Household Power – How It Works

There are a lot of folks that think it terms of two hot legs of power coming into your home and they often refer to them as different phases. This can lead to misunderstanding of how things work and how we wire our generators off gird or as backup in our homes.

We all know about three phase power, and this may be where the trouble starts, some might think that it is two of the three phases delivered into our homes. Make a note that in three phase service, the phases are 120 degrees apart.

Now, let’s look at how the typical home in North America is wired.

Above: Typical Peg Pole Transformer

If you live in an older neighborhood, your house may be served by overhead wire and you can follow the three leads from your breaker box, up thru the power meter, to the mast, and out onto the street where it likely terminates to a round transformer mounted on a pole. If you look closely, you can see that the input to this transformer is fed by only one hot lead, and a neutral tied to ground. This means you have a single phase feeding your home.

What is important to note is the two hot leads are derived from this single phase and they are (single phase service). Since the secondary side of the transformer is center tapped, we can use the center tap (often called our neutral) to divide the voltage in half.

Going back to your breaker box, you will note that these two hot leads run down both sides of your power box, and you will see where the neutral terminates as well. The designers of the breaker box were clever and made the slots for breakers to be powered by the different hot leads. Example “all odd breakers might be powered by the red hot lead, and all even by the black hot lead”. With this scheme, when you plug in a 240 VAC double breaker, you automatically get both hot leads. In addition, as the electrician plugs in 120VAC breakers from top to bottom the loads are distributed across the two hot leads.

With this view of commercial power, we can revisit the PMG pages and see a striking similarity to what comes out of the transformer to our homes and what comes out of the SINGLE Phase generator, this may help you understand why we call it a single phase generator, and why we are better off NOT referring to these hot leads as different phases. It also helps you visualize how to wire in a transfer switch, and to note the similarities between commercial power and the output of the PMG.

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