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I have a Generac 26K generator with a 200 amp transfer switch I have a 200 amp main service. I want to be able to power my entire house in case of a power outage. What size 2 pole breaker do I need to put in the main service panel for the transfer switch.

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    Have you considered what it might take to power your entire house? Commented Feb 16 at 4:18
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    To get a good answer, provide more details: transfer switch model, how mains and generator are connected to your home, etc. I recommend derating your generator by 30%, that means you will have 18KW/75A of useable clean/reliable power. Commented Feb 16 at 5:30

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I want to be able to power my entire house in case of a power outage.

Your entire house? In case someone wants to run a dryer load? Keep that hot tub hot? Charge that EV anyway? No compromises?

NEC 702.5(B)(2) says the generator "shall be capable of supplying the full load that is transferred". That means you'll need to do a NEC Article 220 Load Calculation on the service, such as 220.82 here, and make the generator that big.

Your generator is only good for about 110 amps, so you'd need to have a fairly light Load Calculation, or deploy some alternatives.

First option is automatic load shed devices. These attach to an appliance circuit and monitor it for voltage or frequency sag, the hallmark of a generator bogging, and drop out their load when that happens. A load thus protected can be removed from the Load Calculation.

Second option is a critical loads subpanel so you just don't have the "too much" loads on the generator at all. Now in a case like yours, it's too much work to move dozens of small circuits out of your main panel into a subpanel. It's a lot easier to remove 2-3 loads and put them on a non-backed-up panel.

You can put a "trailer/ranch panel" with thru lugs between your meter and your existing main... and put your non-critical loads in the 8 spaces in the ranch panel. You come off the thru lugs to the transfer switch then onward to your "main" panel.

They even make trailer panels with built-in electric meters, but I'm not a fan of these because of changes coming in electric cars and vehicle-to-home.

The trailer panel is also a great place for surge, solar, geneartor interlock for a smaller generator, etc.

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