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
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.
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
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.
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
One of w_'s special skills is not knowing what his sources say. From
"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
Wirenuts on aluminum wire can fail *because* the spring "conduct major
In addition to not being able to understand his sources, poor w_
apparently doesnt 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
Poor w_. Always certain, sometimes right. But not often.
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
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.
The village idiot still hasnt 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.
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.
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
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.
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.
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