How to reliably secure stranded wire in terminal block?
(This is info already mentioned in the comments on the question — but nobody wrote an actual answer, so I'm doing that. It does also agree with my small experience.)
Do not use any solder at all. Use a wire ferrule, which is a metal sleeve that slides over the (straight) strands and is crimped in place, making a solid end that can be clamped securely by a screw terminal of the type you are using.
Caveats:
Use the proper crimp tool, which will leave a textured surface that is readily gripped by the terminal and resists sliding out more.
Adding the ferrule will increase the size of the wire end, possibly too large to fit in the screw terminal. (This should not be a problem if one of the options you have considered is doubling over the wire.)
This picture shows several sizes of ferrules, two ferrules crimped on 22-gauge stranded wire, and the crimp tool I used ($21 when I bought it).
The plastic part of the ferrule guides the strands in, covers any exposed metal, provides some strain relief, and identifies the size of the ferrule. Caution: ferrule makers do not all use the same color scheme!
Twist it together and double it back on itself. That is, strip twice the terminal depth, twist and bend over. Ensure that the wire then approximately fills the screw terminal. If it doesn't, you need a smaller terminal. There should not be any bare wire (stripped insulation) beyond the terminal itself. Good termination practise should be second nature.
Bad terminations may well initially work, but are liable to fail, heat up and perhaps cause fires. A good termination will last a lifetime.
Basic electrician termination training, this. At least it was when I did it about 30 years ago :)
You MUST NOT solder tin the whole portion that goes ino the connector. Tinning the VERY END is OK to stop it unravelling. See my prior answer on this here
A method of "strain relief" that works well but which is "very naughty" and which people will criticise is:
- Strip wire several times longer than depth of terminal block hole.
- Bend wire back over outside of insulation and wind it in a spiral so it wraps completely around the outside of the insulation several times and extends back by about the depth of the terminal block hole.
- Insert wire into terminal block and screw down clamp or screw onto combined wire end + insulated end.
The screw or clamp both makes contact with the conductor and also clamps the insulation rather than just the conductor. I'll add a diagram if this does not get run out of town on a rail.
I was shown this method decades ago by a man who designed/built/installed taxi meters in taxi fleet cars. I tried it. It worked.
This method is contrary to a number of things you'll have been taught. It does not seem to be an utterly terrible idea in theory and proves to be a good one in practice.
A more conventional method is to either add a sleeve which can be clamped to the cable end or run the insulated part of the cable just before the outer ends under a clamp which is screwed down onto the outer.