[RE-wrenches] Fuse sizing in battery circuits

[Bill Brooks]

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?

 

 

thanks,

Troy Harvey
---------------------
Principal Engineer
Heliocentric
801-453-9434
taharvey at heliocentric.org

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