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I am replacing a dryer outlet with a NEMA14-30, here is the manual) and the wire is 8ga stranded. I screwed the neutral in and screwed, and screwed ... but it never tightened. I also saw that the strands were not all nicely under the screw. Given that 30A will be flowing here I want this connection to be very solid.

Is there something missing here? If it were me, I'd like to surround the stranded wire by a small sheet of copper, so all strands stay together when tightening the screw. Do such things exist and what do I need to look for?

If not, how can it be that these connections are so clumsy in the first place? These squeeze screw connections seem to make it extraordinarily easy to make sub-optimal connections.

Here is a picture where it can be seen that the strands wedge are not fully under the screw. If it were solid copper, it would push it down and have nice contact.

enter image description here

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    When in doubt about your technique for screwed connections (especially so with stranded wire), just grab the wire with a whole hand (not just two fingers) and give it a hard tug (show the wire you really mean business; don't hurt your hand too much, though). If it moves (let alone comes apart), you can congratulate yourself for saving yourself the annoyance of a house fire. A properly tightened connection must not budge at all.
    – TooTea
    Sep 18 at 9:29
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    Just from the picture I cannot determine how the stranded wire is clamped in this receptacle. Is it possible that the wire is not supposed to be pinned by the end of the screw, but instead is inserted into a confining loop which tightens when the screw is tightened? Are you inserting the wire according to the instructions?
    – Jim Stewart
    Sep 18 at 16:45
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    @JimStewart I added the manual (leviton.com/en/docs/55054_Instruction_Sheet_EnFrSp.pdf). It does not say not for stranded wire. But it shows only the screw pushes into the wire.
    – divB
    Sep 18 at 18:06
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    @JimStewart Anecdata: solder flows over time in that sort of situation so it would gradually loosen.
    – Andrew Morton
    Sep 18 at 18:35
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    @JimStewart tinning stranded wires in screw terminals is strongly advised against electronics.stackexchange.com/a/29862
    – llama
    Sep 18 at 19:33

2 Answers 2

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If it were me, I'd like to surround the stranded wire by a small sheet of copper, so all strands stay together when tightening the screw. Do such things exist and what do I need to look for?

Yes. It's called a ferrule, and you need ferrules and a proper crimp tool to apply it. Proper generally implies ratcheting mechanism. Examples (no affiliation, but it's a tool I use and know) is Weidmüller Roto PZ-6.

The purpose is exactly what you state; ensure a good connection. In professional installations ferrules are the norm on stranded wire.

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    I'd say proper crimp tool needs all the emphasis it can get. A properly crimped ferrule is either firmly compressed from all sides into a round-ish (hexagonal) shape, or has several perpendicular notches pressed into it to get a solid grip of all the wire strands. Just squashing a ferrule flat with pliers is not the way and is often worse than no ferrule at all (since the ferrule then takes up part of the clamping force of the screw without transferring it to the strands, so you end up with a barely clamped wire and hotspots).
    – TooTea
    Sep 18 at 9:24
  • Thanks. Seems I can borrow a ferrule clamp from a colleague. Before proceeding, can you comment whether stranded wire with ferrule is definitely OK (and code compliant), given the receptacle in my question (leviton.com/en/docs/55054_Instruction_Sheet_EnFrSp.pdf)? Is it OK to put an AWG#7 ferrule on AWG#8 stranded wire? Strangely the common kits include AWG10 and 7 but never 8!
    – divB
    Sep 19 at 1:48
  • I'm pretty sure that ferruled connections are not approved for use with North American box screw lugs in the normal course of events (i.e. UL doesn't test that configuration at all)
    – ThreePhaseEel
    Sep 19 at 4:15
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    @ThreePhaseEel wow that's crazy. How is one supposed to do this up to code? Note that this outlet does not say it's not incompatible with stranded wire and furthermore, the thicker conductors, the more common stranded. My local HomeDepot had the #10 only stranded and the #8 is only available stranded (as far as I know).
    – divB
    Sep 19 at 17:27
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    @divB -- UL's performance tested the connection and as long as you use correct torque, you should be fine (excessive splayout could very well be a sign that you're overtorquing things though)
    – ThreePhaseEel
    Sep 20 at 3:23
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There are better quality NEMA receptacles (at least in NEMA 14-50) which corral the stranded wire. These are recommended for heavy duty use such as an EV charging connection. One recommended brand is Bryant. I got a Bryant NEMA 14-50 to redo my kitchen range connection (50 A breaker), only to discover it is rated for copper only. I have stranded aluminum and cannot use it unless I pigtail the aluminum or get a copper ferrule which is rated for aluminum conductor. Then I would need the crimping tool to make the proper crimp connection.

Twenty years ago I installed the NEMA 14-50 rated for copper and aluminum that has been in place since then, seemingly without problems, but I have not examined it since.

When I installed it I had the same complaint you describe -- the screw splayed out the strands and it did not tighten as hard as I was used to for solid conductor. I did not use a torque screwdriver. I did coat the aluminum conductor in Burndy Penetrox before makng the connection.

At about the same time I redid the dryer receptacle (30 A breaker) also fitted with a cable with stranded aluminum conductors (smaller diameter than the 50 A range circuit). For the dryer receptacle connection I pigtailed the stranded aluminum conductor with solid #10 copper.* This tightened hard.

In your case you have stranded copper conductor so you could use the appropriate twist on connectors to pigtail this with solid copper 10 AWG. Of course, a clothes dryer does not use use high enough current for long enough to require highest quality in this connection. If you would just tighten the connection according to the instructions (explicit or implied) and put it into service it would almost certainly perform acceptably. However, I do have more confidence in my dryer connection having been pigtailed with solid conductor.

*I used grey Scotchloks twist on connectors filled (by me) with antioxidant compound (Burndy Penetrox) to make the aluminum to copper connection. In my experience this will make a properly performing connection, but Polaris connectors torqued to specification would be much higher assurance of a non heating connection.

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  • Unfortunately there is no better one available for my application (surface mount, same dimensions, 14-30R). I searched a lot and asked here already. If you know of any please let me know.
    – divB
    Sep 18 at 19:25
  • So it would seem that you must either pigtail with solid 10 AWG using twist on connectors or try the connection you have with the screw pressing onto splayed stranded copper. Is the screw tightened as fas as it can go and the strands of wire are not firmly clamped? You do not need antioxidant grease for copper to copper, but it doesn't have any downside AFIK.
    – Jim Stewart
    Sep 18 at 20:44
  • Or crimp on ferrules or tin the stranded conductor with electrical solder (unless tinning is objected to by the experts here or just not approved of).
    – Jim Stewart
    Sep 18 at 20:48
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    Thank you. It seems I can borrow a ferrule tool from a colleague. Seems to be the best route.
    – divB
    Sep 19 at 1:45

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