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The DC Section
This half of the RE system is very different from
the parts available from Home Depot. Your local electrical
supply wholesale company does not stock the DC breakers
required for RE systems. Most of the circuit breakers
available at Home Depot are not rated for DC operation.
Square D has the QO and QOU series that are UL listed
for DC operation up to 48 volts. Some installers use
the QO breakers in low voltage combiner and charge
control circuits. Today’s MPPT solar charge controllers
quite often operate above 100 volts input even for
a 12V battery system. 48 volt systems typically charge
batteries to 60 VDC so QO and QOU breakers are not
suitable. As in the AC section, there are multiple
DC circuits all operating at the same time. They too
require independent circuits, thus eliminating the
possibility to use a standard off the shelf circuit
breaker box as a one stop solution. The DC breakers
required do not lend themselves to the stab in type
of boxes available at your local hardware store anyway.
Some of the DC circuits are:
7. Inverter battery circuit breaker
8. Solar charge controller battery disconnect
9. Solar charge controller PV disconnect
10. Hydro and or wind controller output disconnect
11. DC GFP device (required per NEC soon on all solar installations)
12. DC loads such as a DC refrigerator or freezer
13. Auxiliary monitoring equipment
14. PV combiner breakers
The circuit breakers required in these circuits are
generally rated for 125 VDC. The enormous amounts
of fault current available from the battery bank necessitate
a large interrupt capacity (AIC) for inverter breakers.
25,000 amp interrupt is an industry standard. AC breakers
are not suitable for DC operation due to interrupt
rating and the lack of arc suppression. AC breakers
tend to weld themselves in the closed position when
subjected to high DC fault currents.
