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I will be installing a subpanel with a 100A coming off my main panel which is a standard 200A panel. Is it fine to put that 100A breaker at the bottom of the panel or should I move everything down 2 slots and put that breaker at the top if there is enough slack in the lines to do so?

I heard one time that if you put large draw items on the bottom it can cause excessive heat to the bus bar and lead to failure over time to the whole panel.

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The bus bars, bus stabs, in the main panel will handle the maximum current allowed by the main breaker so where that current comes from doesn't matter. The 100 amp breaker can be at the top or the bottom. In new construction, I've always started at the top and worked my way down so I did the large breakers first to get the larger wires in the panel and out of the way. Since your main panel is existing, install the breaker where it's most convenient.

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  • “The bus bars, bus stabs, in the main panel are rated for the maximum current allowed by the main breaker” That is absolutely not true in general. Stab limits are a thing - check your panel label.
    – nobody
    Feb 4 at 15:02
  • Limits are usually in the 100A to 150A range, so putting a 100A breaker across from another 100A breaker would likely be forbidden even though they wouldn’t exceed the overall panel rating. They’d need to be in different rows.
    – nobody
    Feb 4 at 15:09
  • @nobody Bad choice of words on my part.
    – JACK
    Feb 4 at 18:19
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Nope, that'll make you "zig" when you should've "zagged".

I heard one time that if you put large draw items on the bottom it can cause excessive heat to the bus bar and lead to failure over time to the whole panel.

That idea leads to a conclusion, which is that the breakers should be down the panel largest first, with the 2 largest breakers opposite each other. However, this is nonsense.

What is definitely not nonsense is "stab limits". The stab is the little blade or bar that the breaker clips onto, and serves the breaker on the left and the breaker on the right. We can tell stab limits aren't nonsense because they're stated right on the panel labeling, and must be followed per NEC 110.3(B).

So you can see: there's an inherent conflict between the "pop logic" of bunching big breakers at the top, vs UL/Code requirements of stab limits. Your 100A would be opposite another large breaker of 30-60A, and you'd have 130-160A on that bus stab.

The best thing to put opposite a 100A breaker is a generator interlock or a surge suppressor. In generator mode, the stab cannot exceed generator output x 2 even if it was all going to the garage. After that, lightly-used circuits which for some reason need to be dedicated, such as a 1 amp fridge (common sense) or a 6 amp gas furnace (Code).

Note that the loads in the subpanel need to be included in the main panel's NEC 220.82 Load Calculation. If this is for EV charging, there's a "50A misconception" which is bonkers overkill for home needs. Technology Connections has a first rate video on the topic.

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    @isherwood No, it's "bus stab". Try searching <panel brand> stab limits and you'll see the term "bus stab" or "buss stab" widely used. Nov 10, 2023 at 20:28

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