Outdoor cable for low voltage 12v garden lights

[SkipFeeney]

Hi,

I am planning to install low voltage lights outside running from a transformer.

Is 0.75mm pond flex suitable?

I will be running 5 or 6 1w lights over 20 meters


Thanks

 

[ban-all-sheds]

.75mm² cable has a voltage drop of 62mV/A/m.

You could knock up a spreadsheet to work out cumulative VD given the spacing of your lights, but even if all 6 were 20m away it would only be .62v, which would would surely be OK.

 

[Steven Prentice]

I guess a simple 'yes' is too much of a simple answer for you?

(rememebring of course in your spreadsheet to include for the thermal conductivity of where the cable will run rather than just the volt drop to give you the best ansewr.. which of course all DIYers should know able to calculate)

Just to save you potential propblems later, are you going to include some sort of mechanoical protection for the canle - perhaps to save it from a spade going through it?

 

[ban-all-sheds]

...

(rememebring of course in your spreadsheet to include for the thermal conductivity of where the cable will run rather than just the volt drop to give you the best ansewr.. which of course all DIYers should know able to calculate)

If it looked borderline, you could indeed use a lower conductor temperature.

But I would not expect it to be borderline

{worst case} it would only be .62v, which would would surely be OK.

...

 

[ericmark]

I know the regulations say 3% volt drop for lights, but in the real world depends on the lights, with LED lights rated 10 ~ 30 volt DC one can have a massive volt drop, but it has no effect to the lights, however with cheaper lights using a simple resistor as current regulator then it could where three LED's and a resistor are used be visibly dimmer through the run. Even with a simple resistor 1 LED and a resistor will not show volt drop as much as 3 LED's and a resistor.

Again why also matters, if simply to mark out path end light being dimmer will hardly matter. What often is the limiting factor is size of terminals, 6 mm² may stop one seeing the volt drop, but will it fit terminals?

On a runway, to ensure all lights are the same output, the transformers for each light are wired in series. Not the lights as then if one failed all would fail, but the transformers. In a garden if the lights are all the same wattage and are resistor current regulated I suppose you could wire in series and adjust voltage to suit, however would be better to use a proper driver and have lights in series then if any lamp goes short circuit it will not overload the rest.

But first question is volt drop really a problem for garden lights? I would tend to use suck it and see method.

 

[SFK]

SkipFeeney,
I have 25m of 0.75m2 cable lighting 5 x 3W LED lights without any issues (to my eye they do NOT seem to be getting dimmer). So you should be fine.
I was considering using 1.5mm2 cable as only slightly more expensive, but released that this size cable would be a pain to get into the tiny connector blocks on my LED driver, and felt bad unnecessary wasting Copper.
I have used this cable: https://www.screwfix.com/p/nexans-pond-flex-3183p-3-core-0-75mm-25m-black/8694t
And I have a better (but now out of date) write up of my system here:
https://www.diynot.com/diy/threads/decking-lighting-advice.466740/
SFK

 

[Steven Prentice]

So Eric and Ban-All-Sheds, is the answer still the simple 'yes' that the original question asks for?

I am not sure he will be building a landing strip in his garden, and probably not wanting to get out a text book and spreadsheet for something he might only do once in the next 10- years.

 

[SFK]

Steven,
I found the answer is actually 'yes...probably' and so Eric and Ban-All-Sheds comments are v valid and useful.

When I did the calculations for my 'long wire' 12V lighting system the numbers suggested it would NOT work - especially as LEDs have a sharp cutoff voltage so likely to be off rather than dim.

So once my system was setup I did some testing to see why it was working for me, and found that 1) My LED driver was kicking out 13V when under load (helping me) and 2) My LED lights had an internal circuit that operated down to about 9V.

So if the LED driver the OP chooses has a lower voltage under load and less tolerant LED lights, and he puts more lights at the very end of the wire, he may easily find it does not work.

So my 'feeling' is that 0.75m2 cable should be okay. but Eric and Ban-All-Sheds comments should be fully considered when buying the cable.

SFK

 

[Steven Prentice]

SFK - to be honest I think yours is the answer the question was looking for, can he do it, and from your experience, yes he can
You added a couple of details about what you found and why you did what you did which is helpful too.

