The DryConn Black/Gray waterproof connector medium size are the most popular brand of connectors for 12 volt lighting systems.  They are a corrosion and waterproof connector made especially for 12 volt landscape lighting.

This medium size wire nut fits most lighting applications and has a maximum voltage rating of 600 volts.  The connector size of 1-19/64 X 43/64″ for copper wire type make this suitable in most applications.  In addition, this connector features a temperature rating of 221 degrees Fahrenheit and a silicone sealant temperature range of -45 F to 400 F.  As a result, this brand will stand the test of time.

Check out the Large Black/Blue Connectors when a heavy gauge cable is needed.