Buried junctions (or anything close) should be soldered.
For safety, keep all junctions in grounded metal boxes and use
undamaged solid wire of the correct wire guage and use cable clamps
that fit.
There are many situations where wiring may be difficult to
access. Most notably, it is difficult to access the wiring inside the
junction boxes normally included several inches behind the can of a
recessed lighting fixture (which itself is behind one or more pieces
of trim mounted on drywall). As another example, wire splice kits
labeled "for use behind drywall" are available at Home Depot. Outlets
in hospitals may be inaccessible due to connected life-saving
equipment. Traffic sensors are sealed underneath the pavement. In all
of these situations, splices should be soldered.
All of these situations can be handled legally and safely. Legally,
NEC accessibility rules concern structural obstructions. Where the law
is vague, many jurisdictions explicitly waive NEC accessibility rules
for light coverings such as drywall, asphalt, etc. As for safety, an
inaccessible box is generally safer than an accessible one. Especially
if small children are around. A metal box is more puncture-resistant
than the cable it replaces, and fire does not care if your box is
accessible to a human or not. Safety is a non-issue, but legal
questions sometimes arise. Ask your lawyer to be sure.
Unfortunately, most electricians only know how to splice wires by
twisting and nutting. Such junctions operate only through exposed wire
surface (while solder forms metallurgical bonds). All exposed wire
surface eventually corrodes. The corrosion is accelerated by sulfur
gases from drywall which gets through openings in the tape. Some
quick-connects used in recessed lighting fixtures are better than
twist-and-nut because they maintain extreme pressure against the
wires, which cuts through the corrosion. But beyond that, most
electricians just twist-and-nut.
Even worse, many electricians will use a very narrow definition of
"buried box" which excludes anything that you might be willing to pay
for (like recessed lights, so that they can steal your business
without soldering), but includes anything that would save you lots of
money (like repairing a small segment of a long buried cable). Report
unscrupulous contractors to all applicable agencies.
When I hire electricians, I first ask if they do high-reliability work
for hospitals. If they say yes but they don't solder, then I just call
their bluff (the county licensing department usually gets very
interested).
- junction_expert
25 years electrical AND electronics experience