Hello,
Posted this last week: got no responses. Maybe it was too long lol 
System: Security and 'Home Automation'. All wireless with nrf24l01+.
Problem: Relay will be in a small project box with nrf24l01+. Box will plug into wall outlet, then AC devices can plug into box. This way I can control my AC devices around the house on my website.
How can I have this relay box that is plugged into the wall get power and always run while also being able to control the AC device plugged into my box? I know I could figure out a way but I am a beginner and would like to see what you pros recommend. I don't want to burn my house down.
I have 120VAC-->5VDC converters. I know how to use them and my system already uses them, so that's not the question. The main question is what would be the safest/best way to power my plug?
Thank you!
Original question has more detail: Home Automation - Wireless 'relay control box' - How do I power it best here? - Project Guidance - Arduino Forum
The main question is what would be the safest/best way to power my plug?
Use a relay (mechanical or solid state) with low voltage on the relay-coil. Relays provide isolation, which protects you and the Arduino from the power supply voltage.
You can buy or build a constantly-powered power supply to provide the isolated low-voltage. Any power supply you buy will be isolated, and if you build one with a transformer the transformer will provide isolation.
This way, you have isolation on the power to the Arduino plus isolation from the Arduino's output whatever is being controlled, and that keeps everything safe.
You can put some small amount of electronics (or a relay) in a regular-old [u]electrical box[/u] with regular AC outlets and outlet covers.
I've put a relay in an electrical box. I've also built 4 solid state relays in a 4-gang box with a power cord attached (I actually cut the female end off of an extension cord), keeping the male plug and the cable.) Then, there's a cable with a Molex connector for the low-voltage DC control. In the 4-gang (8 outlet) project, the Arduino and it's power supply are in a separate box. It is difficult to miniaturize this stuff. My control-box with the Arduino inside is actually bigger than the 4-gang power outlet box. It could be made smaller but making it "small" or miniature wouldn't be easy.
I've also built a plug-in 4-channel switch box by mounting two 4-gang boxes back-to-back with 4 regular wall switches on one side and 4 duplex outlets on the other side. Again this box has a cord that plugs into a regular power outlet like an extension cord. (This is just a switch box with no Arduino, no relays and no low-voltage.)