Cable sizes in domestic lighting with 12V transformer

[frediaz]

If I have to increase the normal size of twin & earth cable in a radial lighting circuit to 4mm², do I also have to use 4mm² cable to the wall switches because one is not allowed to reduce the cable size from junctions?

The reason for the increase in cable size is that I put some 12V transformers for sets of halogen lights along the circuit and the current increases, if I am correct in adding the current of each transformer and its lights to the total current in the circuit. I am assuming that I am allowed to use 2.5mm² cable on the output side of the transformer (this cable satisfies the current capacity needed on the 12V side of the circuit)

There are two situations: switches operating lights in the circuit at 230V and the switches operating the 12V lights

 

[electronicsuk]

I really can't believe your lighting circuit requires 4mmsq cable, and even if it really *did* pull that sort of current then I wouldn't advise it, because you'd have a stupidly high rated breaker protecting transformers and light fittings not designed for more than a few amps.

Methinks you've made a mistake somewhere, perhaps you've confused current on the 12v side of the transformer with current on the mains side? Let us know exactly what you're running and then we can help you out.

 

[Adam_151]

Likewise, I think you are somewhat confused,

If I was to install 3 12v 50w halogen lights on transformers on a lighting circuit, I'd want to ensure my lighting circuit could handle an extra (3*50)/230 = 0.66A, which is exactly the same as if I was installing 3 50w mains halogen lights.

However, the cable from the trasformer to the lights would need to be able to carry (3*50)/12 = 12.5A and in doing so would need to drop no more than 12*0.04 = 0.48v so I might find it easier to use three cables from trasformer to lights rather than all in one line... actually it'd be better to install a smaller transformer for each light

Adding the 12.5A onto the 230v side current would make no sense, its wattage thats constant both sides of the transformer (losses excepted), not current... first law of thermodynamics and all that...

 

[frediaz]

That's what I thought, when I got to the circuit with the 12V transformers I started to get confused with the design current calculations. Here is what I intend to do:

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[Taylortwocities]

You've misunderstood things. I agree that the current on the 12v side is 8.3A.

Assuming no losses in the transformer (there wont be much) then assume u have 100 watts on the 230V side. The current drawn will be 100/230 = .43amps per light cluster. So 1mm or 1.5mm cable with 6A mcb will be fine assuming no higher lighting load ia also on the circuit.

 

[electronicsuk]

As Adam quite rightly pointed out, wattage remains constant on both sides of the transformer with the exception of losses incurred in the transformer itself, but these are negligible.

You have 340w of low voltage lighting, this equates to 340 / 12 = 28.33A @ 12v which explains how you came to your result, but on the mains side this is only 340 / 230 = 1.48A. Thus, your total current would be 1.48A + (0.43A * 4) = 3.2A

If you plan to wire the 12v output of the transformer to your fittings in a radial style then heavy gauge conductors might be needed if there's some distance involved. Better to wire in a star fashion with a cable from each fitting to the transformer, then 1.5 should be fine over sensible distances.

 

[Adam_151]

As you seem to spend more time on here than on msn... how did the interview go, Matt?

 

[frediaz]

So that's why people have not been put off installing 12V transformers with halogen spot lights.

Thanks

 

[electronicsuk]

As you seem to spend more time on here than on msn... how did the interview go, Matt?

It went very well, thanks. I should be finding out this week whether or not they want to offer me the job, although I'm still not sure I'll take the offer - I'm due a pay rise from my current employer in the next couple of months and taking the new job would effectively be a pay cut. I'd enjoy it more though, not to mention the prospects for future promotion are far better...