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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

12V lighting in houses must be properly designed

In the Mailbag pages of the April 2012 issue, Gordon Drennan asks if it’s a crazy idea to disconnect the lighting circuit from his home switchboard and connect it to a 12V DC supply, to run LED lights in place of the existing 230VAC lights. I think the idea borders on crazy and should not be attempted.

I do agree with your comments regarding a 12V DC grid installation in the house for powering not just lights but also 12V appliances. Such a system has been successfully incorporated into caravans for at least 15 years and there is no reason why it could not work in homes. But it needs to be properly designed with correctly rated and protected cable and switches. You can’t just shoe-horn it into your existing house wiring without consequences.

I don’t believe that LED lighting technology has yet become as cheap as Gordon states. The Oatley Electronics lights that he refers to sell for $6 each and are a driver and LED kit which would not be an effective replacement for his room lights. LED globes which may suit his requirements retail for approximately $40 each.

SILICON CHIP comments that there are two problems using existing wiring on 12V DC rather than 230VAC – switch and wire corrosion and arcing/fusing of switches. I don’t entirely agree with the solution given. A suitable DC circuit breaker would protect against a short circuit but would not prevent arcing and rapid destruction of the AC-rated switches each time they are turned on or off.

I can think of several more reasons not to use the existing lighting circuit. Lights aren’t the only items connected to the lighting circuit. As well as lights there could be ceiling fans, combined exhaust/heat/light fans, dimmers, PIR sensors and most importantly, hard-wired smoke alarms. Upon disconnection of the lighting circuit from the mains none of these will operate, with possible disastrous effects in terms of the smoke alarms.

Cable used in lighting circuits is 1mm2 or 1.5mm2 and they are usually protected by a 10A circuit breaker. These circuit breakers are selected to match the current carrying capacity of the cable used. This current carrying restriction means that not very many 12V LED lamps could be wired into each circuit. For a 10A circuit, you could only have 120W of LED lighting. Count the number of lights in your home. Would this be enough?

Voltage drop is another factor to be considered. The voltage dropped by say 200+ metres of 1mm2/1.5mm2 cable from a 12V DC source would be substantial and I doubt that there would be sufficient voltage left to run many LEDs.

Although I agree with the concept I can’t agree with the proposed implementation. I hope no-one has attempted to try it.

David Anderson,

Pottsville, NSW.

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