[RE-wrenches] Fuse sizing in battery circuits
Ray Walters
ray at solarray.com
Mon Apr 8 21:19:07 PDT 2013
HI Kent;
I actually used to have info from Trojan on the short circuit current
available from the T105 and L16. I just don't remember it exactly, but
I recall it was around 2000 amps for the T105. I agree its really
shameful how lame most battery manus are at providing info anymore. Its
hard to even get good cycle life tables.
HUP does show their short circuit #s. The 845 AH battery is 9600 amps
and for comparison, the 2490 AH is 29,100 amps.
So yes, we could have trouble.
B&B a manufacturer of sealed deep cycle batteries shows their 12 v 90 AH
battery with a whopping short circuit current of 2300 amps but only for
100 milliseconds.
Midnite shows their breaker AIC ratings and its 10,000 amps for the PV
breakers up to 50,000 amps for the 175 and 250 amp breakers. The class
T fuses I checked are 20,000 amps AIC. Remember that this is all rated
at 125 to 150 vdc, so all of these OCPDs will have even better AIC at
lower voltages. (I wish we could get a chart that showed AIC relative
to voltage.) The other factor in our favor is that the batteries might
be able to exceed the AIC for tenths of seconds, but it can't sustain it
like the grid can.
Sorry that this is not enough info to do a definitive design, but at
least it can get us all thinking about AIC ratings relative to bigger
battery banks.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 4/6/2013 1:21 PM, Kent Osterberg wrote:
> Phil,
>
> Thanks for the code references, they are very useful and point out an
> area where I fear many installations of battery based inverters are
> deficient. You are right the manufacturers have not made this easy!
>
> First, data on the short circuit current available from most batteries
> isn't easily available, if available at all. Concorde publishes short
> circuit current numbers for the SunExtender batteries. Neither Trojan,
> Rolls/Surrette, nor Outback publishes short circuit current data for
> their batteries. Full River publishes an internal resistance number
> that can be used to estimate the short circuit current. Taken as a
> whole, battery manufacturer's certainly haven't made it easy to comply
> with 690.71
>
> Second, sometimes interrupting capacity data on the breakers being
> used in BOS equipment is also missing. Take a look at Outback's
> catalog and try to find the AIC for the breakers used to protect the
> inverter wiring. Is it published anywhere else? Should a system
> designer have to buy the Outback breaker, find out who manufactured
> it, and then search for the correct ratings in the in the breaker
> manufacturer's catalog? Obviously not.
>
> Third, BOS manufacturers often connect circuit breakers for charge
> controllers or dc utilization circuits that have AIC ratings of only
> 5000 amps directly to the battery. So while the inverter may be
> protected with a breaker having an AIC rating 25,000 or 50,000, other
> devices don't. BOS manufacturer's should include on the dc breaker
> panel data showing the lowest AIC breaker that is connected directly
> to the battery. Then installers would have at least half of the
> information needed to decide when a fuse is needed to backup the dc
> breakers. And shouldn't the instructions for BOS equipment point out
> that a fuse may be necessary at the battery.
>
> The short circuit current from a single string of golf cart batteries
> or L16 batteries is probably less than 5000 amps. Fuse(s) are probably
> needed in most installations with larger batteries or battery systems
> with multiple strings of batteries. 2011 NEC 240.21(H) allows
> overcurrent protection to be installed as close as practical to
> battery terminals - even in classified locations.
>
> Kent Osterberg
> Blue Mountain Solar, Inc.
> www.bluemountainsolar.com
>
> On 4/6/2013 7:29 AM, Phil Undercuffler wrote:
>> 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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