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A lot of people seem to discourage "backstabbing" wires (whether those be stranded or solid) to receptacles wherein an inserted wire could later be released by pushing a flat-head screw to some kind of snapping mechanism.

However, I couldn't find a lot of resources about another type of connectors seen in some receptacles wherein wires are also "stabbed" but are secured by a clamp kind of similar to the ones in screw type terminal blocks.

With terminal blocks, it is usually recommended to first crimp stranded wires into wire ferrules before fastening them into the terminal blocks. Is this the same case for clamps in electrical receptacles; or could I just fasten stranded wires directly without using ferrules? What's the best way to connect stranded wires to electrical receptacles?

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    A lot of the bad mouthing of back stabs are from they are usually a cheap spring knife edge to hold wire. They have a bad tenuity for the spring to weaken and tend cause problems, people don't check them till problem happens. The screw type clamps are found on the more expensive(better made) devices. I have a lack of knowledge about stranded wires for screw clamps, but stranded is not allowed for cheap back stabs.
    – crip659
    Commented Apr 24, 2022 at 13:44
  • Backstabs are not rated for use with stranded wire at all, and are not a good idea in any case based on many, many failures that somehow have not resulted in them being banned. As for terminal blocks, it depends on the clamp style of the terminal block.
    – Ecnerwal
    Commented Apr 24, 2022 at 15:34
  • According to UL RTRT.GuideInfo - Receptacles for Plugs and Attachment Plugs database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/…. Terminals of a receptacle are only permitted with crimp-on connectors if identified as acceptable by the manufacturer. Commented Apr 24, 2022 at 16:46
  • "it is usually recommended to first crimp stranded wires into wire ferrules" In what country? It seems that this is more of a EU/UK thing, and doesn't seem (to my knowledge) to be common in the US.
    – FreeMan
    Commented Apr 25, 2022 at 12:44
  • @FreeMan Not at all, as far as I can tell, for building wiring in the US. Quite common inside appliances. Commented Apr 25, 2022 at 17:12

2 Answers 2

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The clamps are great. Not only are they much better than backstabs, they are also better than traditional screw connections, especially for stranded wire. With a screw, it is easy for the wires to get squeezed out, even with solid but all the more so with stranded. With the clamps the wires go straight in and the clamps go straight down. Secure and reliable for any wire type.

At least in the example I found, with UK sockets the wire is placed between the end of a screw and a piece of metal:

UK socket

On the other hand, US screw and clamp uses a curved piece of metal on one side and a flat piece on the other side, as in this Leviton commercial grade receptacle:

Leviton socket

The curved part can hold a solid or stranded wire very well. Much better than the end of a screw (UK) or the head of a screw (cheaper US receptacles) or a spring-loaded backstab.

The clamp mechanism also allows for two wires very easily, unlike ordinary screws (generally only one per screw in the US) or backstabs (definitely only one). However, using both sides of a single screw clamp for wires only works well if the wires are the same type (solid or stranded) and the same size (e.g., 12 AWG or 14 AWG).

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  • I see. So ferrules are not necessary for these clamps?
    – Kosho E
    Commented Apr 24, 2022 at 15:01
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    No. And, based on other questions/answers here (but not on any personal experience or reading of NEC, so not authoritative), my understanding is that ferrules on ordinary wiring (120V and 240V switches, receptacles, etc.) is not a standard practice in the US, but is fairly common in other places around the world. Commented Apr 24, 2022 at 15:14
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    North American screw-and-clamp designs use a notched/bent "clamp plate" system that obviates the need for ferrules, unlike the flat-plate clamps used in other parts of the world Commented Apr 24, 2022 at 16:07
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    @manassehkatz-Moving2Codidact I'd probably put a ferrule on that UK module because grinding the end of a screw onto stranded wire is a recipe for tearing it to shreds. The clamps are ok on stranded because they supply flat, reasonably even pressure, but a bare screw will twist, tangle, and cut the strands of stranded wire.
    – J...
    Commented Apr 25, 2022 at 14:40
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    @J...that's exactly my point. The systems are designed differently, each with advantages and disadvantages Commented Apr 25, 2022 at 14:41
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We can't say.

What can say is the approved instructions that were approved with the equipment you are using.

All the major testing laboratories test a connector in every way it is likely to be used. With solid wire, with stranded wire, with aluminum wire, with a variety of sizes of wire.

If there is a requirement to use a ferrule - that is, if a stranded wire connection cannot pass safety testing without a ferrule, then the laboratory will require that the instructions or labeling say a ferrule must be used. Or they will require that it be labeled "Solid Wire Only" - seen on the backstab connections of every el-cheapo receptacle and switch in North America.

The electrical code requires that you use approved equipment, and that you follow the instructions and labeling.

That is how such questions are resolved.

Obviously this concept falls to pieces if cheap non-approved crud from overseas (or from the wrong end of the Silk Road) is used, which is why it is forbidden.

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