Are wire splices actually fire hazards?

Question

I'm redoing wiring during a remodel and am reading about burying boxes behind drywall, and how it's a fire hazard and not up to code etc. My question - Are wire splices actually a fire hazard in and of themselves?

A lot of responses regarding why to not bury a junction box include that it's a fire hazard. Is it really though? Lots of light fixture, switch boxes, and outlets have the same type of splices with wirenuts going on, but I don't hear these being referred to as hazards?

I understand burying the box is bad (had this discussion in a different thread), but I want to know specifically if burying a junction box with wire splices is a fire hazard and why? Is it just that if something did go wrong, you wouldn't have visibility to see that it was going wrong, vs a box that you can see?

Answer 1

It's a fire hazard because there are a lot of variables and the code (and remember that the code is the law) errs on the side of safety.

EDIT: Consider this answer to another question that was asked today: https://diy.stackexchange.com/a/86019/24137

One answer referenced old knob and tube wiring being spliced inside walls and never showing a scorch mark in 100 years of use.

I'll buy that, because the connections were very solid, professional connections.

But also consider that k&t splices are soldered together in addition to being crimped and/or twisted together. Those soldered connections also protect the conductors within the splice itself from corrosion since oxygen is sealed out (both copper and aluminum wire oxidizes), and from movement. As wire changes temperature, it expands and contracts and this can loosen connections.

Also, it's possible that somebody will pull on one end of the cable or the other from a box, and a wirenut connection is more likely to fail than an intact run of wire is in that situation.

Compared to soldered connections, wire nuts are straight pressure connectors and it's far more likely that a wire nut connection will become loose and overheat or start sparking than it is for a soldered connection to do so.

Also, what if somebody does something like connecting a copper and an aluminum wire with a wire nut? Even the purple wire nuts specifically designed for making those bimetallic connections have a notoriously high failure rate (there are clamp-type connections that are better). Copper and aluminum expand and contract at different rates as the temperature changes, and this can really loosen the connection. Additionally, bimetallic connections like that are subject to greater levels of corrosion, and aluminum oxide is an insulator. Forcing current through an insulator generates a lot of heat (fire hazard). If that kind of failure happens inside a junction box, you'll smell it and be able to find and access it much more easily than if it was sealed up inside the wall. Additionally, the electrical box itself has some nominal fire resistance and code says the box has to extend far enough out to the surface of the wall so that no flammable materials are exposed around the edges of the box.

There are more reasons than these, of course.

So it actually is a fire safety issue, as most of the electrical code is.

Answer 2

The reason you can't bury a j-box is not because the splices are a fire hazard.

It's because it makes it inaccessible for future work. This is a violation of the National Electrical Code and probably any other code around the world. It also violates the law of common sense.

If the joints are a fire hazard they weren't made properly.

Good luck!

Answer 3

You answered it quite well at the end there & that's the law's reasoning as well. But "fire hazard" no, not in the slightest! That's all just fairytale nonsense! Anyone who says otherwise, needs to explain how fire isn't jumping out of every light, outlet & switch box in every building in the world. ONLY IF that were happening would concealment be a "fire hazard".

A "solid" splice is no threat to anything. But yes, a poor & loose splice can arc & create some very localized self-damage. Of course, not always & there are plenty of fires very falsely "attributed to bad wiring". Typically, these are ultimately determined to have been caused by mishandled or defective appliance as well as failed circuit breakers. Usually, if the circuit breaker doesn't trip the fighting wires melt away from each other in just a few arc's, the event's are not the TV & Movie stuff.

I've had 3 places of 90-years old with Knob & Tube wiring & their bevy of low-temperature soldered concealed splices & split circuits & not a single scorch mark, overheating melt nor any defect, failure or threat in the slightest.

ALL of those were places where Window AC's, Space Heaters, Big Vacuums, Hair Dryers & Microwaves were used regularly, for decades. The system wasn't designed for any of it & they all handled the outright abuse for decades like real champs. No melted splices, no burnt off insulation, no fuse box damage & no wires melted apart.