Used Diesel While We Wait for Delivery = Black Smoke

so then how would a main lug panel with a backfed breaker supplying the panel work?

if it is backfed through a breaker, all of the current flowing on the bus has to enter through that position so any position on the bus has to be capable of carrying the full current of the bus.

the larger breakers taking 2 positions per pole solves the problem of the stab being rated for less than the bus. if you are using a breaker listed for use i that panel it will take 2 positions per pole if one stab can't handle the current.

we have drifted way off topic here.

There's two breakers marked A/C. What are they connected to? Is there central air conditioning AND a ductless system for the sun room?

@ChrisJ, @mattmia

Most 100A back fed panels are rated 125 for some reason. Back in the day you could put in a 100, 125, 150 or 200 amp service. Some POCOs will not allow that now. Most everyone will allow a 100 or 200. I did do a 125 once when I was going to upgrade a 60A fused service to 100. This guy had:

Electric hot water heater

electric stove

electric dryer

and was adding a 3 ton AC system

The load calc showed that the 100 amp was large enough but I went with a 125. The POCO allowed it. I had existing 1 1/4" rigid conduit from the weatherhead to the meter and from the meter in the building that was nicely cut into the siding that I didn't want to change that. The meter socket was ok. I called the inspector and he told me meter sockets have no amp rating so go ahead and use it and the conduit was large enough for the wire I needed.

But back to the point. A back fed panel takes up two stabs with a 100 or 125 amp breaker.

150 amp services I have done a few but you can do a 200 for practically the same money in most cases.

At my house I sold I had to change the service drop location as it was illegal since the 1955 house was built. I put a 150 amp meter main on the outside. Theis made the feeder going in the house which has to be 4 wire I got away with I think 1/0 copper instead of 2/0 copper for a 200 and was able to use 1 1/2" emt about 30 feet across the garage to the new ML panel in the basement so it save a couple of bucks and the house has gas heat and HW, stove and dryer