Allan's assessment matches my experience. Interesting note: Outback's IBR enclosures include string-level overcurrent protection "for added safety" according to their data sheet.
http://www.outbackpower.com/downloads/documents/integrated_battery_rack/spec_sheet.pdf http://www.outbackpower.com/downloads/documents/integrated_battery_rack/integrated_battery_rack_installation_manual.pdf Jason Szumlanski On Thu, Feb 6, 2014 at 6:06 PM, Allan Sindelar < al...@positiveenergysolar.com> wrote: > Dave, > I'm not really clear what your question is, but let me take a stab at it > anyway. > > Parallel strings of batteries are routine in PV systems; indeed, far more > common than single strings of large cells. Seldom are the individual > strings separately fused, and I have never seen it required as a Code > issue. The entire battery bank as a single entity is considered a single > current source. Overcurrent protection is sized in accordance with the > battery cable size - or rather vice versa: the overcurrent protection is > sized to the requirements of the inverter(s) and the cables are sized to > the overcurrent protection. > > Having said that, there are at least two cases where it's a good idea to > account for it, whether or not addressed in the Code: > 1) In smaller systems, such as with two or three strings of common > batteries, you will eventually have a cell failure. When that happens, the > amps increase on the other remaining strings. So it is good practice to > oversize series and parallel interconnects for that eventuality. > 2) For similar reasons, in large battery banks we have sometimes added > Class T fuses, appropriately sized to the application, to the B+ output of > each string before combining them on a bussbar. These fuses serve only as > catastrophic protection in case the entire load tries to exceed the > ampacity of one set of cells and thus one set of cables. > > When we have done what I think you have described - two parallel 48V > strings of 4 12V batteries each - we combine them with cables, without > additional string-level protection. That is in accordance with > manufacturers' instructions. > > Allan > > *Allan Sindelar* > *al...@positiveenergysolar.com* <al...@positiveenergysolar.com> > NABCEP Certified PV Installation Professional > NABCEP Certified Technical Sales Professional > New Mexico EE98J Journeyman Electrician > Founder, *Positive Energy, Inc.* > > A Certified B CorporationTM > 3209 Richards Lane > Santa Fe, New Mexico 87507 > *505 424-1112 <505%20424-1112> office 780-2738 cell* > *www.positiveenergysolar.com* <http://www.positiveenergysolar.com/> > > > On 2/6/2014 3:23 PM, Dave Click wrote: > > All, > > I think that in the past, list discussions have talked about fusing of > parallel battery circuits as a best practice, but it's required per 690.9 > right? (text below from 2008 NEC) > > Also, if someone (don't look at me) really wants to use 2 parallel > circuits of 12V batteries, what equipment is recommended to provide > overcurrent protection? The installer is using Outback hardware (FW500?) > and has two battery circuits combined in the battery enclosure and run to a > Big Bus feeding the two 125A dc disconnects for two GVFX3648s. If you've > read this far, this area is more your wheelhouse than mine. Thanks in > advance! > > > 690.9 Overcurrent Protection > (A) Circuits and Equipment. Photovoltaic source circuit, photovoltaic > output circuit, inverter output circuit, and storage battery circuit > conductors and equipment shall be protected in accordance with the > requirements of Article 240. Circuits connected to more than one electrical > source shall have overcurrent devices located so as to provide overcurrent > protection from all sources. > > Exception: An overcurrent device shall not be required for circuit > conductors sized in accordance with 690.8(B) and located where one of the > following apply: > (a) There are no external sources such as parallel-connected source > circuits, batteries, or backfeed from inverters. > (b) The short-circuit currents from all sources do not exceed the ampacity > of the conductors. > >
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