I recently obtained an old 220-V, 28000 BTU air conditioner that was
said to work fine. This morning I decided to power it up and test it
before going to all the work of installing it and an inexplicable
thing happened. The 30-Amp breaker in my shed doesn't trip. The 60-Amp
breaker next to my service entrance, going to the shed, doesn't trip.
But the 100-Amp breaker in my service entrance does!
Is their something really mysterious going on here? Or, does it simply
mean that my service entrance breaker is faster than the other two
Without hanging an ammeter on each, it's impossible to guess, however, it's
possible that the 30 amp was pulling less than 30 amps, and the 60 amp was
pulling less than 60 amps, but all the breakers on the load of the
main,combined, were pulling more than 100 amps, at least momentarily, which
caused the main to trip
If the load (A/C) is shorted the main may trip faster than the branch
breakers. A fully magnetic-trip main and thermal/magnetic branches
have been seen to produce this condition with none of the breakers
Thanks, Mr. E.
I'm guessing you probably solved the mystery. After reading
your comment, I did some Google searching and found that
magnetic breakers trip almost instantly but it looks like
those using thermal technology technology have a trip delay
of about 10 or 15 ms. I'm not sure what type the main,
service-entrance breaker is, but it does look entirely
different than the other two breakers in that it's very
large, while the other two are the small, snap-in type of
I think it's definitely a problem with a dead short and not
an overload. The only things I had running in the house at
the time were a TV and a computer and some lights and a 1/2
horsepower swamp cooler and the AC unit under test was
turned off. When I measure the DC resistance between the two
input poles on the air conditioner, I get 2 ohms which
probably tells me something, I guess. Usually, when you get
a dead short with residential appliances the connections
probably melt before a lot of current can be drawn and the
local breaker trips. The air-conditioner, must have some
heavy-duty connections that don't. That was kind of a scary
experience. I've never had anything like that happen before.
Well I've probably never tripped a magnetic breaker, but otoh, you
might have melted somehing inside but it cooled in a way that left it
Regardless, if it did this when it was turned off, it should be pretty
easy to fix. Either the on-off switch is broken (shorted to the other
hot somehow, or unlikely: it said off but was on and there's a second
problem also), or the problem is in the cord before the switch.
That's a very limited area to search, and you'll probably find sooty
or melted stuff to help you out. And even if not, I see you know how
to use a meter.
Maybe that is the reason they stopped using it, even if the person who
gave it to you honestly thought that it still worked, or was told so.
When you're past 50, that stuff is real easy to forget.
Well, now I have a very difficult confession to make. I tested the
unit using one of my generator suicide cords and grabbed the wrong one
which just happens to present a dead short to 220V power. Being that
I'm pushing 70, maybe I can use age for an excuse, but that certainly
was a stupid thing to do.
In any case, I have it hooked up correctly now and found that the
blower doesn't come on at all and a loud noise that I assume to be the
compressor only comes on sometimes. So, I decided not to fool around
with it anymore and I ordered a portable, Amcor, 12000 BTU unit from
Circuit City for $335.96 (model #ALD 12000E). They have a sale going
on right now, by the way, and shipping is cheap also at only $3.99.
This may be seen as an obviousity....
But how about temporarily turning off all the other brekers in the
service entrance and see if the 100 amp breaker still trips when you
turn on tha AC.
If it doesn't, then as several other have said, your total consumption
may be exceeding 100 amps at times.
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