soldering and shrink tubing NM wire

Question

With exposed NM-B, modern type manufactured after 2000, if you need to join two lengths of exposed NM-B wire then can you point out specific NEC sections, going from NEC ~2016 and later, where wording along with its interpretation, prohibits doing a mechanical connection of a black/red/white 14, 12, or 10 ga. wire followed by a 4% electrical silver solder and then using shrink tubing... for the shrink tubing that which is appropriate for a 90°C rating which i think is just about any (PVC, polyolefin, FEP), and then doing the same with NM-B behind drywall, instead of using a UL listed splice connector and used in accordance with NEC interpretation?

In this case specifically what in the NEC specifically would prohibit doing such a thing? And I am not talking about doing a haphazard mechanical+solder+insulation job; for example I have tested stranded wire by hanging on it from a rafter and the wire will fray and break anywhere other than a mechanical+solder joint (and even just a non-mechanical solder joint).

and for clarification: https://forums.mikeholt.com/threads/splice-kit-approved-for-behind-walls.128894/

put a screw through a wall and straight into a wire. They called out an electrician... https://www.youtube.com/watch?v=rEDHvVPm7fU

any kind of termination can fail if not done properly

https://www.youtube.com/watch?v=hFNE23rCfn0

NM splice demonstration

< https://www.youtube.com/watch?v=GWQCE_mGyFk

being realistic, NM wire inside a wall can get damage. So when it does, everyone here seems to always be saying... have to replace the entire run of NM you can't have a splice inside a wall. Really?

oh, and my point for asking... if "code" NM splicing requires a good mechanical connection of copper to copper for conductivity (which I don't disagree with) then... it doesn't look like those UL listed splice kits have copper connectors within that pierce the insulation to make the connection to the copper wire. Thus, I know "I could" perform a better slice, in many ways.

Answer 1

Technically current carrying conductors are allowed to be soldered (provided you can convince the AHJ that the connections were secure without solder).

110.14 Electrical Connections (B) Splices. Conductors shall be spliced or joined with splicing devices identified for the use or by brazing, welding, or soldering with a fusible metal or alloy. Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder and then be soldered...

But not the ground:

250.8 Connection of Grounding and Bonding Equipment. (A) Permitted Methods. Equipment grounding conductors, grounding electrode conductors, and bonding jumpers shall be connected by one or more of the following means:

(1) Listed pressure connectors (2) Terminal bars (3) Pressure connectors listed as grounding and bonding equipment (4) Exothermic welding process (5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut (6) Thread-forming machine screws that engage not less than two threads in the enclosure (7) Connections that are part of a listed assembly (8) Other listed means

And splices must be in a box:

300.15 Boxes, Conduit Bodies, or Fittings - Where Required. A box shall be installed at each outlet and switch point for concealed knob-and-tube wiring. Fittings and connectors shall be used only with the specific wiring methods for which they are designed and listed. Where the wiring method is conduit, tubing, Type AC cable, Type MC cable, Type MI cable, nonmetallic-sheathed cable, or other cables, a box or conduit body shall be installed at each outlet point, switch point, conductor splice point, conductor junction point, conductor termination point, wiring method transition point, or conductor pull point, unless otherwise permitted in 300.15(A) through (L).

Answer 2

NEC 110.14 Electrical Connections

Soldering lugs shall be identified for the material of the conductor and shall be properly installed and used.

This is clarified in a sub-paragraph:

Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder.

The simplest interpretation is that you must use soldering lugs, and since the material wasn't specified in the question, the lugs would have to be identified to work with the conductors.

Answer 3

334.10 allows concealed NM-B interconnects outside of junction boxes where the body of the conecting device is non-conducting and it is listed for the purpose.

I don't think you will find heatshrink (or tape) that is listed for making concealed splices in NM-B