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Sizing breakers and DC/AC protection
Installation4 min readUpdated: 2026-08
A breaker protects the cable and equipment against excessive current — it must trip before the cable heats dangerously, but not trip spuriously on normal operation.
I_breaker ≥ 1.25 × I_max_circuit25% margin to absorb normal transient overcurrents (e.g. battery charging current)
The breaker rating must never exceed the ampacity of the cable it protects. If the cable is rated for 32A and the breaker is 40A, the cable can overheat without the protection tripping.
| Circuit | Breaker type | Key point |
|---|---|---|
| PV DC | Certified DC breaker (not a repurposed AC one) | A standard AC breaker cannot properly extinguish a DC arc |
| Battery DC | High-current DC breaker | Place as close to the battery as possible |
| Inverter AC output | AC breaker curve C or D | Curve D for high-inrush loads (motors) |
| Main panel | 30mA RCD (residual current device) | Personal protection, mandatory in residential |
SolarDim Pro automatically calculates the PV DC breaker rating from 1.25 × Isc (not Impp), since short-circuit current is the reference sizing value in photovoltaics.
Common mistakes
- Using a standard AC breaker on a DC circuit (DC arcs don't extinguish the same way)
- Rating the breaker above the cable's ampacity
- Forgetting the 30mA RCD on the residential main panel