I can see the confusion over this. We've had this discussion for almost
a decade, and its been made more clear by improved language and by
interpretation by folks like John Wiles and Bill Brooks. The definition
of separately derived system is just not quite clear enough still.
Despite this confusion however, 250.30 which Dave referenced in his
original question is titled:
" Grounding Separately Derived _Alternating Current_ Systems", so it
doesn't apply to DC at all. (whether or not we consider the DC side to
be separately derived) So just to re-answer Dave's original question:
250.30 does Not apply to grid tied PV.
After rereading several parts of the code, I can totally see Jeffrey's
interpretation of the DC side being isolated by the inverter's
transformer (and therefore separately derived). However, I was taught
that a separately derived system has not only its own isolated source,
but it feeds it's own isolated loads (its a complete premises wiring
system). As soon as neutrals are connected together, it's not
separately derived. They need to clear up this question with some
language in 690, 705, or in 100, the definition of separately derived
system.
In the end, I don't believe it matters whether we consider the DC side
to be separately derived or not, as 250.30 only applies to AC, and 690
is now fairly clear about bonding and grounding requirements regardless.
Ray Walters
On 1/24/2012 9:10 PM, JRQ wrote:
Separately-derived means that there is physical isolation between two
circuits in the same electrical system -- usually this involves two
circuits connected by a transformer
The DC side is separately derived in a transformer-based PV inverter.
Most grid-tie inverters have a DC main bonding jumper internally.
Other inverters (esp. small stand-alone inverters) may not, and so a
main bonding jumper will need to be installed.
Jeffrey Quackenbush
NABCEP-certified PV Installer
------------------------------------------------------------------------
*From:* Dave Palumbo <[email protected]>
*To:* 'RE-wrenches' <[email protected]>
*Sent:* Tuesday, January 24, 2012 6:43 AM
*Subject:* Re: [RE-wrenches] Grounding Separately Derived AC System
Regarding NEC Article 250.30 does Informational Note #1 mean that a
standard (no battery) grid-tied PV system is not a Separately Derived
System?
I'm primarily interested on the consensus view of Bonding the Neutral
(grounded conductor) to Equipment Ground in a 60 Amp Fused Disconnect
located between the 240VAC inverter output and AC point of connection
on the load side of the meter in the service disconnect enclosure in a
_supply side connected PV system_.
250.30 Informational Note 1 reads: "/An alternate ac power source,
such as an on-site generator, is _not_ a separately derived system if
the grounded conductor is solidly interconnected to a service-supplied
system grounded conductor..." / I believe a PV system meets that
description as the Neutral stays connected at all times.
//
My interpretation is that a standard net-metered PV system is
therefore not a separately derived system and the Neutral should not
have a bonding jumper to equipment ground in the Fused Disconnect.
Is this the correct interpretation?
David Palumbo
Independent Power LLC
462 Solar Way Drive
Hyde Park, VT 05655
*/www.independentpowerllc.com <http://www.independentpowerllc.com> /*
NABCEP Certified PV Installer
Vermont Solar Partner
24 Years Experience, (802) 888-7194
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