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What the is highest amp breaker I'm able to put on this QO Load center QO20-40m200 series L1?

My objective is to add a subpanel for more space, and the 125amp subpanels have a lot more space compared to 100amp subpanels, but it seems this panel's busbar might be "rated", if that's the right term, for up to 100amp breakers. There seems to be information on terminals No. 6 to 1, but how about the other breakers No. 7 to 20?

There doesn't seem to be much information from the manufacture on this panel that is original to the home build in 1969. Square D/Schneider Electric tech support mentioned they don't have any product info on this product, and their website product search for this model, doesn't seem to have anything on this specific model.

Is the qo20-40m200 series L1 most similar to the now discontinued "QO120-40M200" model? If so, is there anything in the datasheet or other document that could provide more details on single breaker safe capacity?

Additional information on this project, a few of the breakers from the main panel could be removed but we'd like to have the redundancy. A 35 amp breaker for a 24btu 240v mini split system is the catalyst for the additional space. The soon to be somewhat redundant heating for the home is six 240v 1500 watt electric force fan wall heaters that is taking up a lot of space in this smaller panel. We should saving a good amount of energy and lightening the overall load on the panel, hence wanting more spaces than less.

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    You can feed a 125 amp or even a 200 amp sub panel with only a small(40 or 60 amp) breaker. A bigger sub panel does not give more power, just more breaker spaces, so a load calculation might be needed.
    – crip659
    May 30, 2023 at 0:30
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    I do not think that the copper wire handle ties on the top breaker are allowed in code.
    – crip659
    May 30, 2023 at 0:50
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    Panels are rated for the maximum amps they can handle. Usually for the very little cost for big panels compared to the smaller panels, the recommendation is to go big. The problem is if you can fully power a big panel, but then you just use a smaller breaker to feed it. It seems from the label that you can only use up to 100amp breakers in the top spaces. Unless you can find a label for the lower spaces, then that is what you need to go by. Can consider labels as the word that goes above what code says.
    – crip659
    May 30, 2023 at 1:04
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    Labels are part of the manufacturer's instructions, which code says you must follow, actually... If you want spaces, put in a 40-42-space 200A panel as a subpanel. Feed it from a 100A breaker here, unless you really want to go through the hassle of swapping the feed over to it and making this the sub-panel. If you're paranoid that you'll run out of power, get two and run BOTH of them from 100A breakers here....
    – Ecnerwal
    May 30, 2023 at 1:36
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    Some problems with that panel. It looks like extensive use was made of dual singles with approved handle-ties. That is fine for a) MWBCs or b) 240V-only circuits without neutral (white hot wire). However for 120/240V circuits (dryer, range) with neutral, handle ties are not enough, it must be a factory made 2-pole breaker (distinctive in QO for having only 1 handle). That is because common trip is required. May 30, 2023 at 1:56

2 Answers 2

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Your question implies that you suffer a common delusion that "feed breaker must match sub-panel size" - it DOES NOT.

Feed breaker must be sized to protect the wires (but the wires can be oversized - they will still be protected.) Sub-panel must be rated at least as large as the feed breaker. Sub-Panel can also be ANY size larger than the feed breaker.

So, if you want 125A panels "for the spaces" just put in a 100A breaker to feed that and you're good. Or buy a 200A panel for (perhaps) even more spaces. Both of those can very happily and legally be fed from a 100A breaker.

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  • Good point. after a conversation with a retired electrian today, I'm leaning toward replacing the panel to a 42 space panel, and scrapping this whole subpanel idea. Nevertheless, why would the busbar not be able to handle or be rated up to 200 amp, since that is what the panel is rated for? It seems 125amp breakers would work fine for a panel that can handle 200amps. Did 125amp breakers not exist hence not specifically mentioned? Is it common for the best place to place higher amp breakers closer to the main 200 amp breaker shut off?
    – user289394
    May 30, 2023 at 22:43
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    The issue is that it's a somewhat poorly documented (according to your question, and I'll believe you) 54 year old panel. It's a good one, (QO's have a deserved good reputation) but there's a difference between running 200A into the full-width bus-bars on those bolted connections to the main breaker and running more than 100A out of the small tab on the bus that a breaker connects to by a spring clip. You can see two of those just above the 50A dual with nothing connected to it. Notice how much smaller they are than the bus itself they connect to?
    – Ecnerwal
    May 30, 2023 at 23:04
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    If you swap the main feed over to a new panel, there is NO reason not to keep this perfectly good (if poorly documented) QO panel as a sub-panel. It could even have full power available to it if your new panel is chosen to have "feedthrough lugs" (large lugs on the bottom of the bus bars to connect 200A wire to an additional panel, protected by the main breaker in the panel with feedthrough lugs, only.)
    – Ecnerwal
    May 31, 2023 at 12:22
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A bunch of issues:

  • 6 to 1 is not referring to breaker space locations. Yes, sometimes certain breaker locations have particular preferred uses. That's not what this is about. This is referring to wire sizes for 70A and 100A main breakers. The sizing makes sense. 6 AWG copper is rated for 65A at 75 C, which gets upsized to allow 70A breaker since there are no 65A breakers. 1 AWG aluminum is 100A at 75 C. Everything else you might use (copper or aluminum) will be between 6 AWG and 1 AWG.
  • Harper pointed out that there are some handle-tied breakers that probably need to be actual common-trip double breakers. In addition, there are some breakers at the top that are handle-tied with bare copper wire - they couldn't even find a right-sized nail! If those breakers are otherwise OK then replace the handle-ties with proper Square D handle ties - they're all of $7.50 for a 3-pack at Home Depot.

The instructions really don't say much about maximum branch breaker sizes. At the time, there weren't generally many larger than 60A. Since this is a 200A panel, I would keep the large loads (HVAC, range/oven, EVSE if you need that, etc.) in the main panel and you could probably get a lot of usage out of running even a 60A feed to a 100A or 125A or even 200A subpanel. Note that a subpanel doesn't have to be "only" a subpanel - it can be a big "main" panel if the price is right (and it often is!). The only thing that really matters is keeping ground and neutral separate. If you put in a large subpanel but populate it with lots of small circuits and do a load calculation that shows it fits in 60A (that's 60A @ 240V, so 120A of 120V circuits, and based on load calculation/usage and not adding up the breaker sizes) then it really doesn't matter if it has a lot of stuff in it.

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    The “6 to 1” is more specifically talking about type Q1 main breakers that can optionally be installed instead of the default Q2 200A unit. Branch breakers will come with their own instructions and wire size information.
    – nobody
    May 30, 2023 at 2:34

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