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

[Phil Undercuffler]

Dave,

I think the key reference is Section VIII, Storage Batteries, 690.71
Installation.
(C) Current Limiting. A listed, current-limiting, overcurrent device shall
be installed in each circuit adjacent to the batteries where the available
short-circuit current from a battery or battery bank exceeds the
interrupting or withstand ratings of other equipment in that circuit. The
installation of current-limiting fuses shall comply with 690.16.

You need to calculate the amount of current your batteries can provide into
a fault, which is a function of capacity, internal resistance and a few
other details your battery manufacturer should be able to provide.  The
OutBack 175/250A panel mount breakers have an AIC rating of 100,000 at
65VDC, 50,000 at 125VDC.  If the fault current your two strings of
batteries can provide is less than 100,000, you can parallel the two
circuits.  If not, you should provide overcurrent protection on each
string.  Class T fuses can be used to provide current limiting capability
to within that range.

Phil



On Thursday, February 6, 2014, Dave Click <daveclick at fsec.ucf.edu> 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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