If neutrals are connected together on either side of a transformer, then there
no longer is complete physical isolation. The ungrounded conductors would be
isolated, but not the grounded conductors.
My interpretations is that there wouldn't have to be loads on either side for a
system to be separately derived – there just has to be a current-carrying
circuit on either side.
Yes. 690 is very clear about bonding between the DC and AC sides of an
inverter, independent of what is in 250 (and I'm glad they've clarified the
language on this in recent editions).
Jeffrey Quackenbush
NABCEP-certified PV Installer
Peripatetic Solar Technician
________________________________
From: Ray Walters <[email protected]>
To: RE-wrenches <[email protected]>
Sent: Wednesday, January 25, 2012 12:20 PM
Subject: Re: [RE-wrenches] Grounding Separately Derived AC System
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
>NABCEP Certified PV Installer
>Vermont Solar Partner
>24 Years Experience, (802) 888-7194
>
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