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Old 08-07-2012, 04:48 PM   #15
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Originally Posted by Excella CM View Post
The big problem with low voltage wiring is that power is supplied by increased current which can lead to heating, especially when a plug is dirty or worn or loose. Goes with the territory as they say. It is easy to develop enough heat for a fire without blowing a fuse. Keeping connections clean and tight will help assure that no excess heat is developed. Vigilance is key, no set and forget it with RV's and boats.
Not necessarily. 20 amps flowing in a 12v DC circuit has the same heating potential as 20 amps flowing in a 120v AC circuit. In both cases, proper wiring practices and quality components are necessary to prevent fires.


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Originally Posted by Friday View Post
You will find a variety of 'interesting' choices in what wire sizes are spliced-in at the end of some wire-runs... 20A for those wee wires might work in ideal conditions (I don't think so myself though)... but 12v plugs are crap.
It's a stretch at 20a but the point at hand was that the OP's failure was not caused by exceeding the current carrying capacity of the wire.

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Originally Posted by TG Twinkie View Post
Insulation is there for the purpose of protecting the conductor. The current carrying capacity of the wire is based on its size and makeup. Multi strand wire is rated higher in current carrying capacity than solid wire. Contrary to popular belief the electrons flow on the outer surface of the conductors, not thru them. Stranded wire has a greater surface area, thus can handle more current than solid wire if the same gage.
It's this sort of misinformation that makes me want to avoid any sort of electrical thread entirely. While the electron flow does occur primarily on the surface of the conductor at radio frequencies, in DC circuits, it is diffused throughout the conductor. The differences in ampacity between solid and stranded conductors is negligible in the wire sizes typically used in 12 volt circuits.

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Originally Posted by Getahobby View Post
Does this make a case for an inverter and forget about using the 12V receptacles?
No, it makes the case for proper wiring techniques and safe components. It's just as easy to screw up 120v wiring as 12v wiring.

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Originally Posted by Phoenix View Post
Inverters have their own set of hazards. Besides heavy 12-volt DC supply cables, one adds the possibility of electrocution.

Most people don't think 12-volt circuits are very dangerous (unless they have seen a wire glowing like a cigarette lighter heating element). However, it takes very little current at 110-volts AC to stop your heart and kill you.
With the widespread use of GFCIs at this point there are more fatalities from fires of electrical origin than from electrocution resulting from 120v residential circuits.

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Consequently, if you are unfamiliar with both AC and DC safety requirements, it might be advisable to have a professional help you with your wiring projects.
There's some advice we can all take fully on board.

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Originally Posted by wrochdvm View Post
I guess the bottom line here is that the 16 gauge wire was subjected to an excessive load that caused it to burn and the fuse used to protect the circuit was too large to prevent the overheating.
Wrong. If you're going to post and ask questions, pay attention to the answers. The socket burned because it was crap. There's nothing to suggest that a smaller fuse would have saved it. Overcurrent devices, even when correctly applied, do not make up for poor materials and workmanship. Did you measure the current that your vacuum draws? How much was it? Should the fuse have blown?
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Old 08-07-2012, 05:18 PM   #16
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Jammer! It's not misinformation. 12 volts at 20 amps is 240 watts of power or heat capability. 120 volts at 20 amps is 2400 watts of heat capability. A big difference. Now that is misinformation to say they both produce the same amount of heat.
It has nothing to do with radio frequencies. If the wire overheated, it was due to bad connections or excessive load.
I won't get into the True power verses Apparent power.
I am an electrical engineer and I didn't play one on TV.
I wasn't the socket that burned it was the small wire that came with it. They call them pigtails.
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Old 08-07-2012, 08:51 PM   #17
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I had an '84 VW Vanagon that had a fire in the dashboard. Some quick thinking to pull over and disconnect the battery saved the van from burning to the ground.

Ends up a power wire in the dash was rubbing on a bracket and lost its insulation and shorted. The fuse wasn't customary at all. Some crazed engineer put the fuses downstream of the switches in the dash. The fuse was just before the lights/radio/etc. Most of the wiring was completely unfused. Very scary. Don't think that wiring design should have been sold in any market.
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Old 08-07-2012, 09:32 PM   #18
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I found a number of places where the yellow 120VAC wire that Airstream used in my trailer is running through holes in the aluminum interior shell with no grommet or protection. In one place the wire was cut to the conductor next to the shell... it looked like maybe from a knife when the lino was being trimmed...

I'll take an outlet smoking itself out over a 120VAC energized shell anytime...
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Old 08-07-2012, 09:50 PM   #19
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Quote:
Originally Posted by TG Twinkie View Post
120 volts at 20 amps is 2400 watts of heat capability.
While 120VAC x 20 amps AC has instantaneous peak power of 2400; that isn't wattage. Wattage is power over time, so you have to take RMS of current and voltage to get watts in AC. Is "heat capability" peak power? Why is that significant? I thought power over time was what circuit designs were based.
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Old 08-07-2012, 10:14 PM   #20
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Power over time is what I was referring to with the term "Apparent Power" which takes into account the RMS value. You are correct in that 20 x 120 AC will not produce a true 2400 watts of power but it will produce significantly more than 20 x 12 volts DC. When you compare it to pure DC power such as that provided by ten 12 volt batteries wired in series. The DC source would provide about 30% more heat.
In either case if the current exceeds the capacity of the wire. Bad things happen. As seen with the OP's post.
I doubt that even the socket in his trailer was rated for the 8 amps the vacuum cleaner required. So if the wires hadn't melted, the socket may have been next. There is a difference between these 12 volt receptacles and a cigarette lighter receptacle in that the latter is designed for higher current and obviously higher heat.
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Old 08-07-2012, 10:20 PM   #21
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From looking at the picture the pigtail is less than 3' in length per the automotive wire gage rating the wire is capable of 20 amps @ 12VDC up to 7 ft.

