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I currently have an older 125amp QO square D panel with 16 total spaces. I'm trying to update some of the electrical in my house and have some overloaded circuits that I'm now separating and dedicating certain appliances. I don't have a lot of space in my current panel to re-feed some circuits or expand. Few things to note: My calculated total load for my home is approximately 63 amps (Assuming I calculated that properly). I have roughly 1600 SQ ft and all gas appliances. Vast majority of said appliances are less than 10 years old and of higher efficiency. I did base my calculations using the nameplate value on all my main appliances.

I want to add AC this coming spring/summer. Otherwise I don't plan on adding more to the home pending I win the debate to not get a hot tub.
Tandem breakers are not much of an option since it's difficult to find them in the older QO style. Home Depot has them but with a "hook" for the point of attachment and that is not compatible. Furthermore, updating some things to code for GFCI or AFCI requires those to be breakers and not outlets since they would be inaccessible.

Ideally, I would prefer to upgrade the main service panel for one with a main disconnect and significantly more spaces at the very least and running a sub panel to my attached garage for convenience. Upgrading the panel with everything going on right now doesn't seem ideal plus wrestling whether or not to just do a service upgrade at that point and have peace of mind that I will have more than enough room for expansion if needed.

With all that being said. Can I place a 125amp sub panel with my existing 125amp main service panel? Feels weird that the sub panel would be the same amperge rating and having more spaces. Otherwise, I'm wondering if I can use a 125amp rated panel with 100amp breaker and #3 Cu wire to get the more circuit spaces. Also, would anyone recommend if I should place the AC circuit on the main or sub panel?

Looking forward to any advice. Sorry to be long wimded.

***Note: sub panel would be on the same wall as the main panel and very close to one another. The additon of AC would be on the exterior of that same wall right by the service entrance.

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    @George Anderson covered everything pretty good. You mention putting the panels very close together. Just remember to allow for the covers as they overlap quite a bit and inspectors are really sticklers about how they fit against the walls and the screws you use.+ – JACK Apr 14 at 21:19
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    Thanks. I'll keep that in mind. I'm looking up what wire is available in my area. I may just try to run PVC between the panels? I'll probably have them about 18" apart end to end I'm guessing so the panel covers will not over lap. – Elmd13 Apr 14 at 21:49
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I'm pretty sure you are OK with having a 125 amp sub-panel, even off of a 125 main panel. You don't have to supply the sub-panel at 125 amps, just what you anticipate the load will be there. I have a 200 amp sub-panel in my shop, fed at 125 amps. You'll of course need the correct size wires and breaker for whatever amperage you choose. You'll naturally need to treat it as a sub-panel (isolate ground from neutral and provide 4 wire service to the sub). The Square D QO panels are high quality (IMHO) and shouldn't need replacement like Federal or Zinsco panels. You may need to free up some space in your main panel to make room for the breaker to the sub-panel by re-routing a couple of circuits. Not sure if your main panel is full or not. Maybe I've missed something, if so, I'm sure one of the 3 wise men will weigh in!

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  • Thank you for the response. I have 1 space left and trying to free up another but the circuits I already re ran I left slack in the ceiling exactly for this reason so I can move them over when I get a more permanent solution. – Elmd13 Apr 14 at 21:55
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No. You should install a 200A subpanel.

125A subpanels are just too small, and you'll be right back to "my panel is full" far too soon. And the busing doesn't matter as long as it's >=.

Also, since it'll be in the same house, you might as well use a main-lug panel. A main breaker is a waste of money.

I like to see around 48 spaces total. Since you have a 16-space panel now, I'd suggest a 30-space. QO is a top panel but they charge too much for their breakers, and their GFCI/AFCI breakers are annoying to use. HOMeline is pretty cheap and limited though. Siemens is a great choice if you plan a generator or homebrew solar/battery system.

Preferentially put AFCI and GFCI breakers in a non-Square D panel

A weakness of Square D AFCI/GFCI breakers is poor diagnostics. You have to "do the secret dance" to figure out if you got hit with an AFCI, GFCI or overcurrent trip. Other vendors have better options with breakers with LEDs and clear labeling. (remember you cannot put brand X breaker in brand Y panel for technical reasons relating to bus stab fitment).

Further, breakers are generally more expensive in QO panels.

So where you expect to need GFCI/AFCI breakers for future circuits, try to put them in the new panel.

As said, if you have thoughts toward a generator or homebrew solar/battery system, put the circuits you might want on generator into the subpanel.

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  • SqD does go up to 30-32 spaces in 125A bussing, but yeah, at their pricetag, you're better off with 40-42 space panels, and those are only available in 200-225A. As a note: I've been tinkering with a "not-quite-dream-house" in my head, and as a part of it, sketched out the electrical system one-line style...results: 88 spaces spread across 5 panels, with the system about 3/4ths used, and three sets of lugs available for attaching more panels (in addition to room for at least 2 feeders for outbuildings, one of them capable of 125A or more) – ThreePhaseEel Apr 14 at 22:58
  • 3PH and Harp, you guys are doing a "more power" right out of the comedy show "Home Improvement". Let's get a bit more practical for the average guy. – George Anderson Apr 15 at 0:41
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    George, 3PH, & Harp have given some good info. Yes, your sub amperage can be equal to your main. It can even be rated higher (buss), but not fed with a larger breaker than your main. If in doubt, go larger - both spaces & amperage - to leave room for future growth. It's much cheaper to do now than have to replace it later. For panels, I used a 20 space 40 ckt Cutler-Hammer as a sub for my kitchen remodel. After adding 11 breakers, it was more than $100 less than Square D. – Eric Simpson Apr 17 at 17:56
  • Positioning your AC... at 30A+, mine is the single biggest electrical load in my house, so I put it on my main panel as close to the main breaker as possible. If I add an electric car charger (or hot tub) later it'll also go on the (then upgraded) main, next to the main breaker. If you add solar or wind generation, they usually go on the far end of the sub panel to reduce total bus current. – Eric Simpson Apr 17 at 18:03

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