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?
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
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.
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
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.
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.
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
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
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).
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?
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.
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.
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.
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.
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,
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?
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.
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