It it normal that turning on one LED strip reduces other one on the same 24V DC Power Supply?


I have a single power supply 600W, 24V.

I have two LED strips connected to this in parallel.

LED strip A (living room) takes 75 W when fully on and strip B (iota) fully off:

B fully on when A fully off draws 53 W:

When I switch both, the voltage (and therefore brightness of the both strips) is significantly reduced (100 W):

Is this behavior normal? I tried two different power supplies, and the result is the same. It’s annoying that someone in bed room setting the brightness of their strip influences living room strips. The power supply should have plenty of power to supply these.

To provide more on my setup:

  1. the living room ramp (A) has 4 white strips (in parallel)

  2. iota ramp (B) has 3 white strips connected (in parallel)

  3. they are both controlled by Shelly RGBW2 (but the behavior is AFAIK same even without these controllers being there). Shelly RGBW2 should have plenty of throughput for my usecase, that is 45W/channel, 280W/total.

  4. all the strips have the same wattage. It should be 10W/m, but what I observe is that when longer than that, they draw less wattage (to my regret - I believe it makes them less bright?). Specifically, a single channel in ramp B (iota) should draw about 35 W (because it has 3.5m), but it draws either 15 W or 22 W (another inexplicable behavior, see the screenshots below).

  5. I observe the same on a single controller as well (which makes sense given the previous), i.e. having just one channel on makes the strip brighter than when turning on three channels, but maybe slightly less so:

single channel: 15 W:

single channel, sometimes showing 22W:

three channels showing 40W (instead of 3x15 ~ 45W, or 60W)

Is any of this normal? Do power supplies simply behave like this normally and there is nothing I can do about this? Is it the normal LED strip behavior? Can I do anything about this? How can e.g. some infrastructure or home installation work? Thanks!

Sounds like you could be getting some voltage drop in your wiring.

You probably need bigger cables.

It’s hard to say for sure without a diagram of how they are physically wired.

A 600W power supply shouldn’t sag under 150W load. Unless it is a cheap knock-off that isn’t actually 600W.

Are you sure about 600W at 24V output? Because that is 25A and that is a lot. Is the wiring capable of 25A? Because that is what is used to charge batteries of a forklift.

Well. I first bought 1500 W from China, couldn’t really verify. Now I bought two different supplies rated at 600 W from a local company with good reputation. All the same behavior. All the supplies look visually quite well crafted, but that where my assessment ends. The one I am trying now is 24V 600W SLIM-24V-600W.

I like a lot of light (to fight SAD). So I bought tons of LED strips and made indirect light everywhere in the house. I wanted to use a single power supply so I don’t have to have 8 of them, hence it had to be strong. The living room ramp has about 28m of those 10W led strips, so it would be 280W only that, but I hardly get more than those tiny 80W out of it. This is the result… :sob:

All right, this is probably the culprit, I messed up :cry: . I am using CYSY 2Dx1,5mm2 everywhere (which is AFAIK equivalent of 16 AWG). And because of what I said, I am running long cables throughout the house. Specifically, it’s right now about 12m of this cable to the living room strip and then maybe 15-20m to the iota strip. Given this fact, I am getting too big of a voltage drop, am I right?

I tried this. Output voltage measured on the power supply terminals is 23.8 V (that’s expected, being 0.2V under 24V?). I then went up to go to the ramp_iota and measured how much it’s getting when everything is off, and it was 23.6V. Then I switched of the ramp_iota (three channels) and the voltage dropped to 21.3V (11%). I expect that this is the problem then?

Also found this table, it says that for a 300W I can run at top 2.1m long cable to be below 3% drop (I am on 11% on cca 18m):

Do you think this explains the behavior? Is it that when the voltage drop, the led strips won’t/can’t draw more power because voltage goes down and therefore they are less bright?

What are my options here? Is this:

  1. run bigger cables
  2. run shorter cables (probably by having more local smaller power supplies than just one huge)
  3. use power supply with e.g. 48V and then use step down converter to 24v before the strip


# 2m means 2m of cable 2D x 1.5mm2
power supply -> 2m -> 6m -> 6m  -> shelly -> 4 led strips
                         -> 10m -> shelly -> 3 led strips

strips are directly plugged into shelly via 10cm natively soldered wires. As the connections (all other -> symbols in the diagram), I use plain WAGOs.


You are confusing Volts and Watts. As long as your power supply is rated for more than the total power the LEDs could draw if they are all on at full brightness, you are OK.

