[RE-wrenches] Grounding Separately Derived AC System

[Ray Walters]

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 <dave at independentpowerllc.com>
> *To:* 'RE-wrenches' <re-wrenches at lists.re-wrenches.org>
> *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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