Selecting a breaker for a solar array is not a matter of swapping an AC miniature circuit breaker with the right amp rating. Direct current has no zero-crossing, so the arc is far harder to interrupt — what matters is arc-chute design, rated voltage and breaking capacity at the actual installation point.
Why DC breakers are different
An AC MCB relies on the current passing through zero 50–60 times per second to help quench the arc. A DC breaker must stretch and cool the arc purely by mechanical design. Using an AC device on a DC string is a common audit finding: it may switch fine in normal operation but can fail to interrupt a fault.
Rated voltage and string topology
Leave margin on string voltage
Calculate open-circuit voltage at the site’s lowest expected cell temperature, not at STC. A string that looks like 900 V at 25 °C can exceed 1,000 V on a cold, clear morning. Choose a breaker rated above that worst-case figure.
Pole configuration
Positive and negative conductors should both be interrupted unless the design specifically justifies single-pole DC isolation. 2-pole devices keep the string visibly disconnected on both conductors.
Breaking capacity at real fault points
The available fault current at a combiner box is limited by the number of parallel strings — but the point of common coupling with the grid or a battery bank is a very different story. Compare the datasheet Icu value with the calculated fault level at that exact position in the diagram.
| Protection point | Typical device | What to verify |
|---|---|---|
| Per string | 1P/2P DC MCB, 10–15 A | Rated voltage above cold Voc |
| Combiner output | 2P DC MCCB | Icu vs parallel-string fault current |
| Inverter DC input | DC isolator or MCCB | Manufacturer’s own requirements |
| Battery side | DC MCCB or fuse | Short-circuit contribution of the bank |
Before you order
- Worst-case string voltage at minimum temperature
- Fault current calculated at each protective device
- Busbar or cable terminations the device must accept
- Documentation your inspector or EPC actually requires