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.
I may not have read Brian's original post closely enough. If Brian needs
to feed 40 amps of PV derived AC and he wants to do it via the load side
connection allowed by 690.63 (B), he has two choices:
A. He can upgrade all of the distribution to 200 amps from the
meter to his point of connection. The arithmetic is simple: 200 * 120% =
240 amps. 240 -200 = 40 amps of PV feed allowed.
B. He can upgrade all of the distribution to at least 125 amps
capability and leave the 100 amp main breaker. This would require that all
of his distribution be upgraded, from the bus in the main disconnect to the
feeder wire to the sub-panels. 125 * 120% = 150 amps. 150 - 100 = 50 amps
of PV feed allowed.
Logistically, there is not a lot if difference between A and B.
See our web site, http://millersolar.com/, for a point-of-connection
calculator.
Thanks for the advice on Milbank. I will not be needing any 200 amp
breakers as there is no need to up-size a breaker, instead I would be
generating more spare 200 amp main breakers.
William
At 12:20 PM 8/1/2009, you wrote:
William,
I stand corrected. Thank you.
This is not a supply side tap. I should have said it was a tap on the supply
side of the panelboard main breaker. That said , I think it works and meets
code, provided the conductors spliced to the feeder are properly sized. The
panelboard busbar is protected by it's main breaker and cannot be
overloaded by
the solar contributuion.
On your other point, Milbank makes a 200 amp MMC that that can be retrofitted
with a 100 amp breaker. I have done this twice, but not because of a solar
install. We've had a couple 100 Amp, 300', underground service laterals that
required oversized conductors because of voltage drop. The Milbank 100 Amp
equipment is physically too small to do this. In both cases we replaced
the very
expensive 200 amp breaker with an also very expensive 100 amp breaker, all by
Milbank and labeled as such. I was told by my supplier it was impossible to
purchase a 200 amp Milbank MMC with the 100 A breaker preinstalled. The 100 A
breaker is the same physical size as the 200 and fits right in, no
modifications
required. I have two 200 amp Milbank breakers on my shelf if you need
them. The
MMC units are very expensive to begin with. Here in VT / NH a basic meter only
enclosure is more often pared with a separate disconnect enclosure through an
offset or straight nipple. This combination might work for you, substituting a
load center for the disconnect.
Dick Ratico
Solarwind Electric
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