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<font size=3>Dick:<br><br>
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.<br><br>
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.<br><br>
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.<br><br>
William<br><br>
<br><br>
<br>
At 03:47 PM 8/1/2009, you wrote:<br>
<blockquote type=cite class=cite cite>William,<br><br>
I don't know if I've solved Brian's dilemma yet either. But, with regard
to your<br>
example below, if the 100 amp meter main is accepting 20 amps, or even
all of<br>
the 50 amps of PV, that would mean that the house, at that moment,
presented no<br>
load at all, and that all the PV would be backfeeding the grid, turning
the<br>
meter backwards. That, of course, would mean the main 100 amp breaker is
only<br>
handling 50 amps, well below its rating. Likewise the feeder from the
house<br>
would only be carrying the 50 amps from the PV, instead of anything near
it's100<br>
amp rating. The code can be perverse, but it's my understanding that the
point<br>
of 690.64(B) is to ensure none of the distribution components are exposed
to<br>
overcurrent, not to arbitrarily throw numbers around.<br><br>
I was just kidding about offering you those 200 amp Milbank breakers.
I've got<br>
to figure out how to list them on Ebay.<br><br>
Dick<br><br>
--- You wrote:<br>
Dick:<br><br>
I don't believe you have solved Brian's dilemma yet. It is my
<br>
understanding that if one is going to install a load side tape, the back
<br>
feed calculations need to work for every link of the distribution system
<br>
upstream to the meter. In the scenario you suggest, that will not
be the <br>
case. The 100 amp meter main on Brian's project will still
accept only 20 <br>
amps of PV AC feed, still not satisfying his requirements.<br>
--- end of quote ---<br>
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