How many can you power directly from the arduino nano? I've checked posts online that each led takes about 60mA on full brightness. Going by this, if I want 8 RGB leds, the maximum current draw will be 480mA (just under the maximum current that can be drawn from a usb powered arduino, which is 500mA from what I've read).
However, I see youtube videos directly powering led strips of more than 8 RGB leds and they seem to work fine.
What am I missing here?
None safely
An Arduino board is not a power supply. Use an external power supply or power the LEDs from the USB if you must
Documentation for the Uno (and I expect the Nano to be the same) says that the maximum current from the board is 150mA, so one or two.
As Bob said though, they're not intended to be used that way - you need external power.
Thank you. I'll be rethinking my approach then.
Presuming you mean that the Nano is connected to a USB supply - because you should never be contemplating powering it via "Vin" for a serious project - the diode between the USB port and "5V" is I understand rated for 500 mA, so that might work.
But the diode also drops half a volt or so, so that is not such a good idea. Always best just to connect the LED strip directly to a suitably rated 5 V supply and connect the same supply to the 5 V pin on the Nano. At least you are not trying to use the inconvenient UNO!
I believe the maximum current for each WS2812 is actually 55 mA and use that figure for estimation, Note that they draw about 1 mA each when dark.
YouTube? You serious? 
I'm just really trying to add RGB leds on my controller (Edit: arcade stick), that's why I was thinking I can get away with not using an external power supply. Kinda weird having a controller with an external power supply aside from the USB power.
I just really started working on arduino projects. Watching YouTube gave me inspiration but most never explained the full idea behind what they're doing. Had some background in electronics but that was just an intro class.
You started by asking
How exactly do envisage connecting the LEDs to the Arduino ?
I guess I asked the wrong question (or didn't word it properly). English is not my native language.
For context:
I have a zero delay usb encoder (read on pc as a usb game controller). I wanted to add RBG lights to it, which I will control with an arduino nano.
There's a 5v pin on the zero delay usb where I connected the 5v of the arduino nano. The arduino is not really powered through usb but through the 5v.
Right now I have 8 3mm leds of different colors controlled by the arduino on 8 different pins. I added 460 ohms resistors to limit the current draw.
But my problem really is that I plan to change the 8 3mm leds to a WS2812b strip with 8 leds. But looking online, it seems it could never be done without an external power supply. Though I find it weird that there are usb powered deviced with lots of RGB leds that doesn't require external power (like mechanical keyboards and other gamepads).
Will connecting the WS2812b 5v and ground to the same wire to where I connected the arduino nano be enough? I guess how much the usb encoder board draws should also be factored in.
There is a difference between using power from an external source and using power from the Nano
If, for instance, you were to power the Nano with 9V to its VIN pin and then power the LEDs from its 5V pin, the current available would be limited by the capacity of the voltage regulator on the Nano
If, however, you were to power the Nano with 5V to its 5V pin and the LEDs from the same 5V source, which seems to be what you are doing, then the voltage regulator on the Nano will not be supplying current to the LEDs or even the Nano. In that case the current available would be limited by whatever the 5V source could provide but be aware that drawing current from it could make its voltage vary according to load and thus affect the Nano
Hello everybody! First poster here.
I'm in the very same situation. I made a LED sword for a cosplay project in the past and as I couldn't completely understand the theory and calculations to confirm if I would be able to power the amount of LEDs I needed (and was running out of time) I just went and try it, as I noticed the same in YT videos.
So I'm running a 43 WS2811 LED strip at almost full brightness (250) powered directly from an Arduino Nano, with a color wipe animation (LEDs are never at full brightness all at once) and it's working as a charm so far.
The connection is something like this (pic below):
USB Power Bank ---> Arduino Nano USB Port ---> Arduino 5V pin ---> LED strip 5V pin
The thing is, that now I'm starting another sword where I need 76 LEDs, so I'm a little concerned about toasting the strip or the Arduino, should I be worried?
Just to be clear I have little to none knowledge regarding electronics and electricity, so please take this as my personal experience and not as a suggestion to do anything the way I stated before.
Hope it helps!
Regards,
Kaldorey.-
This means the 5V does not have to go through the Arduino's voltage regulator, but it does go through the 'power selector' diode, which is basically just a small Schottky diode. It's usually rated for 500mA. Given that a single WS2811 draws something like 55 ~70mA at full power, this means 7 to 9 WS2811's maximum.
The fact that you're running 43 this way is a testament to the brave little diode that you're apparently running at way above its rating. It'll die at some point, obviously. Alternatively, you managed to get a clone Nano that doesn't have this diode, but that seems unlikely.
Given your circuit, there's no reason whatsoever to run the leds through the Nano. Just make a direct connection from the USB 5V to the led strip 5V to resolve the issue.
That sounds interesting! Do I have to draw it from the Arduino USB or can I draw it directly from the power bank?
Something like this:
Thanks for your reply!
Doesn't matter; it's the same electrically.
Hi,
Yes, BUT, I would be drawing directly from the 5V supply, before the Arduino if you are looking at Amps of LED current.
The Arduino PCB tracks may not be happy with that much current and you could damage/destroy the conducting tracks.
Tom... 
Hey Tom, thanks for your advice!
I guess I'll try something like this
Which is what you suggested if I didn't misunderstanded you 
In the dark the LED's may be way too bright at RGB = 255,255,255.
Setting max brightness to 25,25,25 will reduce (average) current by a factor 10.
A running light will use less current than all on at the same time. If only one color is on, current will be reduced by factor 3.
From experience I know that you may not need a power source rated for all LED's at max brightness at all.
Good news since you plan to use a powerbank.
Still recommended to not run this current through the diode (or worse: the regulator).
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