Correct sizing of AC MCB in ACDBs

One of the essential protection components in ACDB is AC MCB. Many times it is found that an incorrect rating of AC MCB is selected for ACDBs. Overrating and underrating of AC MCBs, both cases can be dangerous for a system. Below mentioned formula should be followed for the correct sizing of AC MCBs.

AC MCB Current Rating (A) = Inverter Max. Output Current x 1.25

For example, if the output current of a 4 kW Single Phase Inverter is 17.4A. Now, AC MCB that shall be added to the ACDB for this inverter should be:

AC MCB (Current Rating, A) = 17.4 x 1.25 = 21.75 A.

The next available MCB in the market is 25A. Hence, 25 A DP AC MCB should be used in this system.

Correct selection of DC and AC SPDs

Surge Protection Device (SPD) is a very critical and important