Subpanel Electrical Fire

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I have a home workshop and needed 220 V circuits for certain power tools and 120 V circuits for lighting and receptacles.
Around the same time our central air conditioning system faded and died. We live in southern California and do not have many really hot days, so we decided to not repair it again and just let it be.
The 220V 50A circuit for the AC happened to terminate on one wall of the shop. So, taking advantage of the situation, I decided to set up a subpanel using the former AC circuit. I am not an electrician, but have done other electrical work around the house, including wiring attic fans, adding new outdoor circuits, etc.
I added the subpanel and ran a 220/20A line (10-2G NM-B cable) to the tablesaw and bandsaw and two 120/20 A lines (12-2G NM-B cable) for new receptacles. The saws are each rated at 220V/13A and are never run concurrently, One of the 120V receptacles was dedicated to a dust collector rated at 120V/18A or 220V 9A.
I have been running this setup for about two years with no apparent problems. I recently removed the cover on the subpanel to check something and found that the neutral wire for the line feeding the dust collector, and connected to the neutral bar was charred for about two inches from the bus. Most of the insulation had been burned off along those two inches. Also, the black plastic around the neutral bus shows signs of having melted around the perimeter of the neutral bus bar.
The circuit breaker on the dust collector line was still engaged. I also tested the cb and found that it does shut off power to the circuit.
I checked the screw which held the charred wire to the neutral bus bar and it was tight I also checked the screw holding the neutral wire for the other 120V line, and it was also tight.
The 50A line from the main box to the workshop subpanel is Aluminum (house is mid 70's vintage) and the Murray subpanel is rated for both Al and Cu wire.
Does anyone have any ideas as to what the problem could be?
Thanks, CW
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Yeah. Aluminium wire.
I had the same thing happen to me 30 years ago in a condo unit that was only 3 months old.
The flash was in an outlet box that was not being used. All the joints were tight but the wire still burned back about 6 inches.
Since then I have only used copper wire.
If you join aluminium and copper you will get a galvanic reaction that could result in a burn.
--
PDQ --
| I have a home workshop and needed 220 V circuits for certain power tools | and 120 V circuits for lighting and receptacles. | | Around the same time our central air conditioning system faded and | died. We live in southern California and do not have many really hot | days, so we decided to not repair it again and just let it be. | | The 220V 50A circuit for the AC happened to terminate on one wall of the | shop. So, taking advantage of the situation, I decided to set up a | subpanel using the former AC circuit. I am not an electrician, but have | done other electrical work around the house, including wiring attic | fans, adding new outdoor circuits, etc. | | I added the subpanel and ran a 220/20A line (10-2G NM-B cable) to the | tablesaw and bandsaw and two 120/20 A lines (12-2G NM-B cable) for new | receptacles. The saws are each rated at 220V/13A and are never run | concurrently, One of the 120V receptacles was dedicated to a dust | collector rated at 120V/18A or 220V 9A. | | I have been running this setup for about two years with no apparent | problems. I recently removed the cover on the subpanel to check | something and found that the neutral wire for the line feeding the dust | collector, and connected to the neutral bar was charred for about two | inches from the bus. Most of the insulation had been burned off along | those two inches. Also, the black plastic around the neutral bus shows | signs of having melted around the perimeter of the neutral bus bar. | | The circuit breaker on the dust collector line was still engaged. I | also tested the cb and found that it does shut off power to the circuit. | | I checked the screw which held the charred wire to the neutral bus bar | and it was tight I also checked the screw holding the neutral wire for | the other 120V line, and it was also tight. | | The 50A line from the main box to the workshop subpanel is Aluminum | (house is mid 70's vintage) and the Murray subpanel is rated for both Al | and Cu wire. | | Does anyone have any ideas as to what the problem could be? | | Thanks, | CW
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On Sat, 05 Mar 2005 14:10:28 -0800, the inscrutable Seawulf

First loosen, then tighten to make sure the screws aren't just frozen. Also check your wire gauge vs. the current running through it.

"DUMP THE ALUMINUM WIRE NOW, BEFORE YOU HAVE A REAL FIRE" he said quietly and unemotionally while cursing the aluminum crapwire.

Resistance in the connection causes wires to char like that.
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Consider yourself lucky you still have a place to sleep.
Get rid of that aluminum crap before the whole damn house burns down.
Also replace the sub panel as well as the branch c'bkrs.
Trust me, you are sitting on a potential powder keg.
Lew
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writes:

Interesting... on alt.home.repair no one even mentioned the aluminum wire. Since it was not near the burnt area, it couldn't have been relevant. They said it had to be a bad connection, even it is seems okay. I would have to agree, as there really can't be another reason.
Heavy gauge aluminum wire (#4 for 50a) is perfectly safe as long as everything is rated for aluminum; though it is prudent to check the tightness of connections now and then, and to use antioxidant paste. It is #12 and #14 that is dangerous.
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writes:

They
to
is
Reminds me of a home repair class I took as a younger man on the mistaken belief that I would learn something useful. During a discussion on the dangers of aluminum household wiring, someone asked why aluminum was bad. The instructor replied "because aluminum is a bad conductor of electricity". Having spend 7 years designing insulators for transmission and distribution power lines, I thought "well somebody had better inform the utility engineers fast", seeing as probably 99% of all T & D lines are strung with aluminum.
todd
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It was somewhere outside Barstow when "Todd Fatheree"
Aluminium wire is a bad conductor (true). This is a different statement from "aluminium has poor conductivity" (false). It's not that aluminium itself is bad, it's the effects of aluminium as acomponent in an overall system.
I'm just grateful post-war Britatin invented the superb ring system, rather than going with aluminium.
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On Sun, 06 Mar 2005 12:18:10 +0000, Andy Dingley

