Use subpanel main breaker slot for outgoing circuit?

Question

We have a small electrical subpanel with only 12 breaker slots. It's rated 125A, which is enough for all the circuits we want, but we will probably run out of slots when adding solar, induction range, electric dryer, heat pump water heater, etc. We can mitigate this to a large extend by using half-width breakers in quad arrangements, but will probably still run 1-2 slots short.

This subpanel also has a main breaker slot that is unused (the subpanel is protected by a breaker further upstream). So my question is, could we put a breaker in the main breaker slot and use it to feed a normal circuit or accept power from the solar array? We would also label it as "Range" or "Solar (not Main)" or something similar.

My guess is that this is not OK due to potential confusion, but I wonder what the code says about it? Electrically it seems like it should work fine.

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UPDATE: for those who asked what model of panel this is: It's a Murray LW1224L1125 Model 2 outdoor load center. I've also attached a photo of the existing circuit breaker setup and subpanel label.

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UPDATE 2: I've attached a photo of what I think is the same model of load center from here. As @jay613 pointed out in the comments, it looks like I made a faulty assumption that the subpanel had both lugs to connect the supply directly to the bus and also clips to attach a main breaker to, like the others. From the photo, it looks like you can only use one or the other approach, not both, so there's no way to add an additional breaker in the main breaker slot on this box.

Answer 1

Update based on the panel label:

It is very clearly drawn (top diagram for main lug, bottom diagram for main breaker) and labeled ("either a main lug or a main breaker") to indicate that you can't use both the main breaker position and main lug feed. It would be possible to design such a thing, but there would be little point when the better answer from the manufacturer would be "move up to the next size panel".

The interesting question, which in a quick search I can't answer but maybe somebody else can, is breaker compatibility. Specifically, there are Siemens breakers that are legitimate for Murray panels (in fact, the Murray label here actually says it is made by Siemens). But what I don't know is whether or not you can transfer the existing Murray breakers to a new (much larger) Siemens panel. In particular, it would be very helpful to be able to move the AFCI breakers, as they are relatively expensive. In fact, those 3 AFCI breakers are roughly on par with either a large replacement panel or all the other breakers (2 quad, 2 single) put together.

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I would seriously consider replacing the subpanel. Panels are cheap - with a quick search I found a few much larger panels with some breakers included for around $100. I would do that sooner rather than later because depending on the existing panel type and what is available at a good price, you may want to switch brands/types, in which case it is better to do that before filling up the panel.

There is one definitely do it and one think twice:

• Definitely replace the subpanel if the existing panel is one of the older known problem panels (Zinsco, Federal Pacific, etc.) or if it is an older type where modern AFCI breakers are not available or if the panel is not actually designed for tandem breakers (for a variety of reasons, many older panels either are not designed for tandem breakers or only for use in some of the spaces).

• Think twice if the existing subpanel is in a location without proper working space (30" x 36"). If it was properly installed without that working space then you can continue using it (and add more breakers, etc.) but if you replace it then you have to have the proper working space, which is sometimes not so easy to do. That being said, since you don't have all that many circuits yet, moving it a few feet to a better location, if necessary, may not be so hard. If you have cables coming from different directions so that some need an extension to reach a new location, you can use the old subpanel box as a giant junction box.

Answer 2

That stop-gap isn't going to fix your root problem.

12 spaces for a 125A panel was a mistake the day it was bought, no offense. You need a larger panel. You're going to get one, once you realize there's no way this will work. Since it's a subpanel and you can fully de-energize it, you're really better off biting down hard and cauterizing the wound now.

We always advise "think BIG" on panels, and given that it seems to be a whole dwelling, a 40-space panel is appropriate. Not a 20-space/40-circuit. A 40-space.

I mean unless you have a 40A feeder, 12-space is just way too small for anything. Everybody gets on fire to save a couple bucks when they're buying a panel. But breaker spaces are cheap (when considering which panel size to buy) and it's a false economy to scrimp on spaces. It really is.

As far as double-stuffing the panel, that's generally "no sale" due to most modern circuits requiring AFCI and/or GFCI, including 240V appliances in many rooms. I'm hearing a lot of 240V things. That's why we don't advise a "20-space/40-circuit".

Answer 3

You can use Siemens breakers, but you'll need to get a bigger panel

The good news is that your existing panel can legally and safely take Siemens QP breakers as they're the designated replacement type for the Murray MP series. The bad news is that that slot labeled "main" on your deadfront is useless to you since your panel's interior is designed to accept a backfed main breaker (in the top left branch positions), not a main breaker kit that replaces the main lugs. So, you'll need to get a bigger panel: since this is a subpanel, you can simply grab a 40-space or 42-space, 200A, main lug Siemens panel and roll with it.