[RE-wrenches] Fuse sizing in battery circuits

Ray Walters ray at solarray.com
Mon Apr 22 22:14:09 PDT 2013 

Hi Troy;

Many moons ago, I looked at the curves as you did, and special ordered 
some 350 amp T fuses, but then I quit making extra work for myself.  In 
a dead short, the 350 is really not going to blow that much faster, 
while the 400 amp blows much faster than a lower rated breaker will trip.
There are several reasons why we use 400s with 4/0 cable.  First and 
foremost is that that is (or was) the fuse and cable size recommended by 
Trace back when we used a lot of class Ts.  If you stick with the 
manufacturer's recommendations, you are doing your job by the NEC.
"110.3(B) INSTALLATION AND USE:  Listed or labeled equipment shall be 
installed and used in accordance with any instructions included in the 
listing or labeling."
The 2nd point, is that cables within NEC are designed for continuous use 
3 hrs or more.  An inverter is not going to exceed the cable rating for 
more than a few seconds during a surge. If something is wrong, it will 
almost always be a higher current short, and the fuse will still blow.  
The idea of a  short circuit situation with an inverter in which a 350 
amp fuse would blow, but not a 400 amp is just going to be rare.  What 
CAN happen though is a normal surge that could nuisance trip a very 
expensive and hard to replace 350 amp fuse.
Third, I'm using class T fuses to protect battery cables that are never 
over 10 ft long.  They usually run in conduit for less than 24 in, so it 
is considered a nipple, and most of the fill and temp adjustment factors 
don't apply: 310.15 (B)3.  Also, since its for short runs and most of 
the circuit is not in conduit, you can use the free air rating of the 
cables. 310.15(A)2 exception.  At least that is how it was explained to 
me.  I never argued this last point with an AHJ, so others could debunk 
me on that, but I simply would show the AHJ the sizing reference in the 
inverter manual, and we're done.
Finally,  Outback still lists the class T fuse 400 amp for 4/0 cable but 
they add a note (for mobile only) since these days class T fuses are 
mostly used in non UL listed fuse holders with no disconnects.  Bolt 
switch still makes UL listed class T disconnects, though.  BTW, I'm not 
advocating for going back to the 90s as Robin mentioned and using class 
T fuse exclusively.  I thought the original thread was about adding 
supplemental protection at the battery terminals or for each string, in 
addition to regular breakers.  In that case you can put in larger fuses 
too, because you're really after the interrupt rating more than the 
regular fuse rating.  The Midnite breaker still takes care of the 
primary OCPD requirements.

I'm sure I missed something, so hopefully others might join in as 
well.   (motor circuit fusing?)

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760

On 4/22/2013 4:58 PM, Troy Harvey wrote:
> Ray,
>
> One thing I didn't understand is how you can pair a 400 amp class-T 
> fuse with a 4/0 cable. The 400 fuse curve looks like the continuous 
> rating is 350A in the graph supplied by Allan. Which is well above the 
> 4/0 ampacity of 250A (copper). Anyone want to explain?
>
> thanks,
>
> Troy Harvey
> ---------------------
> Principal Engineer
> Heliocentric
> 801-453-9434
> taharvey at heliocentric.org <mailto:taharvey at heliocentric.org>
>
>
>
> On Apr 5, 2013, at 10:12 PM, Ray Walters <ray at solarray.com 
> <mailto:ray at solarray.com>> 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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