What kind of wire do I need for 3 LED strips?

Question

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!

Answer 1

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.

Answer 2

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.