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I'm considering purchasing a house and I took a look at the electrical panel. The house was built in 1979 and at some point they added a sub-panel. The breakers in the original panel look like a combination of different vintages, I can't tell if these are all the appropriate breakers. It is a Murray panel, and I recognize the MP-T style breaker in the lower right, but I have never seen the ones with the red handles. It looks like they used to have an electric stove and oven, but those are not currently in use. I would like to potentially put an inductive stove in. There is a sub-panel right next to it, that has plenty of space, but given that it is protected with a 60A breaker in the main panel, I don't think I can add more load there.

Any other red-flags about this panel?

ThanksOriginal Panel Cover Original Panel breakers Original Panel Description Sub-panel Close-up of breakers

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    It isn't so much "different vintage" as "right or wrong type". The subpanel is clearly all Square D breakers in a Square D panel. But the main panel looks like it has different types and very likely some are simply wrong, no matter how old or new. I particularly suspect the pair of 20A breakers immediately above the subpanel feed. As far as subpanel capacity, you may well be able to add more circuits even with the 60A feed, depending on what the existing circuits feed. May 12, 2022 at 1:38
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    On the main panel, is there a paper label under the taped-on directory labeling on the inside of the door? If so, can you get us photos of that paper label please? May 12, 2022 at 3:50
  • @ThreePhaseEel I won't have access to the panel until after we close escrow, but at that point I will see what is below the taped-on directory. May 12, 2022 at 4:21

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Top left doesn't look like a Murray. It looks like a GE maybe -- and it's an A/C unit! Alien breakers are most common on A/C units because the contractors don't care and aren't electricians.

As far as "old breakers", I have to guess that you haven't priced these breakers. You could replace the whole kaboodle for under $100 total, so why wring hands? Just replace any you don't like.

On the Square D "QO" subpanel, that panel uses a completely unique bus design that nothing else can possibly fit in. So it it is impossible to use alien breakers. Every breaker which fits in a QO panel is approved for a QO panel. (note this includes Eaton CHQ and Siemens QD types, which are made exclusively for QO panels).

As far as the QO panel maxing out the 60A breaker, how did you get that? However you got that, it's wrong. The only correct way to ascertain that is to do a Load Calculation on the panel as described in NEC Article 220. The only way that five 20's and a 15 could possibly "max out" a 60A panel is if each circuit is "maxed out" (i.e. each 20A circuit has 16A of grow lights or Bitcoin miners that are running continuously all day all night). Other than that, the breaker handles don't enter into the Load Calculation very much; you just have to crunch the numbers per Article 220 or helpful municipal worksheets that are easier to understand.

You might note that the panel is loaded lopsided; breakers 1,2,5,6 load phase L1, and breakers 2,3 load phase L2. That's not a problem; 1,2,5,6 are nowhere near overloading phase L1, but as you add additional circuits, make an effort to balance them. Panel buses are mapped like this.

One more thing, notice that with all the double-stuffs, every bus stab in the original panel has 90A or 100A stab loading (30+30+30; 30+30+20+20; 60+20+20). Be careful when arranging things, most panels have 125A stab limits.

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  • I have done a load calculation before using Article 220, but it was for a single panel. How do I perform that calc for this main+sub panel? Do I just look at all the loads in the house served by both panels and figure out if the 100A main breaker is sufficient? Or do I have to do something per panel? May 12, 2022 at 4:32
  • Good point about the lopsided loading in the QO panel, I'll keep an eye on that. I'll see if I can just replace all the main panel breakers, hopefully I can figure out from the panel what the correct breakers are. May 12, 2022 at 4:39
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    @ChristianK for the main panel, you do the whole house. For the sub, you consider only the loads served by it. For stuff like the "3 VA per square foot" you just have to guesstimate what fraction of your square footage is served by those circuits. The old main is a straight up Murray and uses Murray MP or Siemens MP or QP type (MP was renamed QP for no good reason I can see). All that stuff is readily available. May 12, 2022 at 6:08

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