[RE-wrenches] Fuse sizing in battery circuits

[William Miller]

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?
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20130405/fb2ca938/attachment-0003.html>