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We're going through the planning phases of installing a pair of Tesla Powerwall units for our house.

We got this via the project contact from their engineering group.

Both Tesla Powerwalls are going on the 200A subpanel that is being fed by a 200A breaker. Without derating, you can only add 40A of generation (whether it is a battery or PV). The continuous amps of the two powerwalls is 60A, which exceeds that. By derating to 175A, you are allowed 65a of generation. So, 60A (Tesla) < 65A (max allowed).

Can someone explain to me how going from a 200A panel to a 175A panel facilitates this? Isn't "more, better"? By going from a 200A down to a 175A, is this what opens up the capacity for the extra 20A that the batteries need to provide (by providing (200A - 175A) 25A)? How does that work?

Note, we already have a 7.5Kw PV system installed (I honestly don't know if its through this panel or not, the breakers are in the other, main, panel outside).

Mind, I don't question the engineers. If it has to be done, I guess it has to be done.

I just don't understand it what is going on here, and what the longer term impacts this has.

Overall we have a 400A setup in the house. I guess it's via two panels (since this one is a 200A subpanel of the other one).

Any insight appreciated.

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    Derating doesn't mean "downgrade a more-capable thing to a less-capable thing". It means "keep the more-capable thing, but treat it as if it were the less-capable thing". – Joseph Sible-Reinstate Monica Apr 10 '20 at 20:59
  • What's your overall configuration look like? Do you want to be able to use the Powerwalls for backup when the grid goes down? Also, what make/model is the subpanel in question? – ThreePhaseEel Apr 10 '20 at 22:03
  • How do I find out @ThreePhaseEel? It's gray. They're all gray. They're always gray. – Will Hartung Apr 10 '20 at 22:29
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    Can you explain that a bit @JosephSible-ReinstateMonica? I clearly don't understand the concept of derating. – Will Hartung Apr 10 '20 at 22:30
  • @WillHartung -- can you post clear photos of the labeling on the inside of the door of the subpanel then please? – ThreePhaseEel Apr 10 '20 at 23:20
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Yeah, that's a thing. The problem is you have potentially two energy sources feeding the panel: The 200A main breaker and the 60A Tesla PowerWall. That's 260A on a breaker bus that is rated either 200A or 225A - I suspect the former, though the label will say for sure.

Now, they give you a mulligan for 20% of the panel ampacity. So they'll let you slide up to 240A on that 200A busing. But you need 260A so that is not enough.

So, one option is to derate whatever breaker happens to supply this panel. Now you have 175A + 60A = 235A = you're inside 240A.

Another way is to go talk to the dealer who sells those lines of panel. They put a large variety of panels on a limited number of panel box sizes. The goal is to find a panel with 225A-rated busing, where the bus assembly will bolt up to the same mounting holes that are in your panel.

Now you'll have 225A x 120% = 270A, which covers your 260A.

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  • So by derating the panel, they're lowering the POTENTIAL draw (since ideally I'm not draing 200A) for the entire set to allow room for the new power source from the batteries. Is that's what's going on? Since I have a "400A" service, I assume that means in reality I have 2, 200A services. And I, in theory, can have 10 circuits on each for 20A each. – Will Hartung Apr 11 '20 at 19:56

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