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My garage and garage apartment has two obsolete 1960s-vintage subpanels which I'd like to replace (photos below). The first one, with 6 breakers, will need to stay where it is since the conduit is the feed from the main panel and it would be an involved task to replace or relocate it. Old 60 amp sub panel

But for the second panel, with 7 breakers (only 5 currently in use), I'm asking if it would be good practice to strip out the breakers, use the shell of the panel as a junction box, and extend the hots and neutrals from the existing breakers through the 1" EMT to the new panelboard and the relocated breakers within? If I do so, may I keep the grounds (only) on the existing circuits tied to the old ground/neutral bar, perhaps with a green ground tie back to the new subpanel? (Yes, I will be careful not to tie the grounds to the neutrals in the new panel. I'll also properly secure the Romex cables to the box in clamps.) Editing To Add: The two panels are physically only about five feet apart.

Old 60A subpanel for apartment

At some point I want to have the garage and apartment entirely rewired, but for the time being I want to upgrade these old installations. I've already replaced one subpanel of similar vintage which likewise had grounds tied to neutrals; if I complete this project it would make the entire installation code compliant in that respect.

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    Is the 30A breaker that's turned on in the lower right connected to anything. or just filling space? Likewise the weird-looking 20A which is shut off on the lower left?
    – Ecnerwal
    Apr 29 at 11:40
  • The 30A and the bottom single pole 20A are both spares. The 2 pole 20A goes to a 240V window unit and used to be a 30.
    – ehbowen
    Apr 29 at 20:45

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That should work. But.

Since it's more than 24 inches, you'll have to use oversized wire if you run all the circuits in one conduit due to derating. See table 310.15(C)(1) A couple (perhaps even one, but it comes 10 feet long and you say it's 5 feet) of 1/2 or 3/4 EMT in addition to the 1" would save that annoyance; compare the cost of the additional conduits to the cost of fatter wires.

You can potentially save some on the derating if you make the 120V circuits MWBCs for the trip over to this junction box. If you are moving rather than replacing the breakers you'll then need handle ties for the pair of breakers feeding each MWBC. What that does is make the Neutral on the run over to this sub-panel NOT a current-carrying conductor for the purposes of derating, so you only need to count the two hots of the MWBC.

So you'd have 2 hots for the 240V-20A, plus 4 hots for the 4 120V-20A circuits, (and I can't see if the other breakers on the bottom connect to anything) for a total of 6 current carrying conductors, requiring an 80% derate (which should not require any increase in size, as you can use the 90°C column for derating THHN), rather than 2 hots for the 240V-20A, plus 4 hots and 4 Neutrals for the 4 120V-20A circuits for a total of 10 current carrying conductors and a 50% derate that would require 10 AWG. If the two outer breakers on the bottom are connected to anything, that will change the math (though the range for 50% derate is 10-20 conductors, so that won't get worse.)

Just make sure the old Neutral bar is well-bonded to the case. While the conduit is a valid grounding path, the green wire is not a bad idea in this case.

As per usual, buy a much bigger replacement panel than you think you'll need. And go ahead and get one with a main breaker, as you'll likely exceed the "rule of six" you're getting by on now.

I'm not clear that you can expect to re-use all your breakers in a new panel - The first one seems to be mostly GE and the second one is things that are not clear (two of which appear in the first panel as well) and a CH breaker, as far as I can see (hard to read clearly even zoomed in all the way.) Physically fitting is not good enough - breakers must be listed for the panel they are used in - some that will "appear to fit" don't make good contact. Some breakers are cross-listed, but in any case your new panel must be compatible with any breakers you want to re-use, or you must not re-use them, and get compatible breakers instead.

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  • The cost difference between 50 feet of #10 and #12 is not enough to matter, IMHO. And I have a NOS in box GE panelboard which comes with a set of breakers. I'll install AFCIs on the circuits which serve the living spaces.
    – ehbowen
    Apr 29 at 20:55
  • Question: What's this 'rule of six?' Hadn't come across that before. So I'll need a main downstream of the 70A feeder from the main panel?
    – ehbowen
    Apr 29 at 20:59
  • IIRC, technically only if it's a separate building. Within the same building the upstream breaker is allowed to apply as the disconnect. If you consider situations where you might want to shut it all down in a hurry and find that seems inconvenient, you can have one even if not required to. Rule of 6 is (under current code) only in Article 225 for outbuildings - used to be applicable to main services as well, has been removed there due to hazard. Must be able to shut down with no more than 6 motions of the hand.
    – Ecnerwal
    Apr 30 at 13:32
  • Ah. Same building, the garage and apartment was an addition to the main home. Looking it up I see it was an NEC 2020 change. My own copy is from 2014. I should update, but.... Unlimited wants, limited resources...as my economics professor once said.
    – ehbowen
    Apr 30 at 15:19

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