Tim - that's exactly what I was looking at - but wasn't sure
whether that part was legally binding. It's not like 12V electrical
work is notifiable AFAICT.

But, it really doesn't matter. Persoanlly, I think it's more useful to write
on the cable with "Sharpie" at regular intervals "SELV, 12V" or whatever, to
avoid confusion...

I suppose it could confuse someone where it comes out of the wall.
Once fitted to transformer + lights you'd need to be a bit of an idiot
not to realise what it was. Though most of us can be idiots outside
our areas of expertise, and each revision of the regs aims to keep
more people out of the Darwin awards.

as to cable, T+E (I'd go for 1.5mm2 if you have a 250W transformer as the
current carrying capacity is closer to the potential short circuit current
(unless the PSU has a short circuit "knee" curve.

But many would use flex for some/all of the SELV, particularly the final
drops. Heat resistant is advisable if the fittings might get hot. I used
silicone cable, with the proviso it's not as tough as PVC but my
installation positions are well guarded.

SELV doesn't have much of a shock risk, but the fire risk is still very real
- so consider it in those terms.

Is standard T+E sufficiently heat resistant?

ELV
doesn't have to pass part-P, does it? I can sheath it brown and blue
if they're that bothered.

Technically yes or at least under some conditions like special locations)
(which might change).

Kitchen isn't yet though?

Thanks.

Cheers,
David.

David Robinson
wibbled on Friday 25 June 2010 09:46

On 24 June, 23:44, Tim Watts wrote:
David Robinson
wibbled on Thursday 24 June 2010 18:33

On 24 June, 14:34, John Rumm wrote:

[snip]

Thanks for all the useful advice John (and others).

on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV
total, or around 1%. So that would be fine.

With lamps that could be up to 50W, then one cable per lamp would be
ok for shortish distances. If you need longer runs (say 5m or more)
then you could go up to 1.5mm^2.

I don't have any 1mm twin and earth at present. I wonder if I can re-
use all the old red+black 1mm I'm ripping out when re-wring?

Line/+ve - Brown,black,red,orange,yellow,violet,grey,white,pi nk or
turquoise Neutral/-ve - Blue

if you want to be pedantic (App 11, Onsite Guide).

Thanks Tim - that's exactly what I was looking at - but wasn't sure
whether that part was legally binding. It's not like 12V electrical
work is notifiable AFAICT.

Check again - I'm pretty sure it is in special locations and kitchens. But
it is stupid, so personally I'd ignore it - but just for the record like...


Is standard T+E sufficiently heat resistant?

Depends on the light fitting. It is rated to 70C max. Some do get seriously
hot out the back, some don't.

ELV
doesn't have to pass part-P, does it? I can sheath it brown and blue
if they're that bothered.

Technically yes or at least under some conditions like special locations)
(which might change).

Kitchen isn't yet though?

Kitchens are mentioned IIRC in Part P (the document).

Cheers

Tim
--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.

In article
,
Adam Aglionby wrote:
12v 20w = 1.66A per lamp. Any cable larger than bell wire can handle
that

Indeed, until voltage drop bites and energy lost goes as heat.

That surely applies to any circuit?

Low voltage high current can melt things nicely...

It's the current wot does this...

--
*When the chips are down, the buffalo is empty*

Dave Plowman London SW
To e-mail, change noise into sound.

Something you have to factor in when designing or extending the LV
wiring side is the type of transformer used and how this relates to
the cable type/size required.

If you are using a conventional heavy magnetic transformer, then you
can use the standard voltage drop figures in the wiring regs to work
out what size cable you need, although as you said, you need a much
lower voltage drop on the 12V side than would be acceptable on the
mains side. (For any sizable runs, minimum size is more likely going
to be dictated by voltage drop than by current carrying capacity.)

If you try doing this with an electronic transformer, you will be
very disappointed with the result. Problem is these generate AC output
at somewhere between 20-60kHz, and not 50Hz. At that frequency, a
physical property called "skin effect" has come into play, which means
the electricity only travels along the outer skin of conductors, so the
bulk of the conductor material is not usable, and the effective cross
sectional area of the wire is very much less than the actual cross-
sectional area. This became very obvious to me when I swapped a
toriodal transformer for an electronic one, and light output dropped
to probably only 1/3rd of what it had been. I had to upgrade the wire
from 2.5mm to 6mm to get the light output back as it was.

