In the 1950's, our house was built with a Zinsco main service panel that ultimately served six two-wire (ungrounded) and two grounded (via AC/BX) circuits. In the 1990's, a new service panel was fitted elsewhere and the branch wiring from the old panel was extended to the new panel; the old panel is now a junction box.

Can I replace this old panel/junction box with a new sub panel and meet 2020 code compliance?

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My foremost concern is whether this is a modification to the branch circuit wiring for the eight circuits mentioned above and as such would require bringing all those circuits up to modern code. I can add AFCI/GFCI breakers in the panel to address AFCI and ungrounded circuit requirements, respectively. But two of the circuits serve the Kitchen receptacles in a manner not consistent with modern code; there is one 15 amp circuit serving countertop receptacles, and another 20 amp circuit serving another countertop, microwave, nook, refrigerator, stove, and hood vent. Obviously that's one heavily loaded 20 amp circuit, and is part of the motivation to fitting a subpanel over by the kitchen so I have panel space for more dedicated kitchen circuits when I remodel in 10 years.

Another, more immediate motivation, to fitting a subpanel here is so that I can use the 50 amp feeder that serves the detached garage subpanel and passes through this junction box as the feeder for this panel. I want to add a 50 amp car charger circuit to the garage, and since I can't send a dedicated circuit in addition to a feeder, I have to abandon the existing feeder and run a 100 amp feeder to the garage instead. Fitting a subpanel at the junction box lets me utilize this abandoned feeder, and also makes more room in my main panel. Win-win-win!

  • One problem I see is that your wires are no longer long enough to reach breaker spots in a new panel, especially a larger one. You can't have splices in a panel, so you'd still need a large junction box. You should make a diagram to clarify how you want to connect things, but note that your system may only branch out like a tree, and must not loop back on itself. You should check the guage of wiring to receptacles in your kitchen and if a 3 wire circuit was run to either. If it was, you can either use each leg to power separate GFCIs, or get a GFCI 2 pole breaker ($$$).
    – K H
    May 6, 2021 at 4:35
  • 1
    @K H, point taken on the wire length, that has to be handled. Whether you can splice in a panel seems to be up to AHJ, other folks on this site haven’t identified NEC requirements that forbid it: diy.stackexchange.com/questions/39798/….
    – aerospark
    May 6, 2021 at 4:51
  • @K H, copy all on tree topology. No issue there. Also no 3 wire circuits from old panel to the kitchen.
    – aerospark
    May 6, 2021 at 4:57
  • @KH - Good comment. I would suggest - and I know this is exterior moving that thing down about a foot.
    – DMoore
    May 6, 2021 at 5:11
  • @DMoore at least where I live there is a requirement for the height of the main disconnect as well, so the height would simply be what it was, although an extra long panel may have long enough busses for the wires to reach bottom breakers depending on how heights work out. You could probably gain at least some length that way though.
    – K H
    May 6, 2021 at 5:28

1 Answer 1


A new subpanel here is legit.

As always, "Go BIG". All reasonably sized subpanels currently on the market are 14.25" wide, intended to fit between two joists. Nobody ever lamented "Why on earth did I buy so many breaker spaces?" but they sure complain about the other thing.

It will need a 4-wire feeder. It looks like there are currently 15 wires in a 1" metal conduit (four MWBCs of Hot-Hot-Neutral each, plus a #8 H-H-N for the 50A circuit). If that is non-flex, that's your ground wire and you only need 3 more wires.

That's a codevio on 2 counts: first the MWBCs are not distinctly identified which hots are with which neutral... and second, 5 circuits in 1 conduit without a 310.15(B)(3)(a) derate - the most egregious being the #8 running at 50A (which only works if it's the ONLY circuit in the conduit; the derate takes it down to 27.5A).

So all those wires must come out of that conduit and be replaced SOLELY with one set of conductors (H-H-N or H-H-N-G if the conduit isn't metal). The existing #8Cu is good for 50A. I would fit the biggest wire you can fit in the conduit - 1" conduit will support #4Al for 65A (breaker at 70A) or #4Cu (85A breaker at 90A).

The eight branch circuits need to be brought into the new sub with appropriate cable clamps. They need to come in at least 1/4" of sheath, then 6" of wire length. After that they can be extended by wire nuts.

50A is plenty to power the 8 circuits given that there will only be 4 circuits on each 50A leg, and they won't all be maxed at the same time. Just the same, think about maxing the feeder.

  • good catch on the 10 current carrying conductors in one conduit. Conduit is FMC back to main service panel. #8 is H-H-N plus #10 G. The ground lands on the old neutral bus and leaves from there to the garage panel. If I use existing #8 for 50 amp service to this panel, do I have to pull out the other conductors from the FMC or is it sufficient to abandon them for derate purposes?
    – aerospark
    May 6, 2021 at 14:14
  • You can leave them in place until the day before an inspector comes :) @aerospark seriously, the way I see it is a) "hey, free THHN wire".... and b) now there's empty space in that conduit, bigger feeder... May 6, 2021 at 18:23
  • And NEC doesn’t require pulling the circuits serving the kitchen up to code as a result of this sub panel installation? e.g., two 20 amp circuits serving countertops and dedicated circuits for appliances fixed in place?
    – aerospark
    May 7, 2021 at 1:33
  • @aerospark not merely for a panel upgrade, which is what that nets out to. May 7, 2021 at 5:14

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