Dick:
Your logic is correct but incomplete. Consider it like this: The buss
bars in the service panel can handle 100 amps (they are rated for that
amount). The main breaker feeds 100 amps into that buss, maximizing it's
capabilities. If you feed an additional 40 amps into that buss bar, you
have the potential of exceeding the ampacity of that buss assembly. If no
other loads are fed from that buss bar, there is no over current
scenarios. Say, however, someone puts more breakers on that buss bar and
draws 140 amps from them. 100 amps comes in from the utility, 40 amps
comes in from the PV and the buss is overloaded.
This is the scenario for which 690.64(B) was written. This will occur only
if the overload amperage is not drawn from a space on the buss bar between
the utility feeder and the PV feeder. If, however, the PV feeder breaker
and the utility feeder are both on one end of the buss and the load is on
the other end, then the loads are additive on the buss assembly.
Some want the code passage re-written to say that if the feeder is on one
end of the bus assembly and the PV is on the other, there is no possibility
of an overdraw on the buss bar and the installation is legal. This has not
happened yet.
William
At 03:47 PM 8/1/2009, you wrote:
William,
I don't know if I've solved Brian's dilemma yet either. But, with regard
to your
example below, if the 100 amp meter main is accepting 20 amps, or even all of
the 50 amps of PV, that would mean that the house, at that moment,
presented no
load at all, and that all the PV would be backfeeding the grid, turning the
meter backwards. That, of course, would mean the main 100 amp breaker is only
handling 50 amps, well below its rating. Likewise the feeder from the house
would only be carrying the 50 amps from the PV, instead of anything near
it's100
amp rating. The code can be perverse, but it's my understanding that the point
of 690.64(B) is to ensure none of the distribution components are exposed to
overcurrent, not to arbitrarily throw numbers around.
I was just kidding about offering you those 200 amp Milbank breakers. I've got
to figure out how to list them on Ebay.
Dick
--- You wrote:
Dick:
I don't believe you have solved Brian's dilemma yet. It is my
understanding that if one is going to install a load side tape, the back
feed calculations need to work for every link of the distribution system
upstream to the meter. In the scenario you suggest, that will not be the
case. The 100 amp meter main on Brian's project will still accept only 20
amps of PV AC feed, still not satisfying his requirements.
--- end of quote ---
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