I am putting RGB LED strip lights in a room, powered by a fairly heavy duty (30A) 12V transformer.

The transformer will also power a Raspberry Pi (volatage will be split to give it 5V)that will be used with a touchscreen to control light colour and function, and a motorised blind. There will also be a relay from the Pi controlling a couple of normal (mains) lights.

Given the "primary function" of the transformer is for lighting, I assume it should be on the lighting ring. (I guess the risk is with "forgetting" the motor etc is on the lighting ring.) Even though there is a high current draw at 12V, it is only 1.5A at 240V. There are only a couple of other lights on this circuit, so I think 1mm2 cable will be fine too.

Is this approach correct?

  • What is a "ring" in this context? Do you mean circuit, as in circuit-breaker connection? Or is there a physical switch you use to switch this off?
    – Fake Name
    Aug 19, 2014 at 10:37
  • Yes - a single circuit, with its own circuit breaker. i.e. Do I put it on the lighting circuit, or on the power circuit (i.e. where appliances plug in)
    – Ben
    Aug 19, 2014 at 11:06
  • PLEASE put your location in your profile! This is very important. Aug 19, 2014 at 11:51
  • In the UK, although "ring" is can be used to refer to circuits generically, especially power circuits (which are often wired as a ring circuit), a lighting circuit is usually wired as a radial circuit, not a ring.
    – John
    Aug 19, 2014 at 13:11

3 Answers 3


Is this approach correct?

Basically, yes.

1mm2 twin and earth cable (I presume that is what you are proposing) could carry up to 16A, depending on where it runs, so 1.5A is well within the headroom (assuming that you aren't talking such long runs that voltage drop becomes an issue).

For lighting, however, I tend to use 1.5mm2 in preference to cable 1mm2.


360W 50 Hz transformer is big and expensive. Conductive losses on 30A will be large at 12V.

A reasonable approach is consider 5% losses in distribution max.

For this you need the average length of cable carrying 30A from source to load. They should not be daisy chained more than 50W per FPC cable unless otherwise suggested by supplier.

Lets assume 10m for L.

use the following formula: R = pL/A

Where p is the resistivity of the material, L is the length and A is the cross sectional area. Copper has a resistivity of 1.72 *10^-8 ohm*m

Using 5% of 360W = 18W = I^2R, R=0.02 Ohm

Thus A= 1.72 *10^-8 ohm*m *10m /0.02ohm=8.6*10^-6 sq.m. Thus diameter of wire = 2.93mm, which is AWG9 !!

Therefore if you use 1 or 1.5 mm wire, it may calculate your losses and consider a star distributed wiring or loop wiring to each 60W section.

I would prefer to use a PC ATX supply with a SMPS instead of a transformer, because the LEDs only conduct during peak voltage mostly between 10 and 14V with series resistors on strings. This is only <1/3 of the sine cycle and thus the peak current will be 3x average of 30Amps, which implies even more distribution losses from this nonlinear load.

AC transformer rectifier is an inefficient solution and wire losses are significant for a 30A Zener like load.

P.s. if you are using a bulky 360VA transformer to power a couple strings, that is of course your choice, not mine.

  • Might have got my terminology wrong. The "transformer" I'm planning on using is this one: amzn.to/1Av6uIo. I think this is a Switch Mode Power Supply. Have to admit I didn't understand much of the above, but are you saying I should use SMPS like this one?
    – Ben
    Aug 19, 2014 at 15:09
  • Yes that is not a transformer but something that will work. Check the LeD specs carefully, some are rated for maximum power at automotive 12V which is actually 14.2 not 12V others may be spec'd for 12 V only. Keep the wires short to each string, not in a loop unless using very heavy wire for DC. The AC load is irrelevant and small. Aug 19, 2014 at 15:43

Yes wiring into your lighting is absolutely fine and would be a more complete solution as opposed to plugging the transformer into your wall socket.

One thing with these 30A 12V transformers is that they are usually open terminal so I would advise you to get an enclosure if you do not already have one.

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