Screw on a wire nut and the steel spring cuts into the soft copper wire.
The tighter you screw the wire nut, the stronger the steel spring holds the wires together.
How do you choose the right size wire nut?
> Does the nut cover past the copper wire?
> Is the nut tight enough that it will never fall off?
If you answer 'yes' to both questions, you have the right wire nut.