Interlocking "Switch" on Main Panel vs. Transfer Power Switch Sub-Panel

Just a word of caution here…When using a generator to hook up to your house…depending on where you live, some municipalities consider the use of an interlocking switch as illegal, such as in Los Angeles! In those cases, a transfer power switch has to be used. In my case, I thoroughly researched this with the city of Los Angeles.

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@jtdavis good advice man! I’m gonna look into what the laws are out here in Michigan.

A transfer switch completely isolates the main panel feed from the utility line thus it requires more materials plus labor to put in. But it can be a secondary master cutoff when the need happens. It also adds more value to the home at resale since it allows the homeowner to select the power source. I did put one in my home right behind the main distribution panel because the wall behind the panel is accessable. And makes it closer and easier to disconnect from the utility power.

A transfer switch’s disadvantage vs an interlock is that it only energizes a limited amount of circuits vs the interlock which allows energizing all 120 V household circuits. Energizing all 120 volt household circuits is very desirable in a power outage rather than having only a few circuits available.

A transfer switch isolates by way of a three way switch with (Utility Power / Off / Generator). It does isolate but only by utilizing a switch.

An interlock system also uses switches in the form of breaker switches. The main incoming breaker is switched off to isolate and the “Generator” breaker is switched on. The only thing the interlock kit does is keep both switches from being placed in the “ON” position at the same time. This is done by a mechanical bar that physically won’t allow the second breaker to be in the on position when the first breaker is.

The Transfer switch isolation is also performed by mechanical means via the three way switch for each circuit. An interlock is perfectly safe when the interlock is in good working order and the user insures that the breaker switches are in the correct position vs the intended function as a back up check. The reason some municipalities won’t allow interlocks is that they assume that the least knowledgeable user would have no idea how to determine if the interlock was functioning or the switches are in the correct position.

California is “Special” and requires specific warning labels vs the rest of the country as well due to their inability to recognize that hazardous conditions exist and caution may be prudent.

Your reply is correct except that the transfer switch is inserted between the utility connection at the meter or master breaker and the main distribution panel. So ALL of the circuits at that panel, either 120 or 240 VAC, are interrupted and later energized by the new power source, such as the Bluetti Fusion panel with 120 and 240 VAC power from the AC300 controllers and all attached B300 batteries. This is what I am doing to my 45 year old reinforced concrete home where I installed a GE 100 A transfer switch in 1995 afer Hurricane Georges hit us, and I attached a 5 kW Onan generator to power my home for 15 days. It was used again after Hurricane Maria hit us in 2017, exactly four years ago this month, and that same generator ran for eight hours a day for 45 days.

So my new Bluetti purchase will be attached to that transfer switch and the generator will charge the batteries instead. That transfer switch works for me since 1995 (26 years) and it will work for anyone who prefers a physically separated form to disconnect from the utility and use a second power source. And it is still a second master disconnect that is better to operate, because it is designed for multiple disconnects and reconnects which can weaken a regular circuit breaker with an interlock. It is simply a huge two pole, two throw switch.

The same thing is occurring in the interlock scenario except it is occurring physically inside the same box and the switches are double pole single throw switches which under backup use will see little wear. I am not saying one or the other is better or bad, merely pointing out that there are two methods each with its own advantages and disadvantages

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