[RE-wrenches] Fuse sizing in battery circuits

Drake drake.chamberlin at redwoodalliance.org
Tue Apr 9 06:15:32 PDT 2013


I just went through this with SMA tech support on the Sunny 
Islands.  The breakers built in to the SIs have a 10,000 AIC 
rating.  The battery bank we are using, one string of Concord PVX 
3050 T, 305 AH clearly won't surge to 10,000 Amps.  We decided that a 
series fuse isn't necessary on the battery terminal.


At 12:19 AM 4/9/2013, you wrote:
>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
>303 505-8760On 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.
>><http://www.bluemountainsolar.com>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 
>>>>>[mailto: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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