[RE-wrenches] Solar accumulation panel (revisited)

Fri Feb 5 14:10:44 PST 2010

Darryl - The argument they are making is that if there was a fault on that
line, and the inverter somehow stayed on, the fault current between the
breakers (the solar interconnection breaker in the subpanel and the subpanel
feeder breaker in the main panel) could conceivably be the equal to the sum
of the breaker ratings.

- Drew

On Thu, Feb 4, 2010 at 5:14 PM, Darryl Thayer <daryl_solar at yahoo.com> wrote:

> I think you have it wrong.  The conductor from the main to the subpanel
> would be limited by the main breaker in the sub panel, or at best the solar
> contribution reduces the current in the feeder circuit.
> But frequently sillier minds prevail.
> Darryl
> --- On Thu, 2/4/10, Andrew Truitt <atruitt at gmail.com> wrote:
>
> > From: Andrew Truitt <atruitt at gmail.com>
> > Subject: Re: [RE-wrenches] Solar accumulation panel (revisited)
> > To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> > Date: Thursday, February 4, 2010, 1:58 PM
> >
> > First - Thank you all for the useful information!  Bill -
> > I'm sorry to hear that this didn't make it into the
> > 2011 NEC but I'm glad its on the radar!
> >
> >
> > In the process of getting a solar accumulation panel
> > approved in a local county we were challenged with something
> > that I have not thought about before.  The first part of
> > NEC2008 690.64(B)(2) states:
> >
> >
> >
> > "The sum of the ampere ratings of overcurrent devices
> > in circuits supplying power to a busbar or conductor
> > (emphasis added) shall not exceed 120 percent of the
> > rating of the busbar or conductor."
> >
> >
> >
> > This means that the conductors between our solar
> > accumulation panel (with [3] 15A 2-pole breakers = 45A/pole)
> > and the 40A interconnection breaker in the main panel must
> > actually be sized to 80% of 85A = 68A.  O.K., fine.  NBD.
> >
> >
> >
> > But what does this mean for connecting to a sub-panel in an
> > outbuilding?  If you have feeders coming from the house
> > that are sized to a 60A feeder breaker in the main panel and
> > you want to connect 30A of solar to the subpanel in the
> > outbuilding, you are out of luck since those feeders are not
> > rated for 72A (60A + 30A * 0.8).
> >
> >
> >
> >
> > Am I interpreting this right?
> >
> >
> >
> > - Drew
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > On Wed, Jan 13, 2010 at 2:09 PM,
> > Hans Frederickson <hans at fredelectric.com>
> > wrote:
> >
> >
> >
> >
> > 690.15, second paragraph supports Drake's method:
> >
> >
> >
> > "A single disconnecting means in accordance with
> > 690.17 shall be permitted
> >
> > for the combined ac output of one or more inverters or ac
> > modules in an
> >
> > interactive system."
> >
> >
> >
> > 690.17 says the disconnecting means can be a switch or
> > circuit breaker. This
> >
> > seems workable, with a couple caveats... The inverter
> > manufacturer
> >
> > directions might specify an individual branch circuit, and
> > you would need to
> >
> > use the larger size wire between the AC combiner box to the
> > fused AC
> >
> > disconnect at each inverter, so you would need to choose
> > disconnects with
> >
> > big enough terminals.
> >
> >
> >
> > Anyone see any other issues with this idea?
> >
> >
> >
> > -Hans
> >
> >
> >
> > ________________________________
> >
> >
> >
> > From: re-wrenches-bounces at lists.re-wrenches.org
> >
> > [mailto:re-wrenches-bounces at lists.re-wrenches.org]
> > On Behalf Of Drake
> >
> > Chamberlin
> >
> > Sent: Wednesday, January 13, 2010 9:59 AM
> >
> > To: RE-wrenches
> >
> > Subject: Re: [RE-wrenches] Solar accumulation panel
> >
> >
> >
> >
> >
> > P.S.
> >
> >
> >
> > In many cases it would be necessary to use a fused AC
> > disconnect to stay
> >
> > within the inverter manufacturer's specifications.
> >
> >
> >
> > At 12:53 PM 1/13/2010, you wrote:
> >
> >
> >
> >
> >
> >         Here is a wild idea.  If you  look at the
> > Enphase inverter systems,
> >
> > several inverters connect together in parallel before going
> > to an
> >
> > overcurrent device.
> >
> >
> >
> >         In all cases, the point of connection to a
> > utility system must be at
> >
> > a dedicated breaker.  If there were multiple string
> > inverters that would
> >
> > connect to one breaker, why not just use a junction box,
> > instead of a
> >
> > combiner panel?  You could put in an AC disconnect for
> > each inverter, if it
> >
> > didn't come with one.
> >
> >
> >
> >         The wiring would need to be sized for the
> > combined output of the
> >
> > multiple inverters, which in many cases wouldn't be an
> > issue.
> >
> >
> >
> >         Drake
> >
> >
> >
> >
> >
> > Drake Chamberlin
> >
> > Athens Electric
> >
> > OH License 44810
> >
> > CO License 3773
> >
> > NABCEP TM  Certified PV Installer
> >
> > Office - 740-448-7328
> >
> > Mobile - 740-856-9648
> >
> >
> >
> >
> >
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> >
> >
> >
> > --
> > "Don't get me wrong: I love nuclear energy!
> > It's just that I prefer fusion to fission. And it just
> > so happens that there's an enormous fusion reactor
> > safely banked a few million miles from us. It delivers more
> > than we could ever use in just about 8 minutes. And it's
> > wireless!"
> >
> >
> >
> >
> >
> > ~William McDonough
> >
> >
> >
> > -----Inline Attachment Follows-----
> >
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-- 
"Don't get me wrong: I love nuclear energy! It's just that I prefer fusion
to fission. And it just so happens that there's an enormous fusion reactor
safely banked a few million miles from us. It delivers more than we could
ever use in just about 8 minutes. And it's wireless!"

~William McDonough
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