user44866: I'm using an Arduino Uno to power some rgb LEDs. 
Each LED takes 20mA (on average). 
I have a strand of 400 LEDs. 
This should be drawing ~8,000mA.
Arduino Uno has a limit on its 5v pin of 500mA. 
**So how many amps is my Arduino outputting? My 8A worth of LEDs are illuminated, just dimly.
Edit:
My Arduino still runs, and the 8,000mA LEDs illuminate, just dimly. I’ve left the setup on for 12 hours, and it all still works.
So it appears the behavior of the Arduino Uno is to limit the amp output of its 5v pin. It does not burn out.
Any explanations for this observed behavior?
Edit 2:
I know how to wire up this setup correctly using power injection. I’m just curious why my Arduino is not burning out as we all expect it to.

user42539: If the 5V source is the barrel jack, (like from a 7.5V or 9V source), than you may be able to get up to 800mA to 1A from the 5V pin. There is a reverse polarity diode that is rated for 1A. The 5V regulator is only rated for 800mA. It does not have good heatsinking, and will overheat and shut before you see 800mA tho.
USB, well, you may get 500mA from that, if the source doesn't shut down first. If it doesn't, then the 500mA inline fuse should open.
You really need a seperate source for 5V for power to the strands.  Be sure to connect the Gnd from the source to the Arduino Gnd.
What kind of RGB LEDs are you using? WS2812B type LEDs?

44866: <p>I'm using an Arduino Uno to power some rgb LEDs. </p>

<p>Each LED takes 20mA (on average). 
I have a strand of 400 LEDs. 
This should be drawing ~8,000mA.</p>

<p>Arduino Uno has a limit on its 5v pin of 500mA. </p>

<p>**So how many amps is my Arduino outputting? My 8A worth of LEDs are illuminated, just dimly.</p>

<p><strong>Edit:</strong>
My Arduino still runs, and the 8,000mA LEDs illuminate, just dimly. I’ve left the setup on for 12 hours, and it all still works.</p>

<p>So it appears the behavior of the Arduino Uno is to limit the amp output of its 5v pin. It does not burn out.</p>

<p>Any explanations for this observed behavior?</p>

<p><strong>Edit 2:</strong>
I know how to wire up this setup correctly using power injection. I’m just curious why my Arduino is not burning out as we all expect it to.</p>
 42539: <p>If the 5V source is the barrel jack, (like from a 7.5V or 9V source), than you may be able to get up to 800mA to 1A from the 5V pin. There is a reverse polarity diode that is rated for 1A. The 5V regulator is only rated for 800mA. It does not have good heatsinking, and will overheat and shut before you see 800mA tho.
USB, well, you may get 500mA from that, if the source doesn't shut down first. If it doesn't, then the 500mA inline fuse should open.</p>

<p>You really need a seperate source for 5V for power to the strands.  Be sure to connect the Gnd from the source to the Arduino Gnd.</p>

<p>What kind of RGB LEDs are you using? WS2812B type LEDs?</p>
