# Cable size for low voltage lights

#### Sfrazi

I will installing 25 outdoor low voltage(12v) lights(mix of spotlights, down lights etc).

Lights are 5w each(total watts = 25x5=125watts).

I will be buying a 150w transformer which will plug into a waterproof socket and the 25 lights will run from one end of the garden to the other.

The last light will be around 30m away from the transformer as there is only the one mains socket so would like to know the size of cable I need to use to avoid a significant voltage drop.

Thanks

125W / 12V = 10.41667 Amp we will consider 5A as an average volt drop over length of cable. Volt drop is given in mV/A/m and we have:-
1mm² = 42
1.5mm² = 28
2.5mm² = 17
4mm² = 10
6mm² = 7
10mm² = 4.2
So taking 2.5mm² as an example 17*5*30 = 2550 mW or 2.550 volt.
Using DC lamps you can get LED lamps with a voltage range of 10 ~ 36 volt, so with a 20 volt power supply it would not really matter what the volt drop is as long as the voltage is above 10 volt they will work full output. Both this site and this site sell bulbs for caravans which are rated 10 ~ 36 volt DC, do note DC not AC. As to if worth using a smoothed DC supply is worth it is down to you.

LED lamps are not all equal, some used pulse width modulated controllers to limit the current others use a simple resistor so it really does depend on what lamp package you buy.

First is the 150 watt transformer a magnetic transformer ( wound coils ) or a Switched Mode Power Supply ( SMPS ). If it is a SMPS then there will almost certainly be a limit on the length of cables that can be connected to the output. Two reasons for this (1) they radiate high frequency electromagnet energy ( akin to wireless signals ) which interfere with radio, TV and other wireless systems such as alarms and home automation. (2) long leads can affect the stability of the switching in the SMPS and result in the voltage becoming unstable.

Assuming one can have a 30 metre long connection on the transformer's output then as Eric has said the size of cable required will depend on several factors.

Good point Bernard I had not considered the problems with HF output from power supplies, why they were ever called "Electronic transformers" I don't know.

I would recommend the DC route.

Also suppose we should point out 12 volt is extra low voltage, 230 volt is low voltage.

Voltage drop could result in lamps furthest from the power source being noticible dimmer than those close to the power source.

A solution is create a ring by adding a cable from the furthest lamp directly back to the power source. This reduces the variation in brightness and shifts the dimmer lamps to somewhere close to the middle of the run of lamps where their being slightly dimmer is not so obvious.

I would install an other mains suppy half way up the garden and use to transformers / SMPS at the point and split the load across them.

Regards,

DS

I will installing 25 outdoor low voltage(12v) lights(mix of spotlights, down lights etc).

QUOTE]

12 volts is not low voltage. 240 volt mains is however.
12 volts is extra low voltage. It has been mentioned (corrected) many times on these forums.

I will installing 25 outdoor low voltage(12v) lights(mix of spotlights, down lights etc).

QUOTE]

12 volts is not low voltage. 240 volt mains is however.
12 volts is extra low voltage. It has been mentioned (corrected) many times on these forums.

Good point Bernard I had not considered the problems with HF output from power supplies, why they were ever called "Electronic transformers" I don't know.

Because there are some stupid ignorant people in the trade. Much the same way they call plugs plugtops

Because there are some stupid ignorant people in the trade.
Ignorant as in un-informed yes, often due to marketing people dumbing down the description of items to make it easier to sell them.
But it is sadly too late to change the world so a polite informative to ill informed DIYers is all that can be done here

I have in the past used a real 12 volt transformer into a full wave rectifier and a smoothing capacitor, the voltage will show somewhere between the 12 Vac RMS and the 16.97 Vdc no load. Likely even on the furthest point with 2.5mm² cable it will still exceed the 10 volt minimum for good quality lamps as linked to.

but in real terms does it matter? If we look at a runway the brightness of the lights is important so current to voltage transformers are used with every lamp to ensure all lamps are the same. However with a garden even if they are dimmer at the end likely it will not matter, all it will do is make the garden look a little longer.

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