Installing outlet in electrical box with two sets of wires? - Home Improvement Stack Exchange

(apologies for possibly very newbie question) I found this setup in an electrical box in my downstairs closet underneath my stairs. It has two sets of wires that seems to be connected serially. My

HVAC Mini Split Systems

How to Secure NM (Romex) Cable to a Metal Electrical Box

Why is my Australian light fixture wired this way? - Home Improvement Stack Exchange

Mark N Guard® by Buddy Tools – Outlet Marker for Drywall Installation – Easy

How To Wire An Electrical Outlet - Addicted 2 Decorating®

Ceiling Box Too Low R/electrical, 57% OFF

How do tap into the power from this receptacle? - Community Forums

Caseta by Lutron Smart Light Switches, Controls & Dimmers

electrical - Tapping into junction box to add new outlet - Home Improvement Stack Exchange

AC WORKS 4-Prong 220-Volt Plug to 120-Volt 15/ 20Amp Household Female Adapter Cord (4-Prong Dryer Outlet to Household W/Breaker)

Tapping Into an Existing Outlet - Fine Homebuilding

Top 2 Things to Consider When Running Ethernet and Power Cable

Ceiling Box Too Low R/electrical, 57% OFF

Adding an Electrical Outlet to an Existing Run - Fine Homebuilding

Electrical Supply - Industrial Supply - Distributor