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I plan on having a subpanel ran from the main 200A box to a garage. In order to future-proof electrical needs, do I need to ensure that the breaker serving the subpanel matches the amperage of the mains breaker of the subpanel?

For example, a 100A subpanel needs a 100A mains inside of it (Hence, the 100A rating). It also needs a 100amp serving it from the main box.

Is there harm in over-doing the amperage (assuming the existing circuitry can handle what's available) like that, but never pulling close to the 100A rating?

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    As long as the breaker in the main panel is rated for the size wire it is fine to go bigger than required. – Ed Beal Mar 31 '17 at 16:47
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The breaker in the main panel is the one that matters

The main job here is protecting the wiring from the main panel to the subpanel. The subpanel also has a rating, but subpanel capacity is cheap so it's very unlikely that the subpanel would be smaller than the wires feeding it. So the breaker in the main panel will be fine.

If the "future proofing" you have in mind is to allow installation of a larger panel later, just do it now. Panels are cheap and often come with a bunch of free breakers. I'm a huge fan of buying the largest panel you can possibly get. You'll thank me later!

The subpanel "main breaker" is just a switch

Its only function is as a shutoff switch, which is required for some subpanels. Nobody cares what its amp value is. You don't want to match it to the breaker ih the main panel, as that would force you into an unnecessarily small subpanel. Go ahead and put a 225A 60-space subpanel if you want.

Obviously if that main breaker has a GFCI or AFCI function, it will do that too. (that's more a thing in Europe or Asia.)

The only exception is if for some reason, the subpanel has a smaller rating than the wire/breaker feeding it; then the subpanel's main breaker would protect the subpanel. But that is a very foolish situation to be in; the difference in cost to a larger panel is typically $30 or $50 tops, and the cost of changing a subpanel is rather considerable.

I want the subpanel breaker to trip first

This doesn't really work. The vast majority of trips are a magnetic trip (a hard short with hundreds of amps of flow). It will be totally random which breaker trips first. For a thermal trip (overload resulting in slow trip), the breaker would have to be significantly smaller to assure it trips first.

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    Go ahead and put a 225A 60-space subpanel if you want. (Tim the Tool Man Taylor grunt) – Shawn Apr 3 '17 at 16:32
  • So essentially, when I upgrade my mains panel to 200A, I might as well put the same one in the garage and ensure the wiring from main to sub matches whatever breaker is feeding that sub? What's with all the specialized subpanels I see at big box stores with 100A or even less? – Shawn Apr 3 '17 at 16:49
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    @ThatWebDude Sure, you could use the same panel. Then your free/bonus breakers that come with the panel could be used either location. A subpanel is a little different from a main in that it must have a separate neutral and ground bar. The big-box stores engage their customers on the illusion of being cheaper. Easiest way to make a subpanel cheaper is smaller breakers/fewer spaces/fewer bonus breakers. – Harper - Reinstate Monica Apr 3 '17 at 18:15

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