First a little possible why:
Back in the olden days of Rule of Six panels, typical electrical service to each home was growing but was not at today's levels. A quick search shows that 100A (up from 60A) was typical in the 1950s. The key reason for Rule of Six panels was to avoid a large main breaker. But in order for that to work, you have to be reasonably certain that total usage will not exceed the total service.
With a (typical) 60A breaker for the lower section, that leaves ~ 40A available for the top. Actually a bit more because you expect that the 60A breaker will itself rarely, if ever, trip. So let's say 60A available for all the other breakers in the top. If you have a 30A breaker for an electric dryer and a 20A for HVAC and a 30A for a water heater, with the understanding that they probably won't be all running at maximum all at the same time, things generally work out OK. There are load calculations and utility analyses and other things that went into all of this, but the bottom line is that the various organizations involved determined that for the typical installation this would work out OK. So the result is to allow, for this type of panel, 3 double breakers for separate loads + 1 double breaker for a total of 4 throws and a reasonable expectation that the total electrical service will not be exceeded. If you change things by:
- Adding 4 more breaker spaces (2 double breakers)
- Allowing 1/2-size breakers, which increases the total number of circuits
- Using one of these top double breakers to feed a large load such as another building, EV charger (didn't exist back then) or tankless water heater (shouldn't exist now...)
then you will likely exceed the total service to the building. The solution is a "heavy up". That, typically, increases the incoming total power to modern levels (e.g., 200A), provides a main breaker/disconnect (possibly inside, possibly outside at the meter), provides a lot more breaker spaces and allows you to connect items that need a lot more power (which is an issue on smaller service even if you have the spaces available).
From a manufacturer's perspective, fewer parts in production at any one time is better than more parts. All the more so with things like breaker panels that have a long lifetime. Murray likely used the exact same bus panel for a variety of panels, including main breaker panels, rule of six panels, etc. They modified and/or added various pieces as needed to meet specific needs. They could have given you 4 more spaces at basically zero cost, but UL would likely not have approved the panel. Similarly, the restriction on the number of half-size breakers in the bottom is based on enforcing usage limits and not on anything physically different about the spaces that do/don't allow half-size breakers.
The key is in the diagram. Your panel is already overloaded, as follows.
The top section is supposed to have 8 full size spaces with 1 breaker per space, or a double breaker per pair of spaces, with a maximum of 6 handle throws to turn everything off. You are OK on the 6 throws (3 regular double breakers, one half-size handle tied pair, two half-size single breakers). But you have a big problem with half-size breakers where, according to the diagram, they are not permitted.
The lower left double breaker of the top section powers everything in the bottom section.
The middle section must be left empty. It is designed that way.
The bottom section has another 8 full size spaces. The 4 on the left and the lower 2 on the right can use half-size breakers. The upper 2 on the right - which currently each have a pair of half-size breakers - are not supposed to have half-size breakers.
So the question is how to make everything work legally. Here is one method. Space numbers are as listed in the diagram.
- Move the 4 half-size breakers from 1 & 3 to 11/12/13/14.
- Move the 4 half-size breakers from 15 & 16 to 17/18/19/20.
- Replace the three 20A and one 15A full-size breakers from 9/10,11/12,13/14,19/20 with four half-size breakers. One catch: They may only be available in 20A pairs and 15A pairs. If that's the case then you will need to either determine if the 15A can be changed to 20A (12 AWG wire throughout the circuit) or downgrade one of the 20A circuits to 15A (OK unless it is a bathroom or kitchen countertop circuit as code now requires those to be 20A - if they are currently 15A that's OK but you can't downgrade if they are already 20A).
- Move the 30A breaker from 17/18 to 9/10.
That now leaves you with an empty double space pair in 1/3. Install a 50A double breaker.
Now for the other problems:
- I don't know if all the breakers are listed for this panel. I do think 2/4 looks a bit out of whack, but whether it is the wrong breaker or just a little "off", I don't know. According to Ed Beal, there are at least some BR breakers in the bottom section that do not belong. Just because it fits does not mean it is correct (and legal and safe) for the panel.
- You may have a real total load problem. A load calculation should be done any time major changes are made. Depending on what you have, and in particular depending on your total utility service, you may have plenty of room to spare or you may be already at the edge. This panel is rated for 150A service. But you might only have 100A service. No way to tell from looking at the panel because it is rule of six rather than a simple main breaker.
- Even if there were no other problems (half-size breakers in the wrong places), you could not simply slap a 60A double breaker in the bottom because the entire bottom section is fed by a 60A breaker. Any new large breakers need to be in the top section.
- Your panel really is nearly full. Once all the above changes are done, you will have a grand total of one half-size space available, and that is only if you replace the full size 30A with a 1/2 size breaker. Which may not be so easy.
- Reading the diagram some more, actually you should only have 6 breakers per leg in the bottom with a 60A sub feed. Which means technically not enough to add everything needed to make the new 50A work - over by 1 breaker.
Definitely time to consider a panel replacement.