Electric Subpanel fire

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

Did you pull a neutral from the sub to the main panel?

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 d> toller wrote:

I don't have an answer for you, just some questions...

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

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

-- 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?

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

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.

WRONG. If there is a restriction, the dust collector motor will draw LESS current because it is moving less air.

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

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)

Bob

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?

The Neutral had a tight connection, as did the other circuit Neutral and the 50A Al feed Neutral.

The dust collector has a dedicated line because of my concern about its

Your rationale about the apparent unbalanced current draw on the Hot and Neutral lines seems well founded. However, it also assumes that the underlying problem was with that circuit.

Based on my own subsequent analysis and that of others on the rec.woodworking group, I now believe that the underlying problem was the connection of the Al 50A feed Neutral to the bus bar. I now believe that the connector problem - well known with Al wire - caused the bus bar to overheat (remember, the plastic around the bar shows signs of melting) and that caused the insulation on the dust collector neutral wire to char - because - I now assume - the insulation was probably in contact with the neutral bus bar. Neither the feed Al line or the other

CDW

John Grabowski wrote:

You didn't move the 50A breaker in the main panel, did you? If you've somehow got both the hots feeding the subpanel on the same phase, that would overload the neutral, too, and some panels allow you to do that. (It would also explain why the AC unit croaked.)

No. Nothing was disturbed on the main panel.

My 220V table saw and band saw have both been operating off the panel for a few years, now. So, that indicated to me the phasing on the panel is correct.

Thanks for your comments, CDW

Interestingly, when I tested the connections on the subpanel, although all of the screws on the Neutral bus were tight, the screws on the two hot phase wires were not tight. I got at least 1/4 turn on each. They, along with all others have been re-tightened. But, it looks like I really need to follow Charley's advice relative the the Al wiring being cleaned and anti-ox paste used, etc. as well as replacing the subpanel enclosure.

The amp meter would be used to measure actual amperage on the active circuits to check for over-amp conditions?

BTW, I did verify the language on the Murray subpanel relative to being rated for AL and Cu wire. The UL Panelboard Marking Guide stipulates the phrase "Use Copper or Aluminum Wire" for this situation. The panel actually reads "Use Cu/AL wire on pannelboard line and neutral terminals." These two statements seem to be the same to me.

Thanks for your comments, CDW

bumtracks wrote:

you need to tighten all the alum connections , both ends the feed at main panel and sub panel connections and would be wise to invest in a simple clamp on amp meter..

Run a new copper wire to the outlets. 18 amps is quite a draw, and the risk of a fire ain't worth going cheap. What would the insurance adjuster think of your setup? 240 V to the dust collector is actually safer in this case, i think. JohnK