The photo at the upper left is a cut-away view of a Wire Nut and you can see the imprecise way this method of connection is supposed to work. (Click on any image to see a larger version.) Standard wire nuts are roughly conical in shape and usually have ridges on their sides so your fingers can get a good grip. Some types have little side wings instead of ridges. Inside the plastic cone is a little square-cut spring that provides tension on the wires to hold them securely. As you tighten the wire connector by twisting it, the spring draws tighter around the wires.
But recently I have found what I think is a superior way to join together 2 or more electrical conductors. It is the Wago line of LEVER-NUTS® which is available at Amazon, Lowe's, Home Depot, etc. In the photo at the right you can see how the Wago 221 model compares to a Wire Nut. The Wago connector is smaller, faster to install, able to accomodate multiple wire sizes including solid and stranded conductors, and in my opinion, MUCH more secure!
Wago Connectors were designed to make the splicing process easier without needing tools. The Wago clamp technology makes for faster installation and eliminates maintenance. All these connectors use clamps to terminate stranded, fine stranded and solid conductors.
Once the wire is inserted into the connector, the Wago clamp secures the conductor and encloses its insulation. This guarantees that the conductor does not come into contact with any live parts, which in turn increases safety.
There is an older model called the "222" which is not made of transparent material. I prefer the Model 221 which is transparent and you can clearly see whether your wire is properly positioned inside the clamping compartment. But, the 222 model does accommodate wire sizes as small as 28 AWG so those could be useful to connect smaller conductors.
If there is a downside to the Wago connectors, it appears that you can only install ONE conductor per section. Wire Nuts and Terminal Strips can accommodate more that one conductor for each connection but these Wago LEVER-NUTS® (for me) will replace the Wire Nuts for everything except the applications where I need a Terminal Strip.
Each Lever-Nut® has two test slots (or ports) that allows you to connect a meter test probe on the connection. COOL! You can see in the photo on the right how a Green test probe is able to slide right into the test slot to test the connection.
As I understand, Wago pioneered lever nuts and received UL certification on the first such device in 2003. Obviously I was a bit "late to the party" in discovering this product! Generic 221 and 222 style lever nut connectors can be found, but Wago is such a leader in this space that the term “Wago connector” is now applied as a general term. In reading the comments from various users it appears that some of the "knock off" or imitator connectors are missing some of the features or the quality of the Wago brand.
As you can see on the bottom left image, there are also "mounting carriers" available for all 221 Series Splicing Connectors (2, 3 and 5 conductors) and can be installed either vertically or horizontally. Secure the carrier by simply snapping it onto the DIN-rail or using screws on smooth surfaces. Flexible mounting clamps simplify both connector insertion and removal.
The "mounting carriers" also allow you to mount 2, 3, and 5 conductor Wago connectors to any flat surface by just screwing down the mounting carrier. You can mount them vertically or horizontally and the Wago connectors simply snap into the carrier.
The photo on the bottom right shows a cut-away view of the Wago 222 connectors. Both the 222 and the 221 models are UL approved and rated for 600 volts at 20 amps.
From my research, I would choose to ONLY use the Wago brand because of the quality issues I noted for the "look-alike" knock-off brands. As in most circumstances in life, "You Get What You Pay For!"
Here is a 3-minute VIDEO that demonstrates and describes the 221 Series of Wago LEVER-NUTS®.
I will be passing some of my assortment of Wago connectors off to my Grandsons for them to put in their tool box for future use. That's just what a Grandpa does!