Jeffrey,

On the load side of the Fused 200 Amp Service Disconnect there is a splice or
splices or distribution block in the conductors which feed the 5 subpanels.  It
seems quite possible that one or more of those conductors or the distribution
block could see the sum of the current from the grid and the current from the
solar. A combination of loads in the 4 subpanels, other than the one backfed by
the solar breaker, could overload the conductor or part of that conductor
carrying that combined current, without necessarily blowing a main fuse or any
of the breakers. The devil is in the details, in this case the size of the
conductors and the way they are utilized.

Ray has a point. It is possible there are existing code violations. These should
be corrected in a manner which facilitates inclusion of the solar.

Dick Ratico
Solarwind Electric

--- You wrote:
The flow of electricity isn't two-way traffic along a conductor. If there are no
loads on the conductor between the main system disconnect and the main breaker
of a subpanel, in this scenario, there can only be up to 200 A coming from the
utility OR up to 60 A coming from the solar system backfed through the subpanel.
The intent of limiting the solar system size plus the utility source to 120%, or
even 100%, of a bus or conductor rating is when that bus or conductor can
potentially see current from both sources simultaneously, it could overheat if
these sources exceed the bus or conductor rating. That's not physically possible
unless there is a load in between both sources. 
--- end of quote ---

