point of connection [RE-wrenches]

[David Buckner]

Oh that code thing... Alright then, thanks Mark! 

On a slightly related but unrelated topic, it sometimes works the same
way with our utility when they are determining whether or not to require
a reverse power relay at a particular site within their networked grid.
We'll measure the building's load for a week or so to determine its
lowest consumption levels.  If that level falls below a certain
threshold relative to the Inverter output rating, then they could force
the RPR to be installed which trips the PV system off before the grid
can be supplied with green electrons (costly, nightmarish, and against
everything the PV industry stands for).  Even when we show them that the
building's low consumption happens at 2am when there isn't a whole lot
of output from the PV system, it doesn't quite sink in.  Wonderful.
Stay away from networked grids is the moral of the story I suppose.

David






-----Original Message-----
From: Mark Frye [mailto:markf at berkeleysolar.com] 
Sent: Saturday, May 03, 2008 11:23 AM
To: RE-wrenches at topica.com
Subject: RE: point of connection [RE-wrenches]


NEC 2005

690.8 Circuit Sizing and Current

(A) Calculation of Maximum Circuit Current. The maximum current for the
specific circuit shall be calculated in accordance with 690.8(A)(1)
through
(A)(4)....

....(3) Inverter Output Circuit Current. The maximum current shall be
the
inverter continuous output current rating.....

(B) Ampacity and Overcurrent Device Ratings. Photovoltaic system
currents
shall be considered to be continuous.

(1) Sizing of Conductors and Overcurrent Devices. The circuit conductors
and
overcurrent devices shall be sized to carry not less than 125 percent of
the
maximum currents as calculated in 690.8(A)...........

David, the Code is set up to force you to size to the inverter rated
output,
not the actual power delivered.

I believe the thinking is that you may only have half the PV installed
now
so the small breaker would "work", but someone else might add more PV
later
without upsizing the conductor and breaker and that would be a problem.
 
Mark Frye
Berkeley Solar Electric Systems
271 Vistamont Dr
Grass Valley CA 95945
(530) 401-8024
www.berkeleysolar.com 

-----Original Message-----
From: David Buckner [mailto:dbuckner at solaresystems.com] 
Sent: Saturday, May 03, 2008 6:32 AM
To: RE-wrenches at topica.com
Subject: RE: point of connection [RE-wrenches]


Wrenches,
If Larry's line side service connection, even with its closer proximity,
becomes costlier than the potential load side connection, could he feed
a
subpanel with the two 30A PV input breakers, then feed into the house
MDP
with a 40A breaker from this subpanel?  This would require calculations
of
DC input and resulting max AC output of the inverters.  If the inverters
won't ever be putting out 20A anyway, then the 40A (continuous duty?)
breaker shouldn't be tripping and code would be met?  I'm asking, not
telling.  Larry, what is the DC STC input anyway?

David


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