OK, I'll bite.
What is the "superb ring system" ?
Rad
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It was somewhere outside Barstow when Eradicate Sampson
Post-war copper shortage, together with a housing shortage owing to wartime bombing and rapid post-war breeding. Everyone wanted houses, with electrics, and with reduced amounts of copper to wire them.
Some countries, including America, switched to aluminium wire. Aluminium was cheap at the time, owing to huge numbers of scrap aircraft. Britain OTOH, just invented the Land Rover as a way of making exportable vehicles that used aluminium instead of steel.
The British solution to house wiring was the "ring" system. The previous "radial" system, as still used in most countries, places a small number of sockets onto separate circuits and fuses each circuit in a central fusebox. Lots of wire, one to each circuit, lots of fuses, and poor safety for over-current faults as the multiple sockets require each circuit to have a relatively large fuse.
Thr ring system still uses copper, but it places all the sockets in a large area (usually one floor of a house) onto one circuit and one fusebox fuse. It uses a single loop of medium-heavy cable, which is a very efficient way of using the scarce material - delivered power for a given copper area is something like 4-6 times that of the US system, depending on whether it's a small flat or larger house. Voltage drop problems are avoided by using a loop, not a radial. The great advantage of the system comes from its better use of "diversity" - it's good to provide many socket outlets, but many of them are little used, and rarely do they carry substantial loads. The radial system has to wire everything up for the worst case, and do it individually.
The downside is a more complex plug on each appliance, requiring a fuse. However this also allows fuses to be related to the real appliance load, not the location. Most of my appliance are fused for just 1A, but I can safely plug a 3kW welder or compressor into the same socket, anywhere in the house.
If you look at the fire statistics for UK domestic fires, by far the biggest problem comes from damage somewhere along a cable (chafing, rodents, or perished rubber). Relatively speaking we get very little trouble from connectors or panels.
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wrote:

electricity".
Sorry, being a "bad conductor" and having "poor conductivity" are the same. My dictionary defines a conductor as "A substance or medium that conducts heat, light, sound, or especially an electric charge". Aluminum conducts electricity just fine. In fact, cost aside, one of the reasons it's used for overhead transmission and distribution is that it's a superior conductor to copper on a per-weight basis. You can certainly make the argument that aluminum (or aluminium) isn't suitable for certain applications, but don't twist definitions just to make a contrary point.
todd
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wrote:

I disagree. I think his statement was exactly correct.
It's as if someone asked why we don't use balsa for plane bodies. A perfectly acceptable answer might be, "it's a bad wood," the implication being that it's an unsuitable wood for plane bodies.
Someone making that statement isn't saying that balsa is bad as a wood ("aluminum is a bad conductor of electricity"), they are saying it's a bad wood for a plane body ("aluminum is a bad choice as a conductor in home wiring').
There's no definition twisting involved. It's semantics. A play on words, and I thought a good one.
- - LRod
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Shamelessly whoring my website since 1999
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He is using "bad" to mean "inferior" rather than "poor". As such, he is correct; aluminum is inferior to copper. However, in standard English, a "bad conductor" means "poor conductivity" and aluminum has excellent conductivity, so he is incorrect. It is not semantics, it is poor English.
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Given that the OP (at least on this segment), Andy Dingely, is from the Auld Sod and speaking the King's English, instead of this bastardized colonial treacle with which we wrestle daily on this side of the pond, I'll defer to his use.
- - LRod
Master Woodbutcher and seasoned termite
Shamelessly whoring my website since 1999
http://www.woodbutcher.net
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"LRod" wrote in message

Wouldna that sair be a unco mistake, wot?
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same.
conductor
that
don't
Same question to you. Why do electric utilites use aluminum almost exclusively for transmission and distribution if aluminum is a "bad conductor".

Where this analogy fails is that unlike balsa wood and airplanes, aluminum is widely used to conduct electricity.

If you stuck with the last part, you'd be correct.

The problem is that the statement that aluminum is a bad conductor is the opposite of semantics, because it's inaccurate. As you correctly said above its "a bad choice as a conductor in home wiring".
todd
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It was somewhere outside Barstow when "Todd Fatheree"
Don't you think that someone who makes a distinction sufficient to use each word appropriately in the same sentence is using it to make a point ?
A "conductor" and "conductivity" are not the same thing at all. One is a bulk property of the material, one is a property of a specific example of aluminium. Although aluminium's conductivity on paper is good, make actual real-world wires from it and it's quite another story -- you need to start worrying about those terminations.
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wrote:

same.
One would hope, assuming they were using them correctly, which you're not.

OK, if aluminum is a "bad conductor" in general, why do utilities use it almost exclusively for transmission and distribution of electricity? You're better off saying that aluminum is a "bad choice for household wiring", which is accurate, than getting too general by saying it's a "bad conductor".
todd
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It was somewhere outside Barstow when "Todd Fatheree"

Because at that scale, they're using far more material with relatively fewer joints. The benefits increase, the costs are more manageable. Their circuits are also powered almost continuously and a flowing current avoids many of aluminium's problems. It's circuits that are cycled for long off periods that are most prone to the typical faults.
You'll notice that telephone companies, another great user of copper wire, never went for aluminium.
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wrote:

So what you're saying, without coming out and saying it, is that aluminum is a "good conductor" for the utilities?

I've never said that aluminum was appropriate for all uses. I just disagreed with the blanket statement that aluminum is a "bad conductor". You'll also notice that the telcos are increasingly moving to fiber. Just gotta get that "last mile". ;-)
todd
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It was somewhere outside Barstow when "Todd Fatheree"

No - I was talking about in-house wiring, not utilitites. You're the one claiming that the OP's problems and near house fire couldn't have been caused by the use of aluminium, because 440kV systems manage to use it.
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