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Like most homes that have a 200amp service at meter box main...can I add a 100amp in that same box to run to my detached garage about 60 feet away? Would upgrading the 200A to a 400A an option and does that cost double on service charges monthly? Maybe add a 100amp to the inside breaker box?

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    maybe or maybe not, depending on load calculation
    – Traveler
    Commented Jun 16 at 3:24
  • Can you post photos of your meter-main please? Commented Jun 16 at 16:02

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The starting point is an NEC Load Calculation. That takes all your fixed loads, HVAC, size of the building, cooking equipment and other stuff and comes up with a number. That tells you how much you currently need to provide to have everything working properly. That number should be something less than 200A (or more specifically, 200A x 240V = 48,000 W) but how much less is a total unknown. Whatever excess capacity you have (measured in A or in W and divide by 240) is what you can send to your garage.

Keep in mind that even if you have a 100A panel (or even a 200A panel!) in the garage, you can provision it for much less power - e.g., 50A or 60A or whatever you can spare based on the utility service load calculation.

If you don't have enough capacity then you have a bunch of options:

  • Change your new loads. For example, EV charging doesn't need 60A. 30A is plenty for almost everyone and even 20A is often enough.
  • Add load sharing and/or load shedding capability so that power is used by large loads (e.g., water heating, EV charging) only if there is sufficient capacity at that moment in time. Since peak usage is rare, this can work very well.
  • Replace inefficient equipment such as resistance electric heating.
  • Heavy-up - go from 200A to 400A.

What will 400A cost? It may come with additional fixed monthly charges, but those depend on your specific utility so we have no idea what they will be. It will likely require some significant upgrades, depending on the current status of your meter, panel, etc. The costs vary considerably depending on the level of changes needed, local requirements (dependent on the version of the NEC adopted in your area) and utility requirements. I would consider it as a last resort, unless you really truly need a lot more power.

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  • Also your existing panel label instructions may limit the amperage of breakers or the amperage of breakers installed back-to-back. Commented Jun 16 at 4:20
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There's both NEC ( National Electricity Code) and Electrical math involved. It comes down to: What are you total calculated current loads in the main panel /and will the authority having jurisdiction ( the local city / county electrical inspector allow it. I'm almost certain after first knowing the loads you currently have on the main panel are established, with that knowledge and traveling to the office where your local inspector resides and talking with him / her you'll know. Don't be afraid to ask if they don't invite themselves to your residence to see your project of interest.

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