Am doing repairs to hot tub circuit, ten year old gfci breakers faulty, in doing panel work noticed that the GE 150 amp main backfed breaker wasn't fitting with cover, in a newer eaton panel. Solar panels were added last to this service, and its got a older 150 amp full width main from a ge panel. on number 2 copper.
I know sometimes the engineering might make exceptions to main sizing in order to not overdrive the buss. I am by no means a authority on solar reasoning, It may have been a loaded ge service without any room for a 2 pole solar breaker? they installed a 200 amp Eaton panel and used existing ? 150 amp backfed main breaker, was number two copper ever used for 150amp service riser conductors, work looks ok, nothing stands out except that the main doesn't sit flat and that it is not same brand as panel, will look into solar contractor. but was trying to see when ampacities may have changed. or other exceptions to solar installs, El Cerrito California

OK, I looked back to the 1978 NEC, and that is the same as the 2017 NEC regarding Table 310.15 (B)(7). #2 Cu is 125 amps for resi, single phase service.

Based on what you said, it sounds like someone did a little 'rube goldberg'.

As to the solar, a 'line tap' is used when in lieu of the solar backfed cb when buss space is not available.

My God that sounds like a mine-field, crselecletric.
How are the solar panel(s) tied into the electrical panel?
Is it through a sub-panel? or some sort of a relay setup where it only feeds voltage when the solar panels put out enough current to over-ride the panel voltage.

I believe the inverter of these grid tie systems handle switching of the solar power into the grid.

I have a '17 HB, and i still don't get most of art 705.....:( ~S~

Trumpy:

A 'basic' on US solar....

The solar output (AC) is utility tied either thru a backfed cb, or a 'line tap' ahead of the main cb. The output is dependent on utility voltage; hence NO utility voltage, no solar output into the grid.

A system with DC output from the panels is connected to an inverter, them a disconnect, and then the utility tie.

A system with micro inverters has the AC output into a solar sub panel, then the disco (if required) then the utility tie.

The technology is changing very quickly, with new equipment which reduces the "parts"; soon it may be an 'all in one'

Note that there are many variations, including the Tesla energy storage system, which is showing up sporadically.