[RE-wrenches] down-sizing main breaker

William Miller william at millersolar.com
Sat Aug 1 17:11:12 PDT 2009


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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Please note new e-mail address and domain:

William Miller
Miller Solar
Voice :805-438-5600     Fax: 805-438-4607
email: william at millersolar.com
http://millersolar.com
License No. C-10-773985
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