[RE-wrenches] Rooftop DC disconnects

William Miller william at millersolar.com
Thu Sep 24 23:39:06 PDT 2015


Chris:



Technically, yes, yes and yes.    The devil is in the details, however, and
the process is not as easy as you have described it: This job has no
combiners and if it did, the combiner wouldn’t be on the roof.  I don’t
know of a good rooftop combiner (other than Soladeck, see below).



Here are the deviling details.



1.    We try to secure PV wiring very carefully under our arrays. This is
to prevent squirrels looking for food, rakes looking to clear leaves and
kids with scrap pipe looking to retrieve Frisbees from engaging in the
leads.  (See:
http://www.millersolar.com/MillerSolar/practices/PV_wiring/_PV_Wiring2.html)

2.    As such, we can’t just reach under a rack and blindly snag an MC
connector and pop it loose.  They are not just swinging in the breeze, by
design.  Accessing them requires removing at least one module for each
string, and knowing how many strings and how they are wired.

3.    We can’t expect fire fighters to fish around under our arrays in the
smoke and confusion in futile hopes of finding the correct MC connectors to
de-energize all strings.

4.    Similarly, we can’t expect the plumber, after accidentally drilling
through the PV conduit, to know how to do this while the conduit is arcing
and sparking and threatening to burn the house down.



I know some of these scenarios are fairly unlikely, but as professionals,
we need to plan for the unexpected.  I believe a well-marked and
conveniently located switch will be the best option, even if that location
is on the roof.  If I were looking for a PV disconnect, I’d look near the
PV.  Particularly if the engraved plaque indicated as much.



Sincerely,



William Miller



PS:  The DC Sunvolt switch arrived today.  It looks solid and we will
install it tomorrow on this roof-mounted off grid system tomorrow.



Wm



PPS:  This discussion branches out in several directions:

1.    The job we are on is a residential, roof mounted off-grid.  Normally
we would have 4 to 8 strings of three into Outback controllers and require
combiners.  Mounting a combiner on the roof is not an option.  All models I
am aware of would stick up an spoil the aesthetics of this modest but
high-end, ocean view home.

2.    Because we had conduit limitations (see below), we opted for the
Morningstar high voltage controller.  With grounded arrays, this limited
the disconnect to for two strings to two poles, making it affordable.

3.    Transformerless inverters are here to stay, but they will require
double the poles of disconnection.  Two poles (one string) = $216.00.  Four
poles (2 strings) = $441.00.



Wm



PPPS: Here’s why we couldn’t use Soladeck on this job:  Soladeck might have
worked here but the roofing is ultra-thick, presidential comp, difficult to
weave a Soladeck into.  Also some idiot preinstalled PVC conduit and we
must have metallic.  Conduit installed is very difficult to install a
Soladeck to as a remodel.  The male adapter needs to be flush with the
decking, easy enough during rough-in, but after the roof is on, it is very
difficult to get to the conduit under the decking to cut in a male adapter



Summary:  Easier said than done.



Wm



[image: Gradient Cap_mini]
Lic 773985
millersolar.com <http://www.millersolar.com/>
805-438-5600



*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org
<re-wrenches-bounces at lists.re-wrenches.org>] *On Behalf Of *Chris Mason
*Sent:* Thursday, September 24, 2015 3:37 PM
*To:* RE-wrenches
*Subject:* Re: [RE-wrenches] Rooftop DC disconnects



I'm not sure I understand this issue. The Morningstar 600V comes with a
disconnect built in, so everything downstream is isolated. Upstream, in the
rare case you need to replace the Morningstar, turn it off, pop out the
fuses in the combiner and work away. If you have a single string
application, unplug an MC4 connector.



On Thu, Sep 24, 2015 at 8:31 AM, Jason Szumlanski <
jason at floridasolardesigngroup.com> wrote:

"​In this scenario, there is no safe way to replace either of the two
Morningstar controllers."



The BEST rooftop disconnect remains... a big thick tarp. Not totally
practical in an emergency situation, but it is pretty fail safe.



Unfortunately for emergency responders, that does not take care of the AC
circuits in a microinverter or AC module system, but ideally the building
main is shut down anyway, and the risk is the same as any other 240V
equipment/appliance circuits.



​Jason Szumlanski







On Wed, Sep 23, 2015 at 4:26 PM, William Miller <william at millersolar.com>
wrote:

Dear Fellow Wrenches



Below is a design conundrum that may resonate with some of you:



We are finalizing a design for an off-grid residential system.  The
customer insist the PV should be on the roof and pre-installed a 1-1/4” PVC
conduit from his roof to a crawl space, in anticipation of a solar
install.  This created real problems, because we all know we can’t pull PV
source or output circuits in (or now, on)  the envelopes of habitable
buildings.



There was no practical way to replace the PVC.  We contrived a method to
sleeve ¾” liquid-tight through the 1-1/4” PVC to the crawl space,
continuing on with EMT.  This is the largest metallic conduit we could
fit.  The distance was greater than 10 feet so we couldn’t use EMT.  Due to
the conduit size restriction, we upgraded to Morningstar 600 volt charge
controllers, allowing us to reduce conductor size.



(As a sidebar, although the Morningstar is listed as a 600 volt charge
controller, we have found no circumstance were we could take advantage of
that high a voltage.  With the currently available high wattage modules, by
the time we added enough in series to get to 600 volts, we were well beyond
the wattage capabilities of the controller.  For sake of design
considerations, I suggest one regard these units as ~300 volt charge
controllers.)



We now have plans for 300 volt PV feeders running down an interior wall and
under the house, with no roof-top disconnecting means.  It is my
understanding none are required.  I am not comfortable with this.

​​

In this scenario, there is no safe way to replace either of the two
Morningstar controllers. Should someone drill through or damage the EMT in
the wall or under the floor, there would be no way to turn off the feeder.



I don’t like putting HU361RBs on a roof.  They must remain vertical and so
they stick up too high and are hard to provide mounting for.  Sola-deck
units are another option, but they require integrating with shingles, not
practical on this job or many others.  I finally settled on a DC-Sunvolt
PV-X16A-4X-RG disconnect as a possible solution.  At $216 it is not out of
range.  The unit will provide means to turn off the feeders for service.  I
will report back on my impressions of the unit.



To distill this scenario, I don’t believe the code requires a disconnect,
but I feel morally obligated to install one.  I’d be interested in
verification of the code interpretation and others response to similar
situations.



I found no other options for rooftop disconnecting means that would be
small, reasonably priced and not present a high profile.  If there are
products I don’t know about, I would be most grateful to receive your input.



While researching the hardware I stumbled upon this article, linked below.
It seems to present a real dilemma, but I am not convinced.  Please remain
skeptical as you read.  It appears all of the links direct you to the same
source.



https://www.linkedin.com/pulse/greatest-debacle-solar-pv-australias-rooftop-dc-isolator-lucas-sadler



Thanks again to all of you for helpful advice and expertise.  I learned
about Sunvolt here, just one of many great suggestions.



Sincerely,



William Miller





[image: Gradient Cap_mini]
Lic 773985
millersolar.com <http://www.millersolar.com/>
805-438-5600




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-- 

Chris Mason

NABCEP Certified Solar PV Installer™

Solar Design Engineer

Generac Generators Industrial technician



www.cometsolar.com <http://www.cometenergysystems.com>

264.235.5670

869.662.5670

Skype: netconcepts
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