Caused some smoking/melting wires looking for ideas of cause on an arduino uno

So I have 12 5v led strips of 40 leds with only 16 leds on at any one point (so not much of an amp draw) on a Arduino Uno. I plugged everything in after uploading a new set of code and the ground/5v wire started smoking/melting once I turned the power on. I hurriedly unplugged the power and seen it pretty much welded the power/ground wires to each other. I used the jumper wire that came with my arduino (a really thin gauge wire, but fit into the arduino pins) and they were soldered into a 18 gauge wire to deliver the power to all the strips. The jumper wires were melted but the thicker gauge wire did not show any melting/bubbling etc. I had a capacitor on the power line and resistors on the data lines of the led strips.

The power is coming from a PSU's 4 pin molex rail (which has 4 wires, 5v-g-g-12v). I tested this overnight and everything was fine. It was only after I unplugged everything and replugged everything in then repowered it that I had this issue.

The only thing I could think of possibilties: 1. I accidentally plugged the jumper power line into the 3.3v pin instead of the 5v on the arduino. Would this cause this issue? I assumed only a higher voltage would cause this possibly? 2. Extremely unlikely but the ground and power made a connection at a point that I covered with electrical tape paste. 3. I plugged the molex in backward (unlikely as they can only be plugged in one way but I know they can be brute forced in upside down which could make it send 12v down the 5v line.

I cut off the melted jumpers, resoldered new ones in, made sure the solder points were covered in tape/hot glue to double up on making sure no contact can be made just in case. The LED strips still run (albeit one strip is not running, I need to do some testing on it to see if its dead), and 1 led on one of the other strips glows green when it should be off. The arduino seems to run fine also as testing the strips is working.

Thanks for any insight, just trying to figure out what happened and not burn my house down.

You were trying to power everything from the Arduino ?

I am confused.

A circuit diagram is required!

I'll get one up soon, sorry about that

You don't have to do anything fancy.

You can draw the circuit on paper and take a photo of it.

sorry for the delay, I work midnights and had to get some sleep. Heres a gif of the lights working before the incident:

I ran in like this for 1 day. Then unplugged everything and ran some new code to test with. Once putting it back in this config and turning on the psu is when the lines melted. Everything is 18 gauge wire except I used male to male jumpers to go from the arduino to the 5v/ground lines coming from the PSU's Molex plug. Heres the schematic:

Obviously the issue is at power " fanout".
the VR or Arduino :
According to the Arduino documentation, the Uno uses a 5-volt linear voltage regulator in a TO223 SMD package
Ams1117 Lm1117 5v 1a
if your leds all together were under it- then the problem in poor wiring

Most LED strings have the capacitor between the positive rail and the negative rail (using correct orientation of course).

Some power supply's are not capable of CONSTANT CURRENT and or VOLTAGE so you should check it under a known load.

I have a few here that say 1.5 A but that will drop rapidly when even loads of less that that are applied.