Can I Replace This Junction Box with a Subpanel?

Question

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?

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!

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.