Dimming is caused by a voltage drop. Voltage drop can be from long cable runs or bad junctions. Since you have not given a diagram of your wiring we can only guess. Measure the voltage at each junction. I personally don’t trust Wago connectors for high current DC connections. I would use old fashioned, properly installed wire nuts.

How are you measuring the power? Are you measuring the input or output of the PSU? Frankly, I wouldn’t bother because the LED strips will draw whatever power they need.

45w /Channel:
Led-strip i.e 7M 10W/ m = 70W

You didn’t mention the Kitchen length in total

However the 28m / 280W is as close to the 300w ( Total wattage ) in your example photo
Meaning a cable length for this is / from the power-supply is 2.1m ( with 1.6mm cable ) , and loss is then 3%

PS: It’s doesn’t say you need a 300W supply, it a table for ( Usage ) meaning , if your strips/lights needs/use 300W ( Total Wattage , of all your lights )

All right, here it is. Does it help?

At which part you think I am confused, please? I believe I get what you say, but in the same time, isn’t it true that the bigger the final device draw, the bigger drop is expected? Because I can see see that when testing it: the voltage drops when I start drawing power and barely before.

The measurements are visible from the Shelly dashboard device, but I believe I saw the same when I used my regular Wattage meter (that you plug in between the power outlet and the power supply).

I see. The table is from driver (shelly in my case?) to the fixture, which is very short in my case, so that’s not the case, thanks.

Nevertheless, the fact is that I do measure about 11% of voltage drop when I get to the LED strip, which is probably problematic, right? And what else it might be than wiring?

Both these LED strips are connected in parallel however in the top version the long run carries the current of both strips increasing the voltage drop to both, thus increasing the power loss.

Physical layout:

Yes, and it’s not to the led strip, it’s to the end of the road, it’s a nature law ( you should look at the online tools for calculation dimensions etc )

And i see you have 2 strips connected to 1 channel, meaning you are pulling the total strip-length( of both ) through that channel 10w/m X Meter X 2 ( you said the Shelly is max 45w / Channel ) … that’s equal to 5m x 10w = 50W

With your wiring with 1.5mm , you end up burning your house down , it’s not the Volt’s that kills you, it’s the Amp.

PS: the ( Driver ) is your power-supply , your Shelly is just a Switch

“When I switch both, the voltage (…) is significantly reduced (100 W)”

STOP measuring power (Watts). The LED strips will draw whatever power they need (up to the capacity of the PSU and wiring). What is important is the current (Amps) capacity of the PSU. According to the specs for the PSU you presented, it can deliver 24 Amps. They also note that if it’s on 8-hrs or more that you should derate the PSU to 17 Amps.

The Shelly could be limiting the power to protect itself. Your LED strips draw about 3A per strip. The Shelly spec is 3.75A per channel, with a total limit of 12A. Four strips would exceed that. You don’t say what LED strips you are using, so my calculations are based on your statements.

Your diagram is barely much to go on. Nothing is labeled. You are showing 4 LED strips per Shelly Channel, yet in your first post you state that in one area you have 4 strips in parallel, and in the other you have 3 strips in parallel. What do you have connected to the Shelly’s? One strip per channel which your drawing shows, or parallel strips per channel?

4 Strips in parallel will require 12 Amps, or 288 Watts. This is the maximum that the Shelly can service.

If you have two power supplies, I would not use a long run of power wires from a single PSU. I would split the power to two power supplies, one for each room. Do not put the power supplies in parallel. Switching power supplies sometimes don’t like this.

Yeah, that’s what I think is the problem, I have the “Bad” design from what you show.

I am not sure what you mean. All 7 individual strips are controlled independently, using ground to control switching, but they do share the relevant DC.

Even if I have all strips at maximum (which would be somewhat rare, but whatever), it would be 400W (280+120). On 24V it corresponds to 16 amps, am I right? From what I found, that seems to be an upper limit for 1.5mm2 cable, but still within the bounds. And 600W supply should be totally fine for that.

I am confused by this statement. Isn’t it clear that because the strips are visible dimmer, they are not drawing the power they need (I can also see that on the meter the power supply is plugged in)?

I am happy to improve my diagram. What is the thing that is missing except the lengths?

What I meant by saying “parallel” was that they are connected to shelly to independent channels. Maybe I am using that wrong. Whatever that is, what you see on the diagram is what I believe my wiring is, and I hope it’s what the official guide says:

They are indeed not running in parallel all the way from the PS as your “Good” label shows.

I have these.

I understand now that I will probably have to install multiple PS on more places.

