Battery Disconnect Sizing

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WastefulMiser

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Is there a code or document available that should be followed when sizing a battery disconnect? Or should it just be sized for the DC load?

For example, 1000AH battery system but the max load was only 50A -- what would be disconnect size? 50*1.25 (or is it 1.15) = 62.5A or 70A disconnect, eh?

Much regard.
 
I think you are on the right track.

I would also make sure the load doesn't list a "Maximum Overcurrent Protection", max fuse size, or something like that. And I wouldn't go below a "Minimum Circuit Ampacity" rating.

And I think the wiring still has to be rated for the circuit breaker size, just like in AC wiring. So you would probably use #6 rated for 65 amps, and use the next larger breaker - 70A.

Steve
 
Also depends on what critical load the battery is supplying. A 15-minute UPS will draw more amps from a battery than the amphour rating indicates.

If the load is a switchgear control circuit, the battery breakers/fuses may be oversized to make sure the switchgear tripping power will always be available.

For a steam turbine DC lube oil pump, some codes do not allow an overcurrent device, similar to the locked rotor requirement on a fire pump feeder.

Best bet is to get the load data and confirm that the selected disconnect/fuse size is adequate for inrush current as well as steady load. If it is a disconnect (no fuses), the amp hour rating may be good enough.

When we lacked data, we have matched the disconnect size on the load, such as a 250A rated input on a UPS.

I checked a couple of IEEE standards and didn't find any recommendations.
 
Make sure your disconnect is approved for DC at the voltage you're using. It needs to be designed to stop a DC arc at fault-current levels, and the physics of that are more challenging than for stopping an AC arc. An AC-rated disconnect isn't sufficient and could be dangerous.
 
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