"Fried" wirenut -- how?

[Percival P. Cassidy]

Pulled a switch to replace it and noticed that

(a) the switch brought one black wire with it but left two other wires (one black solid, the other white stranded -- the bare ends were clearly visible) in the box;

(b) the temperature in the box seemed high -- decidedly warm;

(c) there was a charred black mess that seemed to be "cooked" insulation tape with a small area of something red and hard at the end where the wires went in and the end of a screw sticking out of the other end of this mess.

As I cut away the insulation tape, it became clear that this was a "fried" wirenut, and that what had looked like screw threads was the wire spiral that was gripping (or was supposed the gripping) the conductors.

The first time I ever saw a wirenut, I thought "What kind of a Rube Goldberg kludge is this?" but I now understand that they are NEC approved. So what would cause a wirenut to "fry" like this? Note: I had never noticed any problems with the lights that the switch was controlling, or with any other circuits fed through the wires being held by that wirenut.

BTW, I think the white stranded wire must have been the ground wire to the metal "frame" of the switch. The wirenut fell off the ground wires before I had a chance to take careful note of where the other end of the white pigtail went.

Perce

[Bob]

prob didnt twist the wires together like u r supposed to

[Percival P. Cassidy]

The advertising for some wirenuts (don't recall which) says that twisting isn't necessary. But I have no idea whether it really does vary from brand to brand.

Perce

[Ralph Mowery]

If the wirenut is not twisted on tight there will be a high resistant joint and it will heat up. How hot depends on how much current is being used and how much resistance is in the joint.

I work as an electrician in a large plant and there are thousands of wire nut joints. Most work just fine. We very seldom have any problems with them. We almost never use any tape on them. Sometimes I will put some tape on the motor leads or other devices that have some vibration.

[Reed]

Is the wire aluminum vs copper ? This can and does happen with AL wire. Twice in my house. :-(

Warm cover plates are first clue. Flames are second clue.

[Percival P. Cassidy]

All conductors are Cu.

I'd never noticed that the cover plate was warm.

Perce

[Percival P. Cassidy]

And I forgot to mention (d) the back of the switch body looks to have had a hot (temperature-wise) insulated wire pressing against it.

Perce

[John Grabowski]

Most likely a loose connection caused the conductors to arc which generated the heat.

[Percival P. Cassidy]

Shouldn't the arc-fault breaker have caught this? And wouldn't the lights and other devices fed from that circuit have behaved erratically?

Perce

[AZ Nomad]

It was a loose connection. Power dissapation at the connection is equal to current squared times resistance. Let resistance get to be more than milliohms, and the connection burn up.

[bud--]

AFCIs until this year would only detect a "parallel" arc - H-N, H-G. Starting 1-1-2008 they were required to detect a "series" arc you describe.

If there is an arc you will probably see flickering lights. Some other loads there may be no indication.

As others have said, a loose connection can also produce damage (with no flickering). An arc should give you more heat.

If there was a lot of tape I suspect workmanship. There should be no need of tape with a wirenut. May also have been the wrong size wirenut or a junk wirenut that was not UL listed.

There are a lot of wirenuts I won't use. They are hard plastic outside the spring. The kind I like allow the spring to expand over the wires and have more contact area.

There is no requirement I know of (manufacturer/UL/code) to twist the wires before using a wirenut. It may be a good idea, particularly with the springs that don't expand.

[w_tom]

Let's say wires are not twisted. Then the only reliable connection between wires is the 'spring' inside a wire nut. If spring wires makes contact with every copper wire, then no problem. But with heat, expansion, oxide of copper surfaces, etc, only that wire nut spring carries most (too much) current. Now the wire nut gets hot and wire connection create voltage reductions.

Normally that still is not a problem ... if a wire nut 'spring' has enough turns and makes contact with each wire in enough locations (if wire nut does not require tape to hold it on wires). Sufficient connection does not always happen. A hot wire nut is not a code violation. But you don't want a hot wire nut. Potential for fire or electrical failure increases.

With copper wire, this rarely causes a fire. With aluminum wire, this same poor workmanship too often causes fire. See pictures and technical explanation in:

We don't want wire nuts getting hot even if the manufacture says it is OK. If wires are twisted together, then all wires touch each other in multiple spots without a wire nut. Then add a wire nut to make a good connection better. Wire nut does not get hot by conducting all current IF wires are properly twisted together.

Wire nuts on copper wire should be properly twisted as if the wire was aluminum for a best conducting, low loss, no heat connection.

Does not matter whether code requires wires to be twisted together. All wire connections should be sufficient without the wire nut. Then we add a wire nut to keep that connection sufficient no matter how much corrosion or heating might compromise the connection 80 years from now.

[bud--]

The contact is wire-to-wire.

