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

[Jason Szumlanski]

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 <
allan at 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*
> *Allan at positiveenergysolar.com* <Allan at 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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