TL;DR Go to a Square D dealer (not a big box store) with pictures and part numbers and see if they can provide a form-factor-matching modern, large, replacement panel.
Breaker panel history is...complicated.
The generally best setup is to have a single main breaker rated for your entire service feed (general exception: if you have a Class 320/400A feed to two 200A panels then you just have the 2 x 200A breakers).
However, back in the 1950s and 1960s residential electricity needs (more electric kitchen appliances, air conditioning, etc.) grew quickly but large (> 100A) breakers were very expensive. A solution was the Rule of 6 panel:
- Main lugs instead of a main breaker
- Up to 6 breakers - usually double-breakers - for large loads
- One of those breakers (typically 60A) controlled a lower section of the same panel, often called the "lighting section" since many of those circuits would be at least partly used for lighting. (But also kitchen appliances, general purpose receptacles, etc. - but not the really big things like oven, water heater, dryer, air conditioning - those would get some of the other 5 breakers.)
You have one of those. With a twist. By the time your house came along, 200A was typical and large panels were available. But presumably to save a few $ and to have most of the breakers inside (shorter wiring path, work on the panel in climate controlled space, etc.) instead of a "lighting section" as the bottom of the panel, controlled by one specific breaker in the top, you have a 100A breaker in the top that feeds a subpanel inside the house, where most of your circuits are.
But the panel manufacturer doesn't like extra SKUs. So instead of:
- Small box with Rule-of-6, no lighting section
- Small box with 12 space main lug or main breaker panel
- Medium box with 20 space main lug or main breaker panel
- Large box with 30 space main lug or main breaker panel
- Large box with 30 space Rule-of-6 (top 12 spaces for Rule-of-6 breakers, bottom 18 spaces "lighting section" controlled by one of the top breakers)
etc.
they used one large box for everything. And on top of that, the plastic piece that holds the bus and breakers - they made only a few sizes, so the piece of plastic would be the same for a Rule-of-6 12 space or a 20 space.
Presumably the cost of managing extra SKUs was determined to be more than the cost of the extra materials. So you ended up with a big, mostly empty box.
The good news is that Square D and similar panels are still around. So there is a good chance that a direct replacement can be made in the same outer box. Not that the box itself costs much, but being able to fit in exactly the same space means you can either retrofit the interior, or likely for less money replace it with an entirely new panel (30 or 40 spaces, main breaker, etc.) that will take the existing breakers and have knockouts in just the right place to fit the existing wiring.
Any solution to this problem will require the power company cutting off power. But a direct replacement should be a very quick job, and anything you do after that will not require power company involvement because you will have a main breaker.