With aluminum wire surface oxide may prevent wire-to-wire contact. The wirenut spring is not intended to be a major current carrying element.

Not seen - requirements from any manufacturer, or UL or the NEC that wires be twisted. Apparently none of them are as smart as w_.

As with many other issues all you have is w_?s certainty.

[w_tom]

Which is why wirenuts do not have that metallic spring inside? Wire nuts using only grooves in the plastic cannot be used. Wire nut uses a metallic spring sufficient sized to conduct major currents.

Bud is not posting to provide useful facts. Bud is my troll who routinely posts only to insult.

That wire nut spring must have sufficient gauge and turns to carry current without fire risk. And then copper wires also must be sufficiently twisted so that wire without a wire nut will also safely conduct that current. Bonding by twisting wires and using a wire nut; both must carry current.

If wire nut is not intended to carry current, then a wire nut would only have grooved plastic. Wire nuts use a metallic spring that cuts into (makes electric contact) with each copper wire. Wire nut also must be able to conduct current between copper wires. Spring must be sufficiently sized so that this fire cannot happen:

Bud knows this. But a troll always seeks reasons to post insults.

Twist theose wires together so that sufficient contact exists without a wire nut. Then install the wrie nut so that connection is even better. No reason for a wire nut to get hot if properly constructed and installed. Learn from a worst case reason for wire nut fire in:

[bud--]

. One of w_'s special skills is not knowing what his sources say. From w_'s link":

"27. That is exactly what has happened here. All of the current is passing through only a few segments of the spring and those parts of the spring become red hot."

From elsewhere in the same series: "17. The characteristic heating in a band around the connector shows us where the heat is being generated. In all of those failures shown, from both lab and homes, the heat is generated by current flowing in the connector spring. Here, all of the current is passing from wire-to-wire through sections of the steel spring."

"24. Measurements on newly made aluminum-wired twist-on splices show that most of the current flows through sections of the steel spring. More than 60% typically for an aluminum connection, but less than 10% for an all-copper connection. There is a basic difference in behavior with aluminum wire."

"The key observation is that with this type of aluminum to copper wire splice, most of the current flows through the spring in the connector. In the design of twist-on wire connectors *the purpose of the spring is to maintain tension on the spliced wires, not to conduct electricity itself*."

Wirenuts on aluminum wire can fail *because* the spring "conduct major currents."

In addition to not being able to understand his sources, poor w_ apparently doesn?t know that steel, which the spring is made of, is not a particularly good conductor. .

. Poor w_ gets so upset when someone exposes his misinformation. .

. Still missing - requirements from any manufacturer, or UL or the NEC that wires be twisted. .

How can anyone be stupid enough to completely miss what his source says. The overheating was *because* the spring was forced to be a major conductor. The spring was not the problem. Lack of aluminum wire-to-wire contact was. .

Poor w_. Always certain, sometimes right. But not often.

[w_tom]

To argue and insult, Bud always posts half facts. Wire nuts with too few turns in that spring can heat. Wire nuts that must make up for the bad wire connected (wires not twisted) must carry more current and will get hot. Wire nut must be sufficient to carry the current AND wires must be twisted so they can carry the current. Only then does a superb connection exist.

That paragraph is irrelevant to Bud. He will do what he always does

- apparently an inferiority complex. He will post again only to argue.

Best solution (standard workmanship) is to twist all wires together so that the twist along is a sufficient connection. Then the wire nut is applied to make another sufficient connection. No wire nut, properly installed, should get hot - the connection should be that good. A copper wire nut connection should be installed as is necessary for aluminum wires. Those who actually know reality need not post mockery and insult.

[bud--]

. The village idiot still hasn?t figured out what *his* source says "In the design of twist-on wire connectors THE PURPOSE OF THE SPRING IS TO MAINTAIN TENSION ON THE SPLICED WIRES, NOT TO CONDUCT ELECTRICITY ITSELF."

Nor has he shown requirements from any manufacturer, or UL or the NEC that wires be twisted.

As usual he just repeats the same drivel.

[w_tom]

Wire nuts without that metallic spring also hold wires in place. But wire nuts with springs conduct current - make the junction more conductive - redundancy. As the citation shows, if that spring has insufficient numbers of turns - cannot conduct sufficient current - then a wire nut may get hot; even catch fire.

If Bud was honest, then Bud would also recommend wire nuts with no metallic spring. If honest, Bud should oppose springs as too dangerous and unnecessary. Bud's only purpose is to post insults. Bud is a professional spin doctor - a classic troll - who even posts sales promotions rather than honest technical facts. OP will obtain little useful information from Bud.

Properly installed wires are first twisted together to always conduct sufficient current without a wire nut. Then a wire nut is attached to make that connection even more conductive. A redundant connection so that the junction is always fully conductive and does not get hot.