Every splice in a permanent wiring system has to live inside an enclosure, and that enclosure has to remain reachable without dismantling part of the building. Those two requirements sound bureaucratic until you have chased a dead circuit through a house where somebody ignored them.

The reasoning is straightforward. A splice is a connection, a connection can loosen, and a loose connection generates heat. The box contains that heat and any arcing that comes with it. The cover keeps it contained. Accessibility means that when the circuit fails in twenty years, the next person can find the joint instead of opening walls at random.

Buried splices turn up constantly in houses that have been remodelled by enthusiastic owners. A wall gets moved, a cable is cut and rejoined with wire nuts, and drywall goes over the top. Sometimes there is a box under there and sometimes there is only tape. Either way the joint is now invisible, and it will be invisible right up until the moment it stops working.

Attics and crawlspaces produce a variation on the same theme, where a box exists and is even covered properly, but insulation has been blown over the top of it. That is still a buried box in practical terms. Marking the location, or mounting the box on a joist face where it stays visible, solves the problem at no cost during installation.

There is a related rule about the box being fully enclosed. A splice sitting in an open box with no cover fails for the same reason. So does a box mounted in a ceiling with the cover left off because a fixture was removed and never replaced.

When a circuit behaves strangely in an older house, dropping in and out with no pattern, the hunt usually ends at a joint somebody made in a hurry. Boxes that stay accessible are what turn that hunt from a demolition project into an afternoon.

Where a splice genuinely cannot stay reachable, there are boxes listed for concealment in specific circumstances, and connectors rated for direct burial in others. Those are engineered exceptions with their own listing requirements, which is quite different from a taped joint pushed into a stud bay and covered over.

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