Maybe OT electrical

Start off by realising that 20 watts at 12 volts requires a current of 1.67 amperes.

and 2.5mm (sq) cable is about 7 ohms per kilometre!

It changes it somewhat since voltage drop has a more significant impact on performance. However it does not change the maximum current carrying capacity of the cable itself since that will be dictated by the I^2 R losses.

So 20W at 12V suggests 1.67A. So in terms of the current carrying capacity of the cable, there is nothing to worry about from the cable's prospective.

However, voltage drop on 2.5mm^2 cable will be 18mV/A/m, so for a 20m cable we get 0.018 x 1.67 x 20 = 0.6V, or a 5% drop. For UK wiring regs that would be out of spec since only 3% voltage drop is allowed on lighting circuits.

If you must run the 12V on a long cable like that, then you will need at least 4.0mm^2 (which will be very very slightly out of spec, but not enough to worry about). Note we are also assuming there is zero voltage drop on the mains side of the supply.

(much easier to run mains most of the way, and stick the transformer / PSU nearer the lamp)

So 100 metres would only waste about 1V of the 12V.

That's for one conductor; twin would be 14 ohms/km, = 0.014 ohms/metre. 1.67A would drop 0.023 V/m, = 0.46V in 20m, about 11.5V at the load.

That's for one conductor; twin would be 14 ohms/km, = 0.014 ohms/metre. 1.67A would drop 0.023 V/m, = 0.46V in 20m, about 11.5V at the load.

Plus the same again in the other conductor ...

Assuming 0.0074 ohms/metre. 1.67A would drop 0.012 V/m, = 0.24V in

That's fine for DC, and near enough also at 50Hz.

At the 20-50kHz that an electronic transformer puts out, you'll get next to nothing out of the other end, because skin effect becomes very significant (each conductor looks more like a hollow pipe with much small effective cross- sectional area, because the electricity will only flow near the surface at the higher frequency).

I had to use 4mm² to carry 8 amps just 3 metres. In another place, 2.5mm² extra flexible wire (loads of really fine strands) worked too. (In theory, the stands should be insulated from each other in the wire, but in practice the connection between them was poor enough that it worked without worrying about that.)

You might also end up with a radio frequency interference problem if you attach long wires to electronic transformers.

I have also made use of the skin effect in another application. I wanted a trace heating wire against a mains water supply that I couldn't bury deep enough. I taped a

Christ on a bike.

The voltage NEVER drops to zero except on an infinitely long cable...or one of infinitely high restistance

I ran two 50w/12v spots off 2 meters of T & E - ring mains spec s*it.

They were noticeably dim, and still are...

Bollocks.

Skin effect at hundreds of Mhz perhaps. Not Khz

FMurtz explained :

Impossible !

The voltage cannot drop to zero, because at zero volts there would be zero current flow - zero current flow means no voltage drop.

With a 0.3V drop the only thing dim is TNP. What makes you think he is making it up?

That's just the resistive losses... some of these SMPSU lamp drivers have an output (either AC or rectified DC) of 10s of kHz, and so you can see several additional reactive ohms just from the inductance of the two parallel conductors in the T&E.

This poster is fairly technical (or thinks he is) and often buts in with this sort of stuf on very simple discussions and is being used to make me out to be an idiot for my statement, "

and "Stick to mechanics, current carrying capacity of cable is the same at different voltages and as long as you comply with ohms law you will be all right. "

Copper, room temp, skin depth 0.3mm at 50kHz.

I don't suppose there's any chance whatsoever that you'd admit to being surprised by that, but it caught me out when developing a power line comms system a while back.

Cheers

Yes, the Litz wire at longwave story is surprising. Above MF other losses dominate.