Re: [RE-wrenches] DC Fusing/Breakers for Battery Circuits

[Allan Sindelar]

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 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 office 780-2738 cell 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. _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Change email address & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org

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