Subpanel Electrical Fire

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

Reply to
Seawulf
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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.

--=20

PDQ

| receptacles. The saws are each rated at 220V/13A and are never run=20 | concurrently, One of the 120V receptacles was dedicated to a dust=20 | collector rated at 120V/18A or 220V 9A. |=20 | I have been running this setup for about two years with no apparent=20 | problems. I recently removed the cover on the subpanel to check=20 | something and found that the neutral wire for the line feeding the = dust=20 | collector, and connected to the neutral bar was charred for about two=20 | 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. |=20 | The circuit breaker on the dust collector line was still engaged. I=20 | also tested the cb and found that it does shut off power to the = circuit. |=20 | 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=20 | the other 120V line, and it was also tight. |=20 | The 50A line from the main box to the workshop subpanel is Aluminum=20 | (house is mid 70's vintage) and the Murray subpanel is rated for both = Al=20 | and Cu wire. |=20 | Does anyone have any ideas as to what the problem could be? |=20 | Thanks, | CW

Reply to
PDQ

On Sat, 05 Mar 2005 14:10:28 -0800, the inscrutable Seawulf spake:

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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Reply to
Larry Jaques

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

Reply to
Lew Hodgett

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.

Reply to
toller

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

Reply to
Todd Fatheree

OK, you want the techie explanation, here goes:

1) Aluminum wire oxides in the atmosphere. 2) The oxidization increases the resistance of an electrical connection. 3) The increased resistance generates heat which causes "cold flow" of the aluminum, thus reducing termination pressure, which also increases the electrical resitance of the connection.

It is a never ending vicious cycle resulting in the melting of the aluminum wire about 2"-3" up inside the insulation on the wire and often a fire results.

The above is why aluminum conductor has not been used in buildings for years.

Proper use of antioxidant compounds such as Alnox and larger conductors, above 2/0 minimum, allow use of aluminum in many high current applications, but definitely not branch circuits in housing applications.

HTH

Lew

Reply to
Lew Hodgett

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.

Reply to
Andy Dingley

The quality of the termination of the wires that burned was poor , a result of an untight connection, corrosion of the conductor, or the inside of the terminal, etc. Since this poor connection added a resistance to the circuit, it caused heat to build up at that point. The circuit breaker didn't trip because you weren't pulling more current through it than it was rated for. Whenever you re-use old wiring or panels all of the terminations need to be cleaned thoroughly before they are assembled. In the case of aluminum wire this is even more important. It's also necessary to coat the aluminum conductor terminations with a special grease type compound that has been developed to prevent oxygen in the air from oxidizing the aluminum when it warms up from carrying heavy currents. This has to be done when you do the installation. Now it will be necessary to dis-assemble the problem connections, cut back the wires far enough to find undamaged insulation and conductors. Replace any terminal that shows any damage, and then clean and re-assemble all of the connections again applying the special grease to the aluminum connections. Make sure that you have every connection good and tight before you re-apply power and it should all be OK again. It's a good idea to inspect the panel connections every so often to try to catch this type of problem before it gets this bad. It's a good thing that you caught it when you did. It wasn't your wiring, but the poor electrical connection that caused the heat to build up and damage the wiring.

Reply to
Charley

OK, I'll bite.

What is the "superb ring system" ?

Rad

Reply to
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.

Reply to
Andy Dingley

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

Reply to
Todd Fatheree

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

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

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Reply to
LRod

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.

Reply to
toller

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

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Reply to
LRod

The rating info on the sheathing of the feed cable is very difficult to read over the section of the cable that is exposed to view. But, it appears to be either 8 or 6 gauge. Given that 8 gauge is rated, I believe for only 40 Amps and this is off of a 50A breaker on the main panel, I must assume it to be 6 gauge.

Also, all of the houses in our little development have the same central air conditioning units, so I assume the feed line is original to the house and was accepted as code-compliant in the mid 70's timeframe.

Thanks for your info, CDW

toller wrote:

Reply to
Seawulf

I want to thank all of you who responded. I even enjoyed the playful wrestling with characteristics of Al wiring.

Having thought about this for a few days, now - before reading all of your stimulating comments - I had begun to conclude that the charring of the insulation on the neutral wire for the dust collector circuit was perhaps,not the cause, but the result of the underlying problem. The clue to the problem, I began to think was the melted plastic around the neutral bus bar and the problem was the connection of the 50A feed Neutral to the subpanel Neutral bus bar.

So, after reading your comments, especially Charley's - I believe that to be the case. I think that the faulty connection heated the bus and the dust collector wire had its insulation touching the bar, which would explain why it was so charred and the feed and other circuit neutrals' insulation was not affected.

I also take onboard the comments about the potential danger of Al wire.

Thanks for your help

Charley wrote:

Reply to
Seawulf

I just had a home built and I noticed that the power company used aluminum cable which they trenched underground from the pole to my house.

Reply to
Bob

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.

Reply to
Andy Dingley

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

Reply to
Todd Fatheree

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