12 v led volt drop?

I'm having one of those moments where I cant work out anything.

12 volt led driver feeding garden lighting,  run of 40 metres is proposed. 80 watts total load, led driver is rated at 150 watts. Is it the 12 volt ampage for the calculation, or the mains? I am presuming the 12 volts for the calculation, which gives 7 amps current draw, which , on 1.5mm cable, gives a near 60% volt drop.

Does that look about right?

If so, it is going to need either mains voltage outside lights, or local transformers.


  • Hi, 56.5%, (6.67 V.) A 25 mm cable would be needed just to stay below 5%

  • Is that 40 m of lights, or 40 m to the first light please?

  • There are 9 different lights attached, all at around 5-10 watts each, all branching off along the route, the total length is 40 metres. Even with the first few being compliant, I don't think it will any good for the last few, as the voltage will be down to 6V or less.

  • There might be a bit of wiggle room, by use of larger cable, but not not as big as 25mm! and by increasing the supply voltage a bit.

    Find out what is the MAXIMUM acceptable voltage for the LED lamps, it may well be 14 volts. Starting with 14 volts gives a lot more scope for voltage drop than starting with 12 volts. Assuming a minimum of 11.5 volts at the lamps, with a 12 volt supply allows only 0.5 volts loss. Starting with 14 volts allows 2.5 volts or five times as much.

    Some ELV LED lamps are multi voltage, often accepting a range from 10 volts to 30 volts. Use of these multi voltage lamps can be very helpful, start with 24 or 28 volts.

  • Here is a typical multi voltage LED light. http://www.onsolar.co.uk/12V-10W-LED-Floodlight.htm Many others are available.

  • I have had a bit of a think about this one and I think that the answer is not easy. As you go along the cable, the load decreases, so the VD is not uniform. Moreover, what assumptions can be made about each lamp? If the voltage drops, do they draw less current (rather like a filament lamp) or does the circuitry try to keep the power constant?

    Then if you think about an LED headlamp or torch, they do seem to work with a significantly reduced voltage, albeit dimly.

    I suspect that Alan's original conclusion is correct: mains voltage cable and local transformers, which is rather like the way that DNOs distribute their power.

  • As a rule of thumb the distance for any system to change from being volt drop limited to cable current rating limited is (to within a factor of 1 or 2) the same as the voltage. For systems transmitting much power that also ends up being close to the economic limit for changing voltage as well

    So. 12m for 12V systems (ah.... now then !)

    230m (or maybe 400m) for LV mains,

    (ever wondered why 400/690 is used in bigger factories with sheds of hanger size ?)

    a few tens for km for 11kv lines


    You are well off the very hard end of getting impractical for 12V.

    Personally I would not try and send 12V that far and expect most of it to arrive, except for a very low current signalling system, Any chance of re-adjusting to be a series connection, or center  feeding or supplying at 110V or something ?


  • What about using 24v or 36v supply and lights?