Electric Subpanel 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 in the shop 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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toller wrote:

Yes, the neutrals are all connected to the neutral bus and the grounds are connected to a separate ground bus on the opposite side of the box.
The 50A Al feed Neutral from the main box is connected by the large screw to the top of the Neutral bus. It has no damage, nor do the other Al wires.
The 220V circuit for the saws had been operating from the subpanel for months before the two 120V circuits were added. The two 120V circuits operated without any damage for at least a week or more after they were installed. I know that because I had been removing the front panel periodically assessing options for additional circuits for some time after those 120V circuits were added.
I do not know when the damage occurred after that time. But, I do know that the dust collector has operated and can still operate fine from that damaged circuit Of course, at this point,I have secured power to the entire subpanel until I can determine what caused the problem.
Thanks, CW
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Did you pull a neutral from the sub to the main panel?
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Generally a situation like that occurs when the connection is loose. It simply heats at the location of the bad connection. Try stripping the burned connections off and reconnecting them, use the equipment for a short while then feel the wires and see if they have gotten hot. Some antiox paste wouldn't hurt either

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To create heat necessary to burn insulation, one of two problems must occur. Either too much current is being carried on that wire (which is not likely) or a connection at the point of burning is loose - causing a voltage drop at that connection so high as to create heat. If the latter is true, then what you though was a motor working just fine was actually a motor under undue stress due to insufficient voltage. IOW we have an example of a conclusion not based upon the numbers.
Had you monitored that wall receptacle voltage when under load, the problem would have been apparent then. The reason we usually don't is that such failures are rare. But this is the point. What you observed to be a fully functional system was instead a system with one minor defect. Well such is life and why being a human is so interesting.
If you don't find a loose screw at the point of burning insulation, then we may have a much more serious problem. The question that must be answered - why so much heat on that wire? Maybe a trivial problem (ie loose screw) or maybe symptoms of a human safety problem. Other posters have provided useful background facts.
Seawulf wrote:

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I don't have an answer for you, just some questions...
1) Did you wire the subpanel correctly, with separated neutral and ground buses? 2) The charring was away from the aluminum wire, right?
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Lots of good advice in other's posts. Another remote possibility is that the damage was caused by arcing induced from a lightning strike surge. Otherwise such damage is nearly always caused by a loose and/or dirty connection. Damage over a short part of the wire would be very unlikely to have been from excessive current. Don Young

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You may find that to be a problem when you try to sell the house. Separate issue, I know, but something you should keep in mind.

Does that circuit have both a neutral and a ground? If not, it's *not* suitable for feeding a subpanel, and must be replaced.

That dust collector circuit is a potential problem. Depending on how long you use it at a time, it may meet the Code definition of a continuous load and thus require that the ampacity of the circuit be derated 20% to 16A. This should have been wired with 10ga wire, which can carry up to 24A of continuous load current.

Wow.
No damage to the neutral conductors for the other circuits, right? Then there's something amiss with that circuit, or with what's plugged into it.
I think you oughta have a look at that dust collector motor. You said (above) that it's a dual-voltage motor. I wonder if the jumpers are set for 240V instead of 120V.

That doesn't mean that it will trip at the proper current draw, though.

I hope the wire in that cable is at least 6ga minimum if the conductor temperature rating is 75 deg C or higher, or 4ga minimum for 60 C conductors.

Check the jumpers on the dust collector motor and make sure they're set for 120V operation.
Also check to make sure there's adequate air flow for the dust collector. If it's having to work too hard to draw air through too-small piping, it will draw more current too.
Redo that circuit with 10ga wire. And replace the receptacle: it's probably damaged too.
Check the connections on the cable that feeds that subpanel, at *both* ends of all four conductors. (You *do* have four, don't you, two hots, neutral, and ground?) If the neutral isn't tied down tight at both ends, you may be getting a higher voltage in one leg of the subpanel than the other. Won't make any difference to your 240V tools, but it plays hell with 120V devices. You might have been pumping 150V or so through the dust collector.
If the circuit feeding your subpanel has only three conductors, replace it with the proper cable. Might as well switch to copper while you're at it. For 50A, you need 8/3 with ground (conductor temp rating 75 deg C or higher), or 6/3 with ground (conductor temp rating 60 deg C).
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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CW, Back when I was small we had a similar problem with our main house panel. My father and I smelled burning plastic when we wee in the basement. He removed the front panel and we saw the plastic holding the 2 "phases" had melted. Turns out we were just running too much continuos power through the panel and had to upgrade. We had an electrician replace the panel and upgrade the service. Michelle
Seawulf wrote:

