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Title is pretty much it. I've got a 200 amp main panel and a 125 amp sub panel on the other end of the basement. I need to add four 220v circuits (an air compressor, two network racks, and what's basically a glorified hot plate). The sub panel is in the basement right below the garage and kitchen and the network racks are going to live ten feet from the sub panel in the basement, so I'd rather add circuits there.

The house is about 30 years old, and both the main and sub panels are in good shape. The main panel has six slots available (though they're three per side) and it's far enough away to make it less attractive for these loads, and I'd like to add other circuits to that end of the house eventually.

All the sub panel slots (16 of them) are full. It's got both HVAC air handlers and compressors, a pool pump, and all the kitchen. I can pull it off and replace it with the biggest sub panel I can find (or biggest up to the amperage, I guess), or I can add another sub panel off the existing sub panel (a sub-sub-panel?).

Here's the main panel, and here's the sub-panel. The house is about 5000 square feet, and that sub-panel serves both HVAC systems, the refrigerator, oven, and stove, and the washing machine and (electric, 220V) clothes dryer. It's also serving a swimming pool sub-(sub-?)panel. This sub panel is most of the draw of the house--the main panel is just lights and wall plugs and feeding this sub-panel.

I know I can replace this sub-panel with a larger (in terms of available slots) sub-panel or just install another sub-sub-panel to accommodate the new circuits. An additional panel would go right next to this one, in terms of location. My question is more, "what's the 'better' approach?" Are there good reasons for or against just pulling this sub panel and replacing it with one with more slots? Or will I wish that I'd just put another 16-slot sub-panel right next to this one in a year?

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    What's the NEC Article 220 Load Calculation on the subpanel as it is now? What will it be after the new equipment? How about for the entire service? There's no limit on subpanel size by the way, but those Load Calculations will force you to 400A service eventually. Apr 5, 2023 at 3:40
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    Can you post photos of your existing panels + the square footage of your house and the area servec by the supbanel please? Apr 5, 2023 at 11:44
  • ThreePhaseEel, I'll edit my post with pictures and house size. As far as size served by the panel, how do you mean with respect to HVAC units and other equipment? There aren't any circuits on that panel that serve and 'area', except that the HVAC system serves the whole house.
    – Scott
    Apr 10, 2023 at 16:55
  • If there are no lighting/receptacle circuits on the subpanel, that's fine, we'll just need nameplate data for the HVAC units then. I take it all the kitchen circuits on that panel are for kitchen countertop receptacles and fixed kitchen appliances? Apr 11, 2023 at 4:32

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Well to start off, NEC Article 220 Load Calculations need to be done for the loads on each planned subpanel, and the service at large. (you can't just keep adding more and more and more loads to a house; actually a Load Calc is required at every step, with service upgrades as appropriate.) However, a wide variety of energy management tech is just around the corner, allowing time-shifting of storage loads to avoid the necessity of service upgrades. This video scratches the surface of this, but understates what is possible, and focuses too much on the costly SPAN panel while missing a number of much more economical innovations.

Next let me address a misconception that I think is hamstringing you. But first a question. I drive a 2000 econobox that can barely go 85 mph. Which tire speed rating is required for my car's tires: a) 85 MPH, b) 112 MPH, c) 130 MPH, or d) any, since all >= 85 MPH?

I can pull it off and replace it with the biggest sub panel I can find (or biggest up to the amperage, I guess)

There it is right there. By the way, the answer to my tire question is "any of the above". You were thinking a panel fed 125A (which it wasn't by the way) must be exactly 125A rated. Nope. 125A is only the "never exceed" redline. You are free to use any panel whose rating >= 125A.

The largest panel available is 84 spaces, but the largest panel that is economical is 42 spaces.

But none of that gets you out of the Load Calculation and either the service upgrade or an energy management strategy.

When I say your subpanel isn't fed 125A, what I mean is, your wire isn't 125A. 2/0 aluminum is 135A, 1/0 aluminum is 120A, #1 aluminum is 100A, #2 aluminum is 90A. #2 copper is 115A, #1 copper is 130A. The wire capacity (or breaker, whichever is less) is your actual subpanel capacity that you must use on the Load Calculation.

As long as the Load Calculations are in order, it doesn't really matter whether you add a sub-sub panel, enlarge the subpanel, or run a second subpanel also off the main. Depending on your loads, the Load Calculation may force your hand here.

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Harper has provided an excellent answer regarding the need for Load Calculations. But you've got an additional challenge, Challenger. Google it a bit, and you'll find that while Challenger panels are generally OK, the breakers have issues. There are replacement breakers available (Eaton BR) but you've got, as far as I can tell, a mix of Challenger and other brands of breakers. So this isn't even going to be the preferred situation of "get a new panel of the same type but larger so you can transfer the existing breakers". You can't, or at least should not, transfer most of the existing breakers to any new panel.

In addition, your main panel is also a challenging Challenger. I don't know whether the main breaker is an issue or not, but you may be facing replacement of some or all breakers there as well.

And on top of all that, based on your description and your very full subpanel (including 4 pairs of tandem breakers), I really suspect you are at your limit on 100A in the subpanel. Putting in a huge subpanel won't help if you're just going to be popping the feed breaker when you run everything. Keep in mind that while your air compressor and hot plate are likely short-term loads, your network racks will be running 24/7 so they really impact the total usage.

I would seriously consider the following:

Main Panel

Replace all Challenger or other incompatible breakers with Eaton BR breakers. Read this post for details.

Subpanel

Replace the subpanel with a new, much larger, panel. At least 30 spaces, but more (40 or 42, depending on the brand) is better. It can be a true subpanel (no main breaker) or it can be a "main panel" which includes a main breaker. Make sure that it is a 200A-rated panel. It it includes a main breaker, make sure it is a 200A breaker. Assuming you get a main panel, remove the neutral/ground bond and you may need to add an extra ground bar (depending on the particular panel).

Why, you may ask, should you get a main panel when you only need a subpanel? Because a main panel often comes bundled with extra breakers that make it worthwhile. Since this is a panel replacement, you normally are not required to put in AFCI and/or GFCI on existing circuits, and the bundled breakers are always plain breakers, so that works out just fine. Don't worry about which brand in terms of matching the old panel - almost all the breakers are Challenger that you don't want to reuse anyway.

That will still only get you 100A, which is almost certainly not enough for all your new loads together with the old loads. However, once you have done this, you can move to step 3:

New Subpanel Feed

Run new wire in conduit or cable from the main panel to the subpanel. Then replace the feed breaker with a larger one that will allow you to send more power to the subpanel. The catch is going to be figuring out how large you can go. In a quick search, I found the BR2150 150A breaker. Some panels (but I am 99+% certain not this one) allow feed through lugs to another panel, where the main breaker (200A) protects the feed, provided you use wire rated to handle 200A.

Note that there are some special rules (which I don't totally understand) that basically say "no wire ever has to be larger than the service feed", which may allow you to use smaller wire for a 200A subpanel than you would otherwise need for 200A.

Once you do this, you will finally have enough capacity at the subpanel to be able to add the new circuits.

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