Alex Baker wrote:
If you have a multimeter handy, check the voltage between the two hot
leads from the two outlets. If it is ~ 220, you are in business.
This discussion has gone a long way, and I’m not sure how much actual
knowledge has been applied. Here are the pertinent facts:
Here in the US, a typical residence is served by a single-phase
transformer, which means there are only two high-voltage supply lines
connected to it. This transformer has two secondary windings so
there are three wires from the transformer into the residence: a
common, a +110V, and a -110V.
In the breaker box, each of these three leads is connected to a bus.
Every breaker in the box connects to the common. Every other
breaker connects to each of the other two busses; whichever bus one
breaker is connected to, the breaker just below it is connected to
the other bus. Hence, about half the loads in the house will be
connected to the +110V supply, the other half will be connected to
the -110V supply. Since it’s all AC, both supplies look exactly the
same unless you manage to compare them to each other.
If you need 220V, there’s no need to hotwire anything. You just
purchase a 220V circuit breaker. It’s twice as wide as a 110V
breaker and installs in the same box, but because it occupies two
successive ports it will be connecting to both supply busses. It
typically looks like two regular breakers siamesed together with a
little bar tying the levers together. You connect your 220V load to
the two terminals on this breaker, with only a ground connected
elsewhere. This is not a bodge; it’s the way the system is designed
to work. This is the way dryers and other 220V appliances are
powered. You can just take a quick look inside your breaker box and
quickly see which appliances are 220V.
The ground, BTW, is a separate bus inside the circuit breaker box,
even though the common from the transformer is also connected to
ground. The reason to keep these separate is that the common may get
a significant current flow and therefore a significant voltage drop,
so a few volts may appear on it from time to time. The ground, by
contrast, should never see any current flow (other than when
something goes wrong) and hence should never see any voltage.
Some appliances use both 220V and 110V. For example, an electric
dryer usually has 220V for the heating element but 110V for the timer
and the motor. In this case, three leads are provided, both the 110V
supply leads and the common. From one supply to the other is 220V,
from either supply lead to the common is 110V.
Now, if for some reason you need to combine two 110V power supplies
that originate from different transformers, I suppose it’s possible,
but I probably wouldn’t advise it. I dunno why you’d ever need to do
such a thing, though.
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