On a generator panel, how do you keep the generator from back feeding to all the breakers in the house?
I recently wired in a generator sub panel. It was wired to code with a permit and passed inspection. I did all the work myself so I can answer questions about my particular code compliant and inspection passed installation. This sub panel gets power from two places, the main panel and the generator. It has a big switch, commonly called a contactor, that switches the supplied power from either the main panel or the generator. My generator sub panel has many slots for breakers.
The following describes how the sub panel is wired:
An 80 amp breaker was put into the main panel to supply power to the sub panel. The wire connecting this breaker to the sub panel is sized to carry 100 amps. Remember, and this is important, circuit breakers in your panels are sized to protect the wires they are connected to. They are not for protecting anything connected to those wires. Like toasters or dishwashers. Or people.
For every circuit in the house I wanted to be energized, or powered, from the generator I had to put a breaker into one of the slots in the sub panel. Then the wire for each circuit was disconnected from the breaker in the main panel and connected to the same size breaker in the sub panel with a wire that was run to the main panel from the beaker in the sub panel.
I know this might sound a bit confusing. I will try to simplify a bit. Power from the main panel goes through, in my case, an 80 amp circuit breaker to the generator sub panel. The power passes through this breaker in the main panel to a switch in the sub panel through which all power must flow. From this switch the power is distributed to the breakers in the sub panel. And these breakers i the sub panel are then connected to the wires in the main panel that were disconnected from breakers in the main panel.
What this means is that the sub panel is always energized from the main panel during normal conditions. This also means that the main panel will have several beakers that are no longer connected to anything because the breakers in the sub panel are now supplying power to circuits that used to get power from the main panel.
Now, finally, we get to the question you asked. The switch, or contactor, in the sub panel, will either connect to the generator or the main panel. Since the power from the main panel flows through this switch and then to breakers in the sub panel and then to circuits that are no longer connected to the main panel, it controls where the power comes from and where it goes. The switch has only two options, either main power or generator, it cannot receive power from both at the same time. And since the circuits that are to be supplied from the generator have been disconnected from the main panel and transferred to the sub panel there is no danger of the generator powering, or energizing, the main panel and hence anything connected to it, like somebody fixing a downed power line.
All the circuits described above have assumed a 120 volt circuit, with one hot wire and one neutral wire. If a 240 volt circuit is used it will have 2 breakers. If you look at the main panel in a house in the USA all the 240 volt circuits will have two breakers tied together. So in reality it is two 120 volt circuits that are powering the same device and are switched at the same time using the breakers tied together.
I have not mentioned the neutral and ground wires. They are treated differently and are not switched with breakers nor are they switched by the sub panel switch. The neutral wire is tied to the ground wire in the main panel. The sub panel neutral and ground are both bonded to the main panel neutral and ground.
I know there is a lot of info above and it may be a bit confusing to folks who don't do anything with electricity. However, with a bit of study and reading a few times it should make sense. And even if the connection scheme might seem a little convoluted once a person draws a picture or actually starts to make the connections it becomes pretty clear. Most importantly the way it is done is to prevent electrocution.
Too many times I read here and on other sites messages from folks who want to connect their generator in an unsafe manner and want directions on how to do so. People who insist that nothing can go wrong because they are always in control or their spouse knows just how to do the connection to prevent sending electricity where it shouldn't go. If they know so much why are they asking for directions.
Anyway, I hope the above helps and I will be happy to answer any questions that I can. If you want I would even talk on the phone. The most important thing is to be safe.
Eric
