[RE-wrenches] Fuse sizing in battery circuits

[Ray Walters]

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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