DC breaker sizing checks
DC breaker sizing starts with corrected load current and keeps DC voltage and interruption capability explicit.
Idesign = Iload × FdesignFor known power: Iload = P/VDCIb <= In <= IzUe,DC >= UmaxIcu,DC >= Ik,DCwhere:
- Idesign
- Required design-current basis[A]
- Fdesign
- Entered continuous or design factor
- Iz
- Corrected cable ampacity[A]
- Ue,DC
- Breaker rated DC operational voltage[V]
- Icu,DC
- Rated DC breaking capacity[kA]
The exact documented pole connection, polarity, voltage, and DC time constant govern the usable breaker rating.
How to size a DC circuit breaker
Enter load current directly or calculate it from DC power and voltage. Apply the project design factor, then compare the candidate breaker's current with corrected cable ampacity and separately verify its DC voltage and breaking capacity.
- Use the maximum operating current or power.
- Enter the highest circuit voltage, not only nominal voltage.
- Apply the required continuous-current factor.
- Check Ib <= In <= Iz.
- Verify DC voltage, pole wiring, and breaking capacity from the exact datasheet.
Why DC breaker ratings differ from AC
A DC arc has no natural current zero every half-cycle, so interruption depends strongly on breaker construction, voltage, pole connection, source behavior, and current direction. An AC voltage or breaking rating cannot be assumed to apply on DC.
- Use the documented DC voltage for the exact number of poles and connection diagram.
- Battery and PV sources have different available-fault-current behavior.
- Check bidirectional or polarized operation where current can flow in either direction.
DC breaker selection checks
The current result is only the starting point. Final selection also includes cable protection, ambient derating, enclosure temperature, source time constant, isolation, selectivity, and application-specific PV or battery rules.
Size the connected cableCalculate a PV combiner boxCheck DC voltage drop
DC breaker sizing example
A 32 A continuous DC load with a 125% design factor requires 40 A. A 40 A candidate passes the current relationship only when corrected cable ampacity is at least 40 A; DC voltage and breaking capacity remain independent checks.
Assumptions
- Steady DC load
- Maximum system voltage and fault current are entered for the installation point
- Standard current rounding is a preliminary reference
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Never substitute an AC interrupting rating for the required DC rating.
- Required pole count and series-pole wiring must follow the exact breaker documentation, earthing, polarity, and system voltage.