How to Use Pocket Change to Determine Wire Sizes

What size is that wiring, anyway?

If you’re adding an outlet, you need to use wire the same gauge as the existing wiring. How do you tell if your old wiring is 12-gauge or 14-gauge? Here’s a simple visual. Twelve gauge is about the thickness of a nickel, and 14-gauge is about the thickness of a dime. Also, look at the breaker for the circuit in question to see if it’s a 15-amp or a 20-amp breaker. A 20-amp circuit requires wire that’s 12-gauge or larger.

Check out plans and instructions to change a single-pole switch to a 3 way switch.