Looking to add a 20 Amp electrical outlet right underneath my exterior panel using an available knockout and just want to confirm this is the right way to go about this. (Pics attached of panel) I am planning on mounting this right in between the two existing conduits exiting my panel. I am in Texas if it matters for code reasons. 

  
I need the following:

1. [Outdoor Weather Proof Box. ](https://www.homedepot.com/p/Commercial-Electric-1-Gang-Metallic-Weatherproof-Box-with-3-1-2-in-Holes-Gray-WSB350G/300832220)
2. [GFCI 20A rated outlet](https://www.homedepot.com/p/Leviton-20-Amp-Self-Test-SmartlockPro-Slim-Duplex-GFCI-Outlet-White-R12-GFNT2-0RW/205996739)
3. [Outdoor in use cover](https://www.homedepot.com/p/Commercial-Electric-1-Gang-Extra-Duty-Horizontal-Vertical-Non-Metallic-Weatherproof-In-Use-Cover-16-in-1-Configurations-Clear-WCW1PC/300849073)
4. [1/2 inch offset nipple.](https://www.homedepot.com/p/Commercial-Electric-1-2-in-Rigid-Metal-Conduit-RMC-Offset-Nipple-FRONZ-50-1/316096918) 
5. [Locknut for inside panel. ](https://www.homedepot.com/p/Commercial-Electric-1-2-in-Rigid-Metal-Conduit-RMC-Locknut-4-Pack-FRLNS-50-4/316097902)
6. [CH 20A Breake](https://www.homedepot.com/p/Eaton-CH-20-Amp-240-Volts-1-Pole-Circuit-Breaker-with-Trip-Flag-CHF120/204223325)r to match my panel. 
7. 12/3 wire. 

What I am confused about is the type of wire to get. Do I need THHN or THWN? Or is there a different type I should be looking at all together? 



Thanks in advance!

https://preview.redd.it/caiswnekgraf1.jpg?width=4284&format=pjpg&auto=webp&s=d99970090f9786e6feec5078e7381929881c0a65

https://preview.redd.it/7loxvmekgraf1.jpg?width=5712&format=pjpg&auto=webp&s=6033194092a18c1299653fce0eec24af03594de9

  


Advice on Adding Outlet.