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Assuming you had a main breaker, it should have tripped to prevent that. That can't be his problem, since he has only 40a maximum out of the subpanel.
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WRONG. If there is a restriction, the dust collector motor will draw LESS current because it is moving less air.

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ground
Very interesting. I have NEVER seen an A/C compressor with a 4 wire cable.
Sounds more like your using the ground as a neutral.
Works yes, good practice not even close
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I'm guessing that the 12-2 romex is too small for the 18 amp load. The nameplate reads 120volt/18amps but in reality it's possible that your voltage is only 110 or 115 with a load on. In that case your motor will draw more than 18 amps. The neutral connection may have also been a little loose to cause the charring.
The dust collector should have been wired with a 30 amp circuit or changed to operate at 220 volts.

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SQLit wrote:

I didn't I switched the two. The totally bare - no insulation - wire is connected to the ground bus and the insulated - white - wire is connected to the Neutral bus. IOW, hard to get them mixed up. Thanks, CDW
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toller wrote:

The 50A 220 breaker for the subpanel is on the main panel. Niether it or the individual breakers on the subpanel tripped. Thanks, CDW
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Doug Miller wrote:

It is labled as 120 and I checked the actual wiring agains the 120V and 220V diagrams for the unit and it is definitely wired for 120V.

Tje Neutral was tight at both the Main and Subpanel.

Yes, the 5A 220 feed from the main panel has two black, a white and a ground wire. It is difficult to read the label on the sheathing - red text on grey sheath. It is either 6 or 8 gauge and I can't really make out much more of the ratings on the part of the cable that is exposed.

Thanks for your comments, CDW
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Seawulf wrote:

Sorry for the typo - The 220 feed from the Main panel is, of course, 50A, not 5A. CDW
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John Grabowski wrote:

I get your point. But, if the charring of the insulation on the respective Neutral wire was due to an over-Amperage, (i.e. enough to create sufficient heat for sufficient time)I would expect the breaker to trip. Of course, that assumes the breaker actually trips IAW spec - which I do NOT know to be the case.
Someone else suggested that a more-or-less continuous operation of the dust collector, would, itself, require a de-rating of the cable amperage load spec and, as you suggest, moving to a 10 gauge cable. But, in fact, the dust collector runs very intermittently. I usually have it connected to my small 13 inch thickness planer, which is rarely run for more than 5 continuous minutes less than an average of twice a week. Otherwise, I use it periodically to clean out the bandsaw and table saw accumulations.
Thanks for you help, CDW

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I am also puzzled by the neutral conductor burning, but not the hot conductor. Assuming the neutral had a tight connection it indicates to me that the neutral was carrying more current than the hot. This would only be possible if the neutral conductor is shared by another circuit which is also on the same phase as the dust collector. Is it possible that your planer is on the same phase as the dust collector and their respective receptacles share a common neutral?

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Seawulf wrote:

I wonder if that white wire was nicked or had some broken strands? You were operating it near its maximum rating, and it might have had a bad spot that overheated.
In any case, I would probably rewire the blower motor for 240V and put it on a 2-pole breaker. If you don't have a space for it*, either double it up with the bandsaw, or tie the handles together on those 20A single breakers (use the breaker manufacturer's handle ties, or else pull the single breakers out and replace with a proper 2-pole breaker) and run the dust collector on 240V onthe same circuit as your 120V stuff. (not sure if this meets code requirements or not, but it's close)
*From your earlier description, I assume you have a little 100A panel with 6 spaces, and it's already full with two 2-pole breakers and two single 20A breakers.
Bob
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