Tesla is installing a 12KW system with 1 PW+ and 1 PW. These are feeding a Gateway 2 via 50A and 30A breakers, along with 125A from my main panel. My main panel is an old Square D 200A panel that supports only 3 circuits, with 1 circuit at a max of 125A. I currently have 3 circuits on my main panel, 100A for my house subpanel, 30A for a pool subpanel, and 25A for an AC. The Gateway 2 will receive 125A from the grid via one circuit from my main panel and all circuits will be relocated to the backup subpanel. Tesla's connection from the Gateway 2 to the backup subpanel is currently wired and backfeed as 125A. I've requested many many times to upsize this to 200A, they have so far refused. One claim is that load calculation shows I only need 124A. I told them really? They know for sure it's 124A and not 126A? How can I get them to upsize it? I'm ready to just buy the wire and 200A breaker myself and have it ready for my install in 2 weeks and give it to the installer. Any ideas why what I'm asking for can't be done. Here is the picture:
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There is no technical reason why you cannot have a 200A subfeed from the GW2 to the 225 essential loads subpanel, except they might not be easily available/extra expensive right now. There is a widespread shortage of main breakers right now, so when you can find them they are expensive. Increasing this feed to 200A will give the slight benefit that when you do try to draw more than 125A down the road after your second EV the 3 sources can combine to serve your loads.
If you were to pull 200A through your panel with that 200A feed, then both the Powerwall and Powerwall+ would all work together to serve those loads, especially if Site control was set.
As long as the panel has the main breaker slot, you could change this afterwards if you choose but you won't have the advantage of the Site control to manage that 125A breaker in your main service panel.
From those I have seen, the 200A 4 pole units do not have a hold down, so cannot be backfed.
I have a related question about breaker vs. lugs on a subpanel like this.
In my original installation in 2018, the installer wasn't going to put any breaker at the input to the Generation Panel. I have a Gateway 1 and one set of Gateway Backup lugs goes to an original subpanel and the other set goes to the Generation Panel. My solar was relocated from the main panel to the Generation Panel and of course, the Powerwalls land in the Generation Panel. I thought it was odd that there were only lugs on the feeder to the Generation Panel and mentioned it to the installer. He responded by disconnecting the feeder from the input lugs and landing it on a newly installed breaker, 100A if memory serves correctly. Without this breaker, there would be no feeder breaker between the Generation Panel and the original subpanel that has most of my household loads. The grid input to the Gateway is fed with the 125A breaker that originally fed the original subpanel.
So, the actual question is - does a Generation panel require a feeder breaker? When off-grid there would no feeder breaker between the subpanel and Generation Panel. There is no actual danger of overload because the Powerwalls will shut down if overloaded more than 2x30A and even adding the solar output, the feeder would not be overloaded. I suppose the only overload situation would be 125A grid + 2x30A PW + 40A solar. However, adding a 100A breaker at the top of the Generation Panel would not actually address that situation.
The answer is "it depends" If this panel is exposed to sources that are larger than its wire, and busbar, it does need a main breaker.
Generation panels in general have all the sources inside them, so typically are exposed to power from either the main breaker or from the sources inside them. So if the generation panels subfeed wire, and panel are 125A rated, then it does not need a main breaker as long as the sources within it are less than 125A.
However, if there was a 200A breaker in the main which fed the Gateway then using 125A wire and generation panel would require using a 125A main breaker in the generation panel.
Also if a generation panel has more than 6 sources of power, it needs a main breaker as well for ease of disconnection of power.