3.3v 1](2 5v
3)(4 5v
5)(6 GND
7)(8
GND 9)(10
11)(12
13)(14 GND
15)(16
3.3v 17)(18
19)(20 GND
21)(22
23)(24
GND 25)(26
27)(28
29)(30 GND
31)(32
33)(34 GND
35)(36
37)(38
GND 39)(40
- Voltages -
The Raspberry Pi can provide both 5V (pins 2 and 4) and 3.3V (pins 1 and 17) power. It also provides the Ground pins (Ground or GND) for circuits on pins 6, 9, 14, 20, 25, 30, 34, and 39. These pins are all electrically connected, so it doesn't matter which one you use if you're wiring up a voltage supply.
3.3V pins - Anything connected to these pins will always get 3.3V of power
5V pins - Anything connected to these pins will always get 5V of power
Ground (GND) - 0V, used to complete a circuit
There is no single answer to how much current the 5V power pins can draw as it is reliant on what power supply you are using, and what other components you have attached to your Pi.
The Raspberry Pi 3 will only draw 2.5A from its power supply, and requires around 750mA for boot up and normal headless operation. This means that if you are using a 2.5A power supply, the 5V pins can supply a total current of around 1.7A maximum.
The 3.3V supply pin on the early Raspberry Pi had a maximum available current of about 50 mA. Enough to power a couple of LEDs or a microprocessor, but not much more. All Raspberry Pi since the Model B+ can provide quite a bit more, up to 500mA to remain on the safe side, thanks to a switching regulator. Still, you should generally use the 5V supply, coupled with a 3.3V regulator for 3.3V projects.
- Ground -
The Ground pins on the Raspberry Pi are all electrically connected, so it doesn't matter which one you use if you're wiring up a voltage supply.
Generally the one that's most convenient or closest to the rest of your connections is tidier and easier, or alternatively the one closest to the supply pin that you use.
For example, it's a good idea to use Physical Pin 17 for 3v3 and Physical Pin 25 for ground when using the SPI connections, as these are right next to the important pins for SPI0.
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