 

[JohnW2]

SFK - to be honest I think yours is the answer the question was looking for, can he do it, and from your experience, yes he can

I'm not sure why you are assessing/'marking' everyone's responses - it is surely for the OP to decide which answer(s) are most useful to him.

It's very unusual for me to be writing in support of BAS, but, in the very first response to the OP, he said essentially the same as did SFK, albeit in somewhat BAS-like language. ....

.75mm² cable .... would would surely be OK.

You added a couple of details about what you found and why you did what you did which is helpful too.

Indeed, and BAS also added some details, explaining why 0.75mm cable would 'surely be OK', which the OP might also have found helpful ....

.75mm² cable has a voltage drop of 62mV/A/m .... even if all 6 were 20m away it would only be .62v, which would would surely be OK.

... and, given some other comments, also note that he indicated why use of a spreadsheet was not necessary, since even the worst case (of light positions) would 'surely be OK'.

Kind Regards, John

 

[Steven Prentice]

John, not wanting to get into a big discussion in someone elses post, but reading through the forum it often looks like some responses are not realy answering the question but more showing "I know more than you" type of answer, perhaps a game of one upmanship.

Here for example the Ban_All_sheds and EricMark answers start telling the OP about volt drops which is not what he is asking about (if he did and had the knowledge to know volt drop might be a factor he would surely have mentioned it in the question). Ban-All-Sheds brought up volt drop, EricMark toped this with regulations and volt drop.

Reading the question I believe that the OP wants a simple 'yes' or 'no' answer (neither of these answers gave him a clear answer "would surely be OK" and "suck it and see" ).. which is why I commented.

Help the guy out, he wants to know something other than 'I am cleverer than you' , should he go to screwfix after work this week to buy that cable or not.

 

[JohnW2]

John, not wanting to get into a big discussion in someone elses post, but reading through the forum it often looks like some responses are not realy answering the question but more showing "I know more than you" type of answer, perhaps a game of one upmanship.

Yes, there's a fair bit of that (in virtually every internet forum {or pub!} I've ever known!), but if you've been looking around this forum, you surely must realise that there are many (far, far) worse examples of that sort of behaviour than we have seen in this thread. In any event, ....

Here for example the Ban_All_sheds and EricMark answers start telling the OP about volt drops which is not what he is asking about ... I believe that the OP wants a simple 'yes' or 'no' answer .... which is why I commented.

Fair enough. Albeit there are no real 'safety' issues in relation to a question about 12V wiring, there are some people (particularly BAS) who strongly believe that people should not just be given answers/instructions to 'blindly' follow ("Yes, use that cable", "Put the brown wire in the top left-hand terminal" etc. etc.) without understanding the reason for that answer/instruction. I broadly agree with that, but try to achieve the best of both worlds by a sort-of compromise approach - giving both 'the answer' and a little 'education'. In other words, I might have written something like ....

"Yes, that cable should be OK. The main consideration is how much voltage will be lost in the cable ('voltage drop') and that depends upon the size and length of the cable, and the current flowing in the cable. In your case, with 0.75mm² cable, the worst case would be a voltage drop of about 0.62V, which would probably have no noticeable effect on the brightness of your lights".

[ In passing, I developed that approach decades ago when I was doing lots of oral and practical exams, because of the unpredictability of examiners. If one just gave the 'one word answer', some examiners would say "why are you jumping to a conclusion without first explaining your reasoning?", but if one started by presenting the reasoning, leading up to 'the answer', other examiners would say "Stop waffling - what is the answer?". I therefore developed the approach of 'giving the answer' and then, before they had a chance to interrupt, moved rapidly onto giving the reasoning/explanation! ]

Kind Regards, John

 

[SimonH2]

So Eric and Ban-All-Sheds, is the answer still the simple 'yes' that the original question asks for?

I side with them, because we (based on the information given) CANNOT say that "yes it will be OK" - because the factors mentioned will impact on the quality of "OK". As we don't know what type of LEDs and driver(supply) are proposed, we cannot know how much volt drop can be tolerated - in fact we don't even know that they are LEDs (we can assume so given the 1W rating but it's not stated).
The best way to avoid problems from volt drop is to use a current driven system - eg 350mA or 1A LED units with the appropriate constant current driver. That will, within the voltage capability of the driver, avoid all volt drop problems.