I had a room with a central pendent with 3 x 60W and a pair of wall
lights each with 1 x 60W, and wanted to convert this all to 12V.
I decided to use 35W 12V capsules to replace all the 60W mains lamps.
The transformers are in a metal case in the room above, directly above
one of the wall lights, and for that light, I take the 1mm² cable
directly into the transformer. For the other wall light, at the top
of the run up the wall, I convert it to 2.5mm² to run across under
the floor and up into the transformer. For the central pendent (3 x 35W),
I initially also used a run of 2.5mm² but the voltage drop was too
high, so I changed it to a run of 6mm², and that's fine.

In my case, I wanted to use mains cabling so that I could remove the
12V lighting and go back to mains at some point in the future without
having to replace the cabling. If this had not been the case, I might
have considered other types of cabling suitable for LV high frequency,
such as flat stranded speaker wire (higher surface area to volume ratio,
so less skin effect) or silver plated stranded wire (silver being a
good conductor works very well plated onto copper for high frequencies).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 25 June, 10:34, "Dave Plowman (News)" wrote:
In article
,
* *Adam Aglionby wrote:

12v 20w = 1.66A per lamp. Any cable larger than bell wire can handle
that
Indeed, until voltage drop bites and energy lost goes as heat.

That surely applies to any circuit?

But with LV lighting the currents stack up quick, as NT says a single
20W lamp no great shakes, few 50W on same circuit at nearly 4A a
piece, current climbs quick.


Low voltage high current can melt things nicely...

It's the current wot does this...

The voltage is mosttly harmless , but need to keep respect for the
current and its heating ability, with the cable and making sure
connectors are tight.
Have seen choc bloc melt down because of loose screws with LV lights.

Cheers
Adam


--
*When the chips are down, the buffalo is empty*

* * Dave Plowman * * * * * * * * London SW
* * * * * * * * * To e-mail, change noise into sound.

In article
,
Adam Aglionby wrote:
On 25 June, 10:34, "Dave Plowman (News)" wrote:
In article
,
Adam Aglionby wrote:

12v 20w = 1.66A per lamp. Any cable larger than bell wire can handle
that
Indeed, until voltage drop bites and energy lost goes as heat.

That surely applies to any circuit?

But with LV lighting the currents stack up quick, as NT says a single
20W lamp no great shakes, few 50W on same circuit at nearly 4A a
piece, current climbs quick.

As does adding things willy-nilly to any circuit. If you can't do the
maths it's best left to someone who can. After all, you must have chosen
the transformer to run all these low voltage lights in parallel? And not
by accident - a 50 watt toroidal costs around a tenner, but a 250 watt one
nearer 50 quid. Electronic may not cost quite as much in proportion, but
high current ones are still much more expensive. And


Low voltage high current can melt things nicely...

It's the current wot does this...

The voltage is mosttly harmless , but need to keep respect for the
current and its heating ability, with the cable and making sure
connectors are tight.

Have seen choc bloc melt down because of loose screws with LV lights.

Perhaps the chock block was inadequately rated too?

Low volt lighting circuits don't generally carry any more current than a
ring or cooker, etc. So it's not like they're using currents the average
person hasn't come across before. If they choose to treat them as
unimportant, workmanship wise. since they're low voltage, that's up to
them.

All electricity can be dangerous in various ways. I know of someone who
was injured by a PP3 battery...

--
*I speak fluent patriarchy but it's not my mother tongue

Dave Plowman London SW
To e-mail, change noise into sound.

On Jun 25, 10:57*am, (Andrew Gabriel)
wrote:
Something you have to factor in when designing or extending the LV
wiring side is the type of transformer used and how this relates to
the cable type/size required.

If you are using a conventional heavy magnetic transformer, then you
can use the standard voltage drop figures in the wiring regs to work
out what size cable you need, although as you said, you need a much
lower voltage drop on the 12V side than would be acceptable on the
mains side. (For any sizable runs, minimum size is more likely going
to be dictated by voltage drop than by current carrying capacity.)

