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I have a few circuits in my house that I would like to be able to power during an outage with a portable battery backup system (something like the Ecoflow Delta Pro). I'm trying to understand if what I'm envisioning is possible with either a transfer switch or some sort of interlock/power inlet box setup.

My setup is as follows:

  • 200A main service panel on outside of house
  • 100A sub-panel in house, fed by 100A breaker on main service panel
  • Various 15A circuits in house sub-panel that I would like to backup (most critically, gas boiler for heating during a winter outage)

Here's a schematic of the main panel and circuit breakers on it:

panel diagram

Due to the way things are physically configured, installing a full transfer switch in the house near the 100A sub-panel would be somewhat disruptive and more expensive due to where the switch would need to be located. By far, the easiest place to install a transfer switch/inlet box and plug in a battery in a backup situation would be near the main service panel outside. Since these portable battery backups supply a max of 30A, I am wondering if it would be possible/safe/to-code to install a 30A transfer switch between the main panel and the 100A house sub-panel, so the house panel is fed either from the breaker in the main panel or the portable battery. 30A is definitely not enough to power every load in the house, so in a power outage, I would plan to shut off all non-essential circuits in the house and monitor the load on the battery.

Is this even feasible? I spoke with an electrician and he seemed to imply I needed a whole-house battery ($$$$) for what I was describing, which is vastly more than I am willing to spend for a maybe once-per-year backup scenario. I'm hoping he just didn't understand what I was asking.

jat255
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2 Answers2

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I am wondering if it would be possible/safe/to-code to install a 30A transfer switch between the main panel and the 100A house sub-panel

The 30A transfer switch is only rated for... 30A.

In backup mode, this would not be a problem: if the generator trips and shuts down above 30A, then the backup side of the transfer switch will never be forced to handle more than that.

In normal use, however, your 100A panel is expected to be able to draw 100A, and that will go through the other side of the 30A transfer switch, and it can't do that. That's the issue here.

bobflux
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This setup would be exactly the same as a home generator setup that trasnfers loads to a generator. Numerous examples of this exist and the proess is fairly straightforward.

Loads you want to power externally are moved to a sub-panel with their own breakers. There are other ways to do this but this is the most straightforward. This sub-panel can be powered by either the main panel or from an external source, in this case a battery. The loads attached to this panel SHOULD BE NO GREATER THAN THE EXTERNAL SOURCE CAN SUPPLY. This is not for safety of the circuit, but to keep from overloading your backup source. This is quite normal, not everyone can install a whole-house 200 amp generator. Many people will have smaller units. My local Blue/Orange stores carry switches for these panels, because I live in hurricane land.

Note that the simplest solution, used often by people with portable generators, is an interlock on the main panel which allows a breaker that goes to a plug you conect a generator to to be turned on only if the main breaker to the utility company is off. You plug in the generator, turn off the main breaker, turn on the generator breaker, and manually choose the breakers in the panel you want to power. Non-essential breakers are turned off. I have this at home, I have to turn off all the large loads (flip the breakers) since my house is all-electric and my generator won't run water heaters, A/C, stoves/etc. all at the same time. Without the interlock there are a hundred ways to do this wrong and kill yourself with a suicide cord or kill a power worker by energizing their lines from your house.

If you have a sub-panel, when you lose power automatic transfer is possible, there are transfer switches that will do this for you when you lose power. If not automatic, you walk to the garage/basement when the power goes off and flip a switch to the external (battery) source.

You are now operating all the loads on the sub-panel from your backup source. Some panels let you monitor loads, this is important when you are supplying 240V, each leg needs to be balanced. In your case you could conceivably send 120V only, and have one less thing to worry about.

Tiger Guy
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