Two breaker boxes -indoor and outdoor

Question

We have a meter main box outside the house that is 200 amp. It has the main shut-off up top, (a breaker that we don't know what it goes to and is shut off) and a double 30 amp for an outdoor 220 plug for a welder. The Indoor box also says 200 amp and breakers total about 345. My question is "does each box handle 200 amp separately, or is it just a total of 200 amps for whatever we want to do inside or outside?

If the main box has room, we want to do a spa box for a hot tub and run wires (110) for a pool pump.

Answer 1

There are some variants (e.g., 400A, a.k.a., Class 320 service, which normally gets 2 x 200A boxes), but what you describe typically means:

• 200A total service
• 200A main breaker outside protects everything
• Individual smaller breakers outside for specific (generally large) uses
• The full 200A feed is sent along (after the 200A breaker) to the inside 200A panel

So you almost certainly only have 200A total to use, however it gets divided up. The next step is what is called a Load Calculation. A Load Calculation is a very specific means of figuring out how much power you need overall and in each panel. It is not:

• Add up the breaker handles. 345A > 200A and that's OK (though your actual number is probably something lower because 200A is at 240V and many of your breakers are single breakers at 120V).
• Look at your power bill and average the usage by day (totally off, need the peak) or even take peak demand from your power bill (if your utility provides that number, because it varies year round for various reasons).

It involves adding up certain standard loads for kitchen and bathroom required circuits, certain factors on cooking equipment (there is a presumption you aren't using everything at once all on "high"), highest of heating or cooling HVAC (which varies a lot depending on the efficiency and type of HVAC equipment), a certain amount per square foot of space, and any other large loads (EV charging, water heating, etc.). It is a non-trivial exercise, commonly done by electricians when determining needs for a panel or service upgrade.

So you first need a Load Calculation for everything you have installed right now (both inside and outside, though the 200A main breakers inside and outside doesn't itself count for anything - that's protection not actual usage, it is the branch circuits that matter). Then you see how much is left over and determine whether the circuits you want to add can fit in the balance. If they can fit, great - add them. If they can't fit then there are a number of load management things you can do, particularly as described for EV charging in this wonderful "canonical" post.