[RE-wrenches] Fuse sizing in battery circuits

Sat Apr 6 07:29:36 PDT 2013

Ray,

690 does offer the protection, it's just that most maufacturers haven't
made it easy.

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.

The telcom industry has been doing this for years.  Their norm is to have
each string of batteries on a shelf, and terminate at a breaker.  Each
string then is combined at a bus bar or plate, to help ensure equal current
on each string.  The loads and charging sources all come to that same bus
bar.  It has some advantages, as individual strings can be monitored and
serviced without taking the entire system down.

FWIW, OutBack makes a battery rack for AGM batteries with series string
overcurrent and disconnects on every string.  It's also to my knowledge the
first UL1741 Listed device available -- the other stuff I've seen is either
not listed, or is trading on old 508a standards.  AGMs might not be for
every installation, but for jobs where they are appropriate this could be a
good, code-compliant solution.

Phil Undercuffler
OutBack Power

On Friday, April 5, 2013, Ray Walters wrote:

>  I just finished a rewire and we kept the Ananda power center, Allan :-)
> Back when we used class T fuses more they were always over sized relative
> to Heinemann breakers.
> 400 amp class T fuse = 250 amp breaker= 4/0 cable
> 200 amp class T fuse = 175 amp breaker= 2/0 cable
>
> I still think class T fuses are superior in some ways to breakers: they
> have better interrupt capacity and trip faster in a short circuit condition.
>
> However, If you spend too much time thinking about and looking at the trip
> curves, you'll break your brain and special order some weird fuses and
> breakers.  (anybody need a 350 amp class T fuse?)
>  Just follow the inverter manufacturer's recommendations and all will be
> well.
> On the other hand, William has brought up a topic I have harped on for
> years:  Having the OCPD in the cabinet doesn't protect the majority of the
> circuit.  Class T fuses at the battery terminals do, but they're not rated
> for the corrosive environment.  I have thrown a few class T fuses away that
> had acid eating away at the ends.
> I don't have the solution, but I will continue to point out that this is a
> real problem. Dropping a wrench across the battery terminals can lead to a
> spectacular failure that not only can cause a fire, but might even cause a
> battery explosion, yet NEC offers no protection.
> We use insulated wrenches from experience, and hope for the best.
>
> R.Ray Walters
> CTO, Solarray, Inc
> Nabcep Certified PV Installer,
> Licensed Master Electrician
> Solar Design Engineer
> 303 505-8760
>
> On 4/5/2013 8:29 PM, William Miller wrote:
>
> Friends:
>
> Good topic.  Some questions:
>
> 1. Most manufacturer's present an installation guide that shows one OCPD
> in the battery circuit and that is in the BOS cabinet.  This means the
> battery leads are unprotected.  Do we need an OPCD at the battery terminals?
>
> 2. Class T fuses are generally recommended for this application.  The data
> shows them as "fast acting."  Is this a problem?  Will they act too fast
> and open during normal surge loads?
>
> Thanks in advance!
>
> William Miller
>
>
>
>
>
> Troy,
>
> Overcurrent device size is matched to the conductor size. The inverse time
> constant nature of an overcurrent device can typically handle the surge
> currents as long as conductor sizing has truly been done correctly for the
> conductor. Circuit breakers are preferred to fuses because they can be
> reset.
>
> There has been volumes written on this issue. The constant current at
> lowest battery voltage should be used, plus the ac ripple content on the
> battery circuit. This is usually a much larger conductor than your average
> designer will plan for. The best thing is to look at Midnight, Outback, and
> Schneider and see what size overcurrent devices they require for their
> products. That will give you a good clue as to how to size the conductor
> and overcurrent device.
>
> Bill.
>
> *From:* re-wrenches-bounces at lists.re-wrenches.org <javascript:_e({},
> 'cvml', 're-wrenches-bounces at lists.re-wrenches.org');> [
> mailto:re-wrenches-bounces at lists.re-wrenches.org <javascript:_e({},
> 'cvml', 're-wrenches-bounces at lists.re-wrenches.org');>] *On Behalf Of *Troy
> Harvey
> *Sent:* Friday, April 05, 2013 3:38 PM
> *To:* RE-wrenches
> *Subject:* [RE-wrenches] Fuse sizing in battery circuits
>
> I've got a question about battery string fusing. Typically we size the
> wire from the batteries to the inverter based on continuous rating
> procedures (max power/efficiency)*125%.
>
> However a 6kW inverter, can peak at 12kW for 5-10 seconds, doubling the
> source current. That is no big deal for the wire, because it is a short
> time frame... little heat will be generated. However, in fusing the
> sub-strings, you need to account for that peak surge current so you don't
> blow fuses all the time. But if you put a 500-1000 amp fuse on a 4/0 wire,
> above the max surge draw of the inverter, the wire will be under-protected
> for its ampacity rating. Any thoughts on the catch-22?
>
>
>
>
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