I'm surprised this hasn't come up before but I did a search here and didn't find anything, so here goes.

I need to power a dozen 12VDC LED deck post lights, each drawing 1.6 watts. I want to place the transformer in a basement utility room where I have an outlet and run 20' of 12/2 stranded wire to the first light. From there, the rest of the lights are spaced out along 160' of the same wire. I know I need a transformer capable of providing at least 19.2 watts (12 lights x 1.6 watts) but I've seen some references to 300 watt transformers because of the wire length regardless of the number of lights. How do you calculate the current draw of the lights plus the resistance of the wire to determine the required transformer size?

  • a cheap 2A wall wort, like the ones that comes with routers and hard drives should suffice. – dandavis Dec 7 '20 at 7:35

How do you calculate the current draw of the lights plus the resistance of the wire to determine the required transformer size?

You do not need to

At that low of a current draw voltage loss will be negligible. The big concern about voltage loss with outdoor lighting was important with incandescent and halogen bulbs, not LED.

The calculator I used would put end-of-line voltage at 11.59 in your setup. Your LED bulbs will operate correctly down to minimum voltage required to power the fixture. Typically anything higher than 8.5 volts is acceptable; check your specific fixture.

  • Thanks for your comments. These are the lights we plan to install. I haven't found any technical specifications on minimum voltage, but I think they'll work. – RayW Dec 6 '20 at 17:27


The resistance of the wire will, if anything, reduce the amount of current/power drawn in a fixed voltage lighting system. The lights at the far end might be a bit dimmer as a result. It won't be a big deal, and using a higher wattage supply will make no difference at all.

A higher wattage supply at the same voltage will provide exactly the same voltage and current (thus, wattage) into the system as one of merely adequate wattage. A higher voltage supply will risk damaging the lights. And the resistance/current here are not enough to worry about anyway. If we caculate as if ALL of your lights were at the far end of 180 feet of 12 gauge wire, they'd see about 11.1 volts. If we put them all at 20 feet they's see 11.9 volts (assuming 12.0 volts into the system in both cases.) On average we might expect them to see 11.5 volts.

They'll work fine at those voltages, it's normal and expected in landscape lighting. If you wanted to oversize something that would make a very trivial difference in the performance of the system, upgrade the wire size. I doubt you'll be able to see the difference. (10Ga .vs.12Ga would raise the average voltage from 11.5 to 11.68 volts.)

If we assume that your light drivers are fancy enough (unlikely) to manage to pull full-wattage despite voltage drop, the additional wattage for the wire is all of 0.8 Watts, so a 20 Watt supply is still plenty.

  • I think 20 watts for the transformer is risky. You'd be running it at it's maximum capacity. I'd like to see some head-room both to reduce strain on the transformer as well as provide for expansion later. – George Anderson Dec 6 '20 at 15:41
  • A DC power supply that can't provide its maximum rated capacity is a piece of junk. Expansion was not mentioned in the question. – Ecnerwal Dec 6 '20 at 15:49
  • Just a difference of opinion. Around here, advice is almost always "go big" get more than you think you'll need (esp. when it comes to sub-panels LOL). BTW, aren't LV transformers for outdoor lighting A/C on the LV side? I don't think most of them get rectified to DC. – George Anderson Dec 6 '20 at 16:08
  • The lights we plan to use are made by LMT Mercer and they offer a 50 watt transformer that would work but seems pricey. Their "system" involves a bunch of fixed length patch cords and splitters I'm not fond of. I plan to run the standard 12/2 wire and tap off of it for each light. I'd rather just run any 20+ watt transformer than splicing into a proprietary system and voiding the warranty. – RayW Dec 6 '20 at 17:36

LED lighting is such a game changer for both low voltage and line voltage installations.

A bit of History: With LV wiring using old style incandescent bulbs at 12 volts required careful sizing and layout of wiring. Voltage drop was a huge consideration. Even an 18 watt bulb at 12 volts would draw 1.5 amps. Connect 10 of them on a run and you're drawing 15 amps! Common practice was to run the supply to the center of a run and limit the number of lamps to maybe 3 on each side. 12 ga would be the main supply and maybe 14 or 16 ga to the "sides runs". Lots of larger wire needed...expensive. Also transformers were "multi tap" meaning you had various voltages available (12, 13, 14, 15, etc). You would connect to a high voltage tap to account for voltage drop in a long run. Lots of planning and expense.

The LEDs you are proposing could easily all be run on one 16 ga cable, start to finish. A small (under 100 watts) transformer would be fine as well. You will be drawing less than 20 watts with your 12 LEDs, but having a say a 60-100 watt transformer would give you room for expansion later. Outdoor lighting can be addictive!

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