circuit breakers and switches

Hi Bob,

I will answer this with North American components... Other countries/electrical codes may have variations in numbers.

First, figure out the rated/maximum voltage and current that you expect for each wire. And when you have multiple energy sources (such as 3x 200 AH @ 48 VDC power walls). Now--Many Li Ion batteries can output around 100% of their AH rating (i.e. 200 Amps @ 48 volts) and that would be 600 Amps rating to the "battery bus"... But your friend may only want to run the 10 kWatt max inverter(s) as loads.

A 10 kWatt inverter may draw max continuous current:

• 10,000 Watts * 1/0.85 AC inverter eff * 1/42 VDC typical batt cutoff voltage = 280 Amps max inverter draw

So, you may want to rate each power wall for less that 200 amp (smaller wiring, smaller breaker/fuse, etc.) because your loads and charging current is less than 200 Amps each. Say you plan on 100 Amps max charging/discharging current, then 100 amps * 1.25 branch circuit derating = 125 Amp minimum rated branch circuit... Or go ahead and keep 200 Amps * 1.25 (or whatever PW is rated for) on each PW branch circuit.

As always, first read the installation manual. It should have the details on DC bus (and other connection) requirements.

You can see that you need to have a simple "line drawing" that lists the load/charging current for each branch circuit and do the calculations for each. Typically assuming the "common battery bus bars" are the "source/sink" of "unlimited current--And the fuse/breaker for each branch circuit is designed to carry rated current and protected against short circuits/etc. (fuses and breakers are there to protect wiring--They are not there "prevent connected equipment damage"--I.e., the AC inverter may blow from too large of load, but the DC power breaker may have tripped in time to "save" the AC inverter's electronics.

You can use fuses or breakers for protective devices (OCP--Over Current Protection)--But Breaker can be very nice to use as on/off switches too when servicing/bypassing a problem/debugging. Your choice.

The other major specs. to pay attention to pay attention too... The OCP voltage (48 VDC, 240 VAC, etc.) and the maximum interrupt current... In the US for example, we use 10,000 AIC rated breakers/fuses for our homes. This assumes that the pole transformer can only supply a maximum of 10,000 Amps into a dead short and our house breakers can "safely disconnect" with 10,000 Amps or less of current.

Your Power Wall should list what the max output current is and/or what breaker/fuse ratings to use. And remember that the "battery bus bars" will have 3x PWs connected--So those breakers need to be able to "interrupt" the max Battery Bus Bar current. Much of this depends on the PW design--I would guess they have a BMS (Battery Management System) and other possible internal fuses/safety components/BMS software limits... Again, read the installation manual closely.

Another issue that has "arisen" over the years is the AC vs DC circuit breaker (and fuses). Standard "home breakers" are rated for 120/240 VAC @ XX Amps @ 10,000 AIC... It would "seem" that using these same breakers on a 48 VDC battery bus would be "safe"... And that is WRONG!

DC current is much more difficult to "interrupt" with a switch/breaker/fuse. DC Current tends to "sustain" arcs very nicely at anything over ~12 VDC--DC Arc welders are "very nice" to use with 20-40 Volt output.

Therefore, you need to make sure your breakers/fuses are rated for your bus working voltage... Or else a simple breaker trip or cycling off power can easily set your breaker/electrical panel on fire if running an AC rated breaker on DC circuits (some OCP are rated for both 120 VAC and 24 VDC as an example).

The other issue is that DC current has a "direction" of flow. Many (usually older designs) DC breakers have a "polarity". And that works just fine in "simple circuits" that have a source (battery bus bar) and a destination (a load).

Where that does not work well is for electrical systems where the DC can flow either direction. Batteries (both charge and discharge), inverter-chargers (both inverter drawing, and DC current flowing back to the battery bank). And even "protecting" solar arrays (array combiner boxes for 3 or more parallel solar strings) can have two flow directions--"Forward" in normal operation and "reverse" when there is a solar panel short.

And it turns out that running normal current "backwards" thru a polarized circuit breaker can easily set it on fire when you simple turn on the breaker to service it (i.e., turn off a solar string).

A recent discussion about polarized DC breakers:

https://forum.solar-electric.com/discussion/comment/426128

And a more detailed discussion about fuses and breakers:

http://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

Its a start. Details matter--Questions?

-Bill