Wire Gauge Amps Ratings for 12 volt Automotive Systems

I would consider that marginal but that is the rating.

30 years ago I was taught electrons ran on the outside of the wire and the stranded wire was used in cases where flexing was important.
However; stranded wire has a higher RF resistance example center conductors on coax is solid wire.

In this case I suspect poor connections were probably the culprit regardless of the electrons running on the inside or outside of the wire.
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Old 08-07-2012, 10:59 PM   #22
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If the wires overheated from high current flow they would be burned along much of their length. But they're only burned near the 12v socket. This indicates the socket overheated, and that was most probably caused by a poor connection in the 12v socket.

That is also the simplest explanation, seeing as there are probably millions of these in use in autos and RVs. My sense is there is nothing wrong with the wiring or fusing.

The cigarette lighter socket is a lousy connection, in this case easy to loosen during vacuuming. A vacuum would be a poor choice by the vacuum builder to install such a connector on.

I would install a new 12v socket and throw the vacuum away (not to be taken as an approval of cigarette lighter connections).

My background is troubleshooting and repair of aircraft electrical systems.

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Old 08-08-2012, 01:37 PM   #23
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Originally Posted by TG Twinkie View Post
Jammer! It's not misinformation. 12 volts at 20 amps is 240 watts of power or heat capability. 120 volts at 20 amps is 2400 watts of heat capability. A big difference. Now that is misinformation to say they both produce the same amount of heat.
The amount of heat produced in a length of wire is I * I * R, where I is the current, and R is the resistance of the wire. The voltage between that wire and, say, ground, is immaterial.

Electrical fires caused by undersized conductors and poor connections usually result from relatively amounts of power dissipation (heat) compared to the power being transmitted. It doesn't take more than 10 or 20 watts to melt a connector, and so enough resistance to cause a 1 or 2 volt drop is enough to do it on a circuit such as this, powering a 10a or so load.



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I am an electrical engineer.
Where did you go to school?
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Old 08-08-2012, 05:05 PM   #24
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The amount of heat produced in a length of wire is I * I * R, where I is the current, and R is the resistance of the wire. The voltage between that wire and, say, ground, is immaterial.
This is the most general equation:

120v at 20amps delvers far more power (watts) than 12v at 20amps.

Starting with your equation, Joules law for power (not heat because time is omitted).


But given that fixed resistance, the current depends upon the voltage applied. (Ohms law):


Substituting:


As the voltage goes up across a fixed resistance, so does the power.


Now that is what Twinkie was saying
BUT..... it is heat generated across a fixed resistance that is likely the answer to what caused the insulation on those pigtail wires to melt. HEAT = POWER * TIME


As garry said the pigtail wires appear sized properly for 20 amps over that short distance. I cannot imagine Airstream nor their suppliers making such a mistake. Looking at the picture, as dkottum points out, the insulation is not melted because the wire overheated from over-current, otherwise it would be melted along the entire length. It appears the socket or the socket terminals heated the wire up at that end. A poor connection between the socket and the inserted plug, (or a cheapo Chinese plug) could create enough resistance to heat the insulation to its melting point.
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Old 08-08-2012, 06:32 PM   #25
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wow

a couple questions for the OP: 1) what was burning? Just the insulation of the pigtails? 2) if you remove the fuse in question, does it de-energize the circuit that burned? 3) Did the "...melted..." vacuum plug (paraphrased) when removed do any damage to the receptacle? ie: is the receptacle dead shorted (faulted) now, or is it deformed in any manner? 4) is the vacuum still operating correctly?
5) was your wife able to remove the vacuum plug from the receptacle when she first complained/mentioned it to you, or did you have to remove it? I guess the real question is: was the vacuum plug melted into the receptacle?
My pea brain says the vacuum is the root cause; its plug caused deformation of the receptacle; said deformation caused a continuing resistive fault of the receptacle, ultimately causing the pigtails to become fusible links. Now, why the fuse did not operate is beyond me, unless this particular circuit is not protected the way we all think it is/was. I believe a 20amp fuse will operate before a 16 gauge wire will burn, but it is after all, a pea brain.

oh, and nice math stuff, it humbles me.
ol' pea-brain bill
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Old 08-08-2012, 07:17 PM   #26
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pea soup

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My pea brain says the vacuum is the root cause; its plug caused deformation of the receptacle; said deformation caused a continuing resistive fault of the receptacle, ultimately causing the pigtails to become fusible links. Now, why the fuse did not operate is beyond me, unless this particular circuit is not protected the way we all think it is/was. I believe a 20amp fuse will operate before a 16 gauge wire will burn,
What is great is that if enough of us put our pea brains together on the WWW we can convince ourselves of pretty much anything - like pea soup.

I agree that it could well be a resistive fault caused by a poor connection or a maybe a faulty plug. To answer your question as to why the fuse did not blow..... the current flowing through a high resistance fault was probably less than 20 amps but high enough to slowly overheat the metal contacts of the socket and transfer that heat to the wire and insulation on the pigtails.

The 16 gauge wire was not hot because of excess current - it was hot from being physically connected to a mini soldering iron. Come to think of it the end of that wire does look like a few of my less than stellar soldering jobs.
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