My knowledge of this is very basic, so I would really appreciate it if I can get some help. I am trying to connect 3 LED strips around a room in crown molding. The strips are 3528 LEDs, 12V DC, 1.5 Watts/foot, and are 16 feet each. I figured that to connect all 4 in a series I needed a power supply that is around 70-80 Watts, so I bought a 120 Watts, 10 Amp, 12V DC to AC adapter. When I connected the series, the last 2 strips were very dim, I am guessing because of voltage drop. I have never connected in parallel before but I am guessing that is what I need to do.

My questions are, what kind of wire do I need to run if my room is around 48 feet in perimeter? And will running it in parallel help with the dim at the end? And finally do I need to get a different power supply if I change it?

Thanks a lot!

  • 1
    Yeah, you shouldn't ever wire lighting in series unless you actually need the voltage drop.
    – isherwood
    Aug 8, 2016 at 17:44
  • 1
    @isherwood He's connecting them nose-to-tail but they're not actually in series. There's a + and - busbar running down each LED strip and everything's in parallel. (well, groups of 3 LEDs are in series with a resistor.) Aug 8, 2016 at 18:00
  • How is the dimming explained, then?
    – isherwood
    Aug 8, 2016 at 19:02
  • Voltage drop down the internal busbars of the LED strips. What are you expecting for $7? LOL They're simply not meant to go long distances without feeder. Aug 8, 2016 at 19:50
  • Thanks, I'm doing this for my baby's nursery and my knowledge with electricity is changing outlets and light fixtures, so I apologize if my explanation was inaccurate :) my husband's knowledge is also as bad as mine, so we've been trying to figure it out
    – May
    Aug 8, 2016 at 21:35

2 Answers 2


what kind of wire do I need to run if my room is around 48 feet in perimeter?

Check out http://www.calculator.net/voltage-drop-calculator.html. Plug in numbers until you get an acceptable value. For example, 14 AWG copper at 12 V and 2 A load (1.5 watts/ft * 16 ft / 12 volts = 2 amps) gives you 11.52 V at the end. If that's within spec for your LEDs then 14 AWG copper it is.

And will running it in parallel help with the dim at the end?

Yes. With 4 strips in series the total voltage drop across all 4 is 12V, so each of the 4 necessarily sees a much smaller drop (average 3V each because 12V / 4 strips = 3 V/strip). In parallel they would each see 12V and the limiting factor becomes how much total current the power supply can provide.

And finally do I need to get a different power supply if I change it?

Doesn't sound like it. Worst case your four strips require 64 feet * 1.5 watts/ft / 12 volts = 8 amps. Your power supply gives you 25% extra padding. Should be more than enough.

  • Those ampacity calculations are flat wrong in more ways than they are right. OP is slightly misdescribing them and you are repeating his errors, I sense a lack of familiarity with these products. Aug 8, 2016 at 18:17
  • @Harper I am not familiar with the products at all and have only the OP's numbers to go by. Substitute correct numbers as needed then apply the same logic (Use the calculator, if power supply > requirements = it's sufficient, this all holds. The rest is just numbers, feel free to help out by supplying the correct ones.)
    – Jason C
    Aug 8, 2016 at 18:29
  • That ampacity calculator is faulty; it is distorting the answer to conform with NEC rules for mains voltage (120/240/480). Aug 8, 2016 at 18:44
  • @Harper I see. Even with DC selected from the drop down, and 12 V entered in the text box? It's always agreed with my own measured values in the past. To me it looks like the only potential issue with my answer is just that the tail end of those led strips is also part of their power bus (assuming your description in your answer is correct, I have no reason not to believe it except the OPs observation of dimming) and so end to end isn't actually in series.
    – Jason C
    Aug 8, 2016 at 19:35
  • Thanks Jason, I honestly stated this is not my area of expertise at all. Perhaps my use of the word series was in inaccurate, I basically connected them end to end. The amp is listed on the box, so I am assuming it is correct.
    – May
    Aug 8, 2016 at 21:40

A rule of thumb for 3528's is 2 amps per 16ft (5m) section. They are rated to connect up two strips (not 3) nose-to-tail (4 amps through their internal bus-bars). I recommend limiting it to 3 amps (1-1/2 strips) but even then you'll have noticeable fade. To avoid it, you need feeder wire. 18AWG intercom cable is ample, as it is nominally good for 6-7 amps. Anything heavier will be difficult to attach to the strip ends anyway. I prefer 20.

Option 1: Ring (no feeder)

Wire the LED strips in a total loop. Always, black (-) to black... and red (+) to red. The power supply feeds the loop in both directions. Yes, there are redundant current paths. That helps carry the load and evens out fading.

However you'll readily notice fading between the power supply location and the opposite side.

Option 2: Ring, with feeders

Lay it out as in a ring, but also add 18-20 AWG feeder wire. (18 AWG intercom wire probably being the cheapest choice). Splice it in wherever makes sense - at the ends of the strip runs, or in the corners if you are cutting.

So imagine your room is a clock face with the power supply at 12:00. I'd run feeders to the LED strip junctions at 4:00 and 8:00. Or if you are cutting for the corners, I'd feed there instead at 3:00 and 9:00 (wouldn't bother with 6:00).

Since each LED is no farther than 1/2 strip (8 feet) from a feeder, there'll be no visible fading and you'll get max light out of the LEDs.

  • Thanks Harper, that is actually really helpful. Just so I'm sure I understand right, by a ring, you mean the beginning and the end of the loop is connected to the power outlet as well?
    – May
    Aug 8, 2016 at 22:49
  • 1
    Yup. So power has two ways to get to any LED. Mind you, loops like this are totally OK with DC power like this, but should not be used with AC utility power. Aug 8, 2016 at 22:51

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