Yes, that would be a good idea , like closer to the fixtures

Your latest picture, for sure dont look like (reflects) your hand drawing diagram, and now you also added ( a few meters ) to the equation ( thou still missing the distance to the strips, + whatever length they have , seems to be 10m long )

With this and your latest answers in mind , i really doesn’t have more to say

If a circuit needs 3 Amps, it will consume 3 Amps. No more, no less. But the LED is not the whole circuit. Yours has the Shelly controller, the wires and the Wago connectors. Each of those introduce a small amount of resistance and burn up some of the available power as heat. And as I said, the Shelly may be limiting the power to protect itself from overheating.

Measuring the power that the PSU consumes does not tell us much. You need a proper ammeter (most DVMs have Amp settings) to determine what the LED strip is drawing. You can connect a single LED strip to the PSU and use the DVM to determine the current draw of the LED strip without the rest of the wires, connectors and switches. It will never draw more power than this.


Your issue is in the single run of cable that is shared between both strips and runs back to the PSU. The voltage drop happens at the point the cable forks to the two strips. Make that a much thicker wire.

600W is a big PSU and assuming it is a reasonable unit and the voltage doesn’t change at its output you have ample power. Although adding more separate PSU’s would fix your issue it isn’t necessary.

watts = volts x amps
You can’t force something to take more watts other than by raising the voltage. It is the voltage dropping along the length of the ‘shared’ cabling that is your issue causing the strips to dim.

True, however if the top-right Shelly +Strips is the (Living Room) with 4x7m ( 10w/m ) , his pulling from first junction-point ( 15m, to shelly, then X-meter to the 4 X 7m-strips ) , in also a 1.5m2 wire , i believe there is quite some voltage drops there as-well
280W , 12 Amp i.e 20meter through 1.5m2
400W, 16 Amp i.e 8Meter through 1.5m2
Nothing adds up

Beside ( putting All on 1 card ) is not always a good idea ( and in this case , certainly high gambling ) , i would prefer that not All lights goes in dark, in LivingRoom and Kitchen, if i.e the PS fails ( Waiting a week for a new to arrive )

Thanks everyone who helped, it’s clearly the voltage drop on cables, especially the one going from the PSU to the first branching.

I gave up on having one strong PSU and running thick cables everywhere. I will have multiple smaller PSUs (I have good spots to put them at). The cables are in conduits, I could change them, but it sounds like more trouble than it’s worth it.

As soon as I shortened the cable length, increased the width of cables where possible, etc., everything behaves as expected.

A bit unrelated note:

As much as I value the help of everyone who has helped me and was patient and explained to me what’s wrong, I really dislike communication norms of “shouting orders” such as using uppercase letters with exclamation marks or being passive-aggressive. I am still glad that even those who do this helps, and it’s valid that given you are the one putting your time in this, it’s your “right” to set up the norms. But I also want to give feedback that it is annoying and doesn’t help at all. I myself am trying not to be like that when I am helping.

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Then you should see my handwriting , total mix of small / capital letters , In between each other , tilting forth and back ( totally chaos, many would say ) , beside i even use friendly-names etc. like this everywhere, not only in my HA. i.e LivingRoom, TechCorner, TechWall, Battery PanRoom, Nettto Price Month , All my Home Devices, etc etc, i could go on forever.

PS: yes and no, it’s related , i can also get annoyed … on people who get annoyed , or get irritated if / when other people i.e misspell words or not using the (correct) wordings etc , some gets annoyed or irritated on such, this to the extend so they totally loose the context , and neglects facts etc.
My advice to these people is, chill out. We are all stuffed and exposed with Capital Letters even in BeTween daily, in i.e all commercials here and there, which we seems to handle without getting annoyed, but sorry if this behavior makes you react in a negative way, i have since childhood been writing this way
Maybe i refused to follow the Norm ever since i started in first grade, or i for some personal reasons didn’t really care whether i spelled correct or had a ( Nice ) handwriting

You have marked this as complete so that’s great. Not sure if you completely understand why, but a brief look at your schematic and what should be remembered is that every electronic device on the circuit requires it’s own power to operate, this includes the shelly devices. Also since you only have 1 set of wires coming ftom your power supply these wires need to be large enough to handle the sum of all the current required by every device since this is the only circuit path for the devices to obtain power.

Hey. Yeah, I do understand that, thanks for checking though :slight_smile: .

I did buy several smaller PSUs, shortened the cable length where possible and tada, everything is working fine.

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