If you try doing this with an electronic transformer, you will be
very disappointed with the result. Problem is these generate AC output
at somewhere between 20-60kHz, and not 50Hz. At that frequency, a
physical property called "skin effect" has come into play, which means
the electricity only travels along the outer skin of conductors, so the
bulk of the conductor material is not usable, and the effective cross
sectional area of the wire is very much less than the actual cross-
sectional area. This became very obvious to me when I swapped a
toriodal transformer for an electronic one, and light output dropped
to probably only 1/3rd of what it had been. I had to upgrade the wire
from 2.5mm to 6mm to get the light output back as it was.

I had a room with a central pendent with 3 x 60W and a pair of wall
lights each with 1 x 60W, and wanted to convert this all to 12V.
I decided to use 35W 12V capsules to replace all the 60W mains lamps.
The transformers are in a metal case in the room above, directly above
one of the wall lights, and for that light, I take the 1mm² cable
directly into the transformer. For the other wall light, at the top
of the run up the wall, I convert it to 2.5mm² to run across under
the floor and up into the transformer. For the central pendent (3 x 35W),
I initially also used a run of 2.5mm² but the voltage drop was too
high, so I changed it to a run of 6mm², and that's fine.

In my case, I wanted to use mains cabling so that I could remove the
12V lighting and go back to mains at some point in the future without
having to replace the cabling. If this had not been the case, I might
have considered other types of cabling suitable for LV high frequency,
such as flat stranded speaker wire (higher surface area to volume ratio,
so less skin effect) or silver plated stranded wire (silver being a
good conductor works very well plated onto copper for high frequencies).


would be good to put that on the wiki somewhere.


NT

On 25 June, 13:43, "Dave Plowman (News)" wrote:
In article
,
* *Adam Aglionby wrote:

On 25 June, 10:34, "Dave Plowman (News)" wrote:
In article
,
* *Adam Aglionby wrote:

12v 20w = 1.66A per lamp. Any cable larger than bell wire can handle
that
Indeed, until voltage drop bites and energy lost goes as heat.

That surely applies to any circuit?
But with LV lighting the currents stack up quick, as NT says a single
20W lamp no great shakes, few 50W on same circuit at nearly 4A a
piece, current climbs quick.

As does adding things willy-nilly to any circuit. If you can't do the
maths it's best left to someone who can. After all, you must have chosen
the transformer to run all these low voltage lights in parallel? *And not
by accident - a 50 watt toroidal costs around a tenner, but a 250 watt one
nearer 50 quid. Electronic may not cost quite as much in proportion, but
high current ones are still much more expensive. And *

Back in the day, guess mid 80s one of the companies doing the new
fangled LV lights , replacing PAR38 for retail display, ran a roadshow
for installers.

One demo was in the car park, 12V lamp on 240V , graphically
demonstrated need for trafo and cost of wiring mistake.

Other one remember was 250W torrodial and loop of bell wire racing the
25A fuse, bell wire went first with some smoke in protestation.





Low voltage high current can melt things nicely...

It's the current wot does this...
The voltage is mosttly harmless , but need to keep respect for the
current and its heating ability, with the cable and making sure
connectors are tight.
Have seen choc bloc melt down because of loose screws with LV lights.

Perhaps the chock block was inadequately rated too?

The tightening was inadequate on a substanial block, seen it happen
several times in last 20 odd years.


Low volt lighting circuits don't generally carry any more current than a
ring or cooker, etc. So it's not like they're using currents the average
person hasn't come across before. If they choose to treat them as
unimportant, workmanship wise. since they're low voltage, that's up to
them. * *

Its low voltage but high current, people including electricians just
see the LV bit, the current even on a single 50W lamp will happily set
something on fire.

All electricity can be dangerous in various ways. I know of someone who
was injured by a PP3 battery...

C`mon theres got to be a story ;-) ?

Cheers
Adam


--
*I speak fluent patriarchy but it's not my mother tongue

* * Dave Plowman * * * * * * * * London SW
* * * * * * * * * To e-mail, change noise into sound.