Allan:
I whipped up three drawings that indicate three overload scenarios possible
with incorrectly sized distribution equipment. They are on our web site
at: http://millersolar.com/case_studies/case_studies.html Click on Point
of connection. Click on any drawing to see a larger version.
I hope this helps with understanding the concept.
William Miller
At 06:19 PM 8/1/2009, you wrote:
William,
I have been following this back-and-forth thread and need you to please
explain one thing:
Your 140A point makes perfect sense (from the point of 690.64 in requiring
that the busbar to which both utility and PV feeds are connected. No
question there. But you write: "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." It seems
to me that there's still no way to exceed main disconnect current rating.
* 100A utility (using 100A subpanel main) + 40A PV tied into subpanel
busbar = 140A, which is OK if the subpanel busbar is rated at least 125A
(residential) or 150A (commercial).
* 100A + 40A tapped into subpanel ahead of 100A main breaker:
* maximum current passing through 100A main breaker = 100A. Adding
40A of PV only reduces amount through conductors from main AC panel to 60A.
* no AC load in daytime = 40A of current back to main AC load center
* some subpanel loads in daytime reduces 2.b. above
* At the main panel, maximum current flow through breaker feeding
subpanel is 100A, as any PV reduces load at breaker. However, main panel
must itself meet 690.64, as it's still possible to feed 40A of PV
(daytime with no subpanel load) into main panel. So same busbar
requirements apply in the main panel as in the subpanel. But that's all.
The subpanel feeders don't need to be upsized.
What am I missing here, please?
Allan Sindelar
<mailto:al...@positiveenergysolar.com>al...@positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
<http://www.positiveenergysolar.com>www.positiveenergysolar.com
----------
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of William Miller
Sent: Saturday, August 01, 2009 6:11 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] down-sizing main breaker
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
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