[RE-wrenches] Supply Side Interconnections - Maximum Backfeed
Dave Click
daveclick at fsec.ucf.edu
Thu Mar 4 17:34:41 PST 2010
Sometimes utilities will set a max too- here in Florida the
investor-owned utilities have a rule that you take (DC at STC) x 0.85 to
get the "AC equivalent" size of the array. That size then can't exceed
90% of the service rating. Not sure if this is an issue in other states.
Scott McCalmont wrote:
> Going all the way up to the rating of the service might not be such a
> good idea. The utility may use smaller wires for the service drop than
> you would expect based on the ampacity tables in the NEC. Considering
> the example of a 400 A service:
>
> - PG&E (local utility) requires 350 kcmil Aluminum conductors.
> - Table 310.17 rates 350 kcmil Aluminum for 395 amps before temperature
> correction.
>
> With load diversity the smaller wires will be fine, but the output of a
> PV system is continuous. Running the output of a system sized at 400
> amps through the wires on a hot summer day could overload them.
>
> So... it depends.
>
> Scott
>
> Scott McCalmont, P.E.
> McCalmont Engineering
> San Jose, CA
> (408) 204-0318
>
>
> On Thu, Mar 4, 2010 at 3:02 PM, Michael Kelly <mike at solarflair.com
> <mailto:mike at solarflair.com>> wrote:
>
> Kirpal,
>
> That was always my interpretation as well. So in the case of a 400A
> service, I can backfeed 400A when utilizing a supply side
> connection. Others have thoughts on this?
> - Mike
>
> --
> Michael Kelly
> Project Manager / Project Engineer
> NABCEP Certified Solar PV Installer ™
>
> SolarFlair Energy, Inc.
> 11 Mayhew Street
> Framingham, MA 01702
> Direct Mobile: 617-899-9840
> Main Phone: 508-293-4293
> Main Fax: 508-293-4003
> mike at solarflair.com <mailto:mike at solarflair.com>
>
>
> On Thu, Mar 4, 2010 at 4:16 PM, Kirpal Khalsa <solarworks at gmail.com
> <mailto:solarworks at gmail.com>> wrote:
>
> Mike.....i may be wrong....but I think i am right....you are
> limited by the wire size feeding the supply and/or the
> transformer size which supplies the service.....depending on the
> physical location of the tap you may be limited by the enclosure
> and the enclosure ratings as well
> would love to hear any other ideas...
> Cheers,
> --
> Sunny Regards,
> Kirpal Khalsa
>
> NABCEP Certified Solar PV Installer
> Renewable Energy Systems
> www.oregonsolarworks.com <http://www.oregonsolarworks.com/>
> 541-218-0201 m
> 541-592-3958 o
>
>
> On Thu, Mar 4, 2010 at 12:50 PM, Michael Kelly
> <mike at solarflair.com <mailto:mike at solarflair.com>> wrote:
>
> Hi All,
> Next to grounding, I find the calculation of the maximum
> amount of backfeed current allowed in a supply side
> connection to be the most vague issue in Article 690. Let's
> assume we have a 400A, 480/277 service in a commercial
> building, a 400A main service disconnect, and the
> jurisdiction is under NEC 2008. What is the maximum
> backfeed current allowed in a supply side connection? The
> only place the code points you to is 230.82(6), which is no
> help, or to Article 705, which is no help either. I know
> that in the draft version of NEC 2011, there is
> an expansion of 705.12(A) which states "* *Supply Side. An
> electric power production source shall be permitted to be
> connected to the supply side of the service disconnecting
> means as permitted in 230.82(6). The sum of the ratings of
> all overcurrent devices connected to power production
> sources shall not exceed the rating of the service." That
> is a helpful sentence from the Code, but as it is in draft
> form, what do we do under NEC 2008? What have you used as a
> maximum backfeed current for supply side connections?
> Thanks,
> Mike
> --
> Michael Kelly
> Project Manager / Project Engineer
> NABCEP Certified Solar PV Installer ™
>
> SolarFlair Energy, Inc.
> 11 Mayhew Street
> Framingham, MA 01702
> Direct Mobile: 617-899-9840
> Main Phone: 508-293-4293
> Main Fax: 508-293-4003
> mike at solarflair.com <mailto:mike at solarflair.com>
>
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