Pathway lights - 12v or 240v - any electricians/help?

[sidekickdmr]

I have about 100m of pathway I want to light.

All I want is a small little 2W LED spike light every 5 or so meters, so only 20 in total, and only about 40watts total too.

However with the length of run, and voltage drop, can I do that on a 12V system?

large transformer and large core cable may well help?

Saves having to buy armoured cable, pay a pro to instal it, and dig up 100m of solid stone path if I can.

Thanks

[Mr Pointy]

If you wire the run in 2.5mm as a radial then you'll drop roughly 5v (maximum, an approximation) at the end of 100m. If you can position the transformer in the middle, or wire it as a ring, then you would halve this drop to around 2.5v. You can choose to raise the transformer voltage & risk overvolting the nearest lights or run the end lights undervolted, so they may work or they may not.

[sidekickdmr]

So if i understand that right

20 of these lights (60W)

https://www.amazon.co.uk/Landscape-Spotlight-Green...

Using this wire (2.5mm 2 core)

https://www.edwardes.co.uk/en/products/2-5mm-3182y...

And this transformer (100W):

https://www.amazon.co.uk/Voltage-Outdoor-Lighting-...

I may find the first LED is brighter and the last one dimmer but should work ok?

[Alucidnation]

What's a 'small little'?

Anyhoo, 240V.

Next..

[mikeveal]

To understand the 12V case, you need a few formulae:

P=VI, Power in Watts = Volts * Current in Amps

V=I*R, Volt drop in your cable = Current in Amps * Cable resistance in Ohms.

& R=R/m * l , Resistance of your cable in Ohms = resistance per meter * length in meters.

We need to work out the voltage at each lamp and see if it's significantly lower than the 12V.

If you use cable like this https://www.tlc-direct.co.uk/Products/CASPEAK79.ht... each conductor has a copper area of 2.5mm squared.
Here https://www.tlc-direct.co.uk/Book/5.3.6.htm TLC helpfully tell us that 2.5mm squared copper cable has a resistance per meter of 7.41milli Ohms.

Step 1, how much current in the cable from the transformer to the fist light...
Answer: all of it!
P=VI, so I=P/V
For each lamp 2W is drawn
I=40 / 12 = 3.33A


Conventionally we think of the current flowing down the 12V wire and returning on the 0V wire. The cable has a resistance, so a voltage will be developed across it as current flows through it BOTH on the 12V AND the 0V conductors.

The voltage dropped on the 12V side is
V=I*R
& R=7.41 milliOhms/ meter * 5 meters = 37.05mOhms.
V = 3.33A * 37.05mOhms = 123.48mV
The voltage dropped on the 0V wire.

So from the transformer to the first light, you lose 2* 123.48mV = 247mV
The voltage at the first light is 12V-0.247V = 11.75V


Moving to the next lamp, again we have 5m of cable. This time the current is lower as we're only powering 19 2W lamps.
I=38/12 = 3.166A
And the voltage drop is:
V=IR= 2*37.05mOhms *3.166A =234mV
But this time the voltage at the start of the 5m run is 11.75V. So the voltage at the second lamp is 11.75V - 0.234V = 11.51V


If you keep this going, the voltages at each lamp work out as follows:
Lamp 1 11.753
Lamp 2 11.51835
Lamp 3 11.29605
Lamp 4 11.0861
Lamp 5 10.8885
Lamp 6 10.70325
Lamp 7 10.53035
Lamp 8 10.3698
Lamp 9 10.2216
Lamp 10 10.08575
Lamp 11 9.96225
Lamp 12 9.8511
Lamp 13 9.7523
Lamp 14 9.66585
Lamp 15 9.59175
Lamp 16 9.53
Lamp 17 9.4806
Lamp 18 9.44355
Lamp 19 9.41885
Lamp 20 9.4065

9.4 / 11.75 *100 = 80%

So assuming , the lamps are fairly linear, lamp 20 could be 20% dimmer than lamp 1.

If this is not acceptable, either run MUCH fatter cable (it won't be economical) or use 220V (armoured) cable and lamps. As already suggested running a second cable from lamp 20 back to the transformer will help a bit, the losses aren't halved though..

Being PH, someone's bound to point out that this is an approximation. As the voltage at each lamp drops, the current could increase, decrease or stay the same depending on the type of lamp,

[Schmeeky]

^^ This is one reason I love PistonHeads!!

