DC ring mains for lighting, revisited

Question

Traditionally ring mains for lighting have been (relatively) low current AC mains typically at 110/230VAC.

It makes sense these days to replace that with lower power, lower voltage wiring to more diectly drive LED lights (and wallwarts repolacements?).

The question being, what is the optimimum voltage? I assumed previously that 12VDC would be best but I am seeing lighting products and dev kits running from 48VDV (not to mention 24VDC).

Obviously, double the voltage, halve the current. But at some voltage point various regulations kick in (depending on jurisdiction).

Righ now, in my house, the ring mains is fused at 5A per floor. At 12VDC that would be a 60W limit for LED lighting. Just about enough for me, but 24DVC would provide 120W which is more than enough.

Are there any standards in place for this kind of repurposing of existing wiring?

Ultimately, I am looking to run almost everything from solar/battery

Answer 1

It makes sense these days to replace that with lower power, lower voltage wiring

I don't think it does. As you say, double the voltage, halve the current - so for going down from 240V to 12V you multiply the current by 20.

Right now, in my house, the ring mains is fused at 5A per floor

In the UK, domestic lighting is wired with 1.0mm or 1.5mm cable, which is safe for carrying at least 12A so there would be no problem increasing the fuse to do this. But using it to carry 10A over, say, 30 metres around a house lighting circuit is going to result in a voltage drop of about 13V in the cable alone, plus any additional drop in switches and junctions.

Because lighting circuits aren't wired in rings as standard, what you'd end up with is a completely different voltage at the start of the lighting circuit run compared to the end, massively significant for 12V or 24V LEDs. You'd have to fit much larger cable and that would offset most if not all of the savings in using low voltage lights; the massive loss of power in the wiring would offset anything left.

The other practical considerations are that it would be unrealistic to have low voltage DC for most household appliances, so you'd effectively have two separate power systems, and they'd need to be safely isolated from each other. And, of course, you'd need something to generate your low voltage, high current DC in the first place.

Regarding standards - in the UK, BS7671 recognises low voltage circuits as SELV:

Extra-Low-Voltage system (i.e. normally not exceeding 50 V a.c. or 120 V ripple-free d.c.) which is electrically separated from earth and from other systems in such a way that a single fault cannot give rise to an electric shock.

I'm not aware of any standard for repurposing mains wiring for SELV, and the earthing arrangements would need to be changed to ensure electrical separation.

Answer 2

In terms of best power per conductor cross section, you want the highest voltage practical, until some other consideration comes into play.

If you want touch-safe wiring, then 48v (or thereabouts) is what most countries in the world regard as safe.

While there are many products on the market designed to use 48v, you might find the fact that there are many more at 12v (for automotive use) sways you in the direction of using 12v, at the cost of more expensive wiring.

Unfortunately, there is no standard, and it's impossible to decide what an optimum is without bringing in your own subjective weighting of cost versus available products.

Also consider the voltage drop at the outlets. The cable area has to increase as the square of the current to keep the fractional voltage drop for any given power the same at the outlets (as the current is bigger and the voltage is smaller). So with a 12v system, you'd need 16x the copper area to achieve the same percentage drop as with a 48v system.