I'm thinking of doing the same - fortunately I only need a run of about 5m..

[Teddy Lop]

mikeveal said:

If this is not acceptable, either run MUCH fatter cable (it won't be economical) or use 220V (armoured) cable and lamps. As already suggested running a second cable from lamp 20 back to the transformer will help a bit, the losses aren't halved though.

230v would be fine, all things being equal, but 230 means RCDs and any damp, rodent damage, vandalising gardeners etc means the lot trips out and stays out and you're going fitting to fitting trying to find the issue. I buy gu10 fittings and convert them to 12v for this reason - typically an install of 20 spike lights will have the 230 side at a few transformer boxes fixed up off the ground at strategic locations.

A 50v or therabouts based system would be the ideal compromise.

[eldar]

Solar powered ones. No wires, simple, easy and reliable.

https://www.amazon.co.uk/Outdoor-Yacikos-Security-...

[Mr Pointy]

sidekickdmr said:

So if i understand that right

20 of these lights (60W)

https://www.amazon.co.uk/Landscape-Spotlight-Green...

Just to highlight that those lamps are 3W not 2W so the losses are considerably larger & from the extensive analysis above the voltage on the final lamp would be around 8.1v (about 70% the brightness of the first if they are linear). I would suggest that if possible you split the chain & feed the lamps with more than one power supply. It might not make a massive difference but it will help.

One thing that might save you is if the lamps aren't linear in brightness in whcih case they might be ok, but you'd have to buy a set to try.

[V8Rush]

Mr Pointy said:

Just to highlight that those lamps are 3W not 2W so the losses are considerably larger & from the extensive analysis above the voltage on the final lamp would be around 8.1v (about 70% the brightness of the first if they are linear). I would suggest that if possible you split the chain & feed the lamps with more than one power supply. It might not make a massive difference but it will help.

One thing that might save you is if the lamps aren't linear in brightness in whcih case they might be ok, but you'd have to buy a set to try.

I'd have thought the LED lamps would have a constant current supply inside for the ~3v that LEDs actually want, so the voltage drop may make no difference at all.

[peterperkins]

V8Rush said:

I'd have thought the LED lamps would have a constant current supply inside for the ~3v that LEDs actually want, so the voltage drop may make no difference at all.

Certainly that's the type you need to be getting.
Then forget all the voltage drop stuff.
It would run off a piece of 1940's bellwire.
OK just use sensible 1mm mains cable..

[Mr Pointy]

V8Rush said:

Mr Pointy said:

Just to highlight that those lamps are 3W not 2W so the losses are considerably larger & from the extensive analysis above the voltage on the final lamp would be around 8.1v (about 70% the brightness of the first if they are linear). I would suggest that if possible you split the chain & feed the lamps with more than one power supply. It might not make a massive difference but it will help.

One thing that might save you is if the lamps aren't linear in brightness in whcih case they might be ok, but you'd have to buy a set to try.

I'd have thought the LED lamps would have a constant current supply inside for the ~3v that LEDs actually want, so the voltage drop may make no difference at all.

Possibly, but it may not be a single LED - it could be four in series to add up to 12v, or there may just be a dropper resistor (less likely). On both the 12v & the 240v panels there can be many lEDs wired in all sorts of ways:
https://www.youtube.com/watch?v=uIspnsBp3o4
https://www.youtube.com/watch?v=C-AfmLggE28

All hail Big Clive.

Edited by Mr Pointy on Thursday 24th January 10:56

[V8Rush]

Mr Pointy said:

Possibly, but it may not be a single LED - it could be four in series to add up to 12v, or there may just be a dropper resistor (less likely). On both the 12v & the 240v panels there can be many lEDs wired in all sorts of ways:
https://www.youtube.com/watch?v=uIspnsBp3o4
https://www.youtube.com/watch?v=C-AfmLggE28

All hail Big Clive.

Edited by Mr Pointy on Thursday 24th January 10:56

Unlikely, but possible. I've taken a few of the chinese '12v ac/dc' ones apart and they've all had boards like this in
https://www.aliexpress.com/item/10W-DC-12V-24V-LED...

[sidekickdmr]

Problem solved, decided to go for commercial quality extendable 240V overhead festoon lights.

The extra cost of the hardware is offset by the lack of time/labour connecting and running cables

Thank you for the very detailed responses though, Pistonheads is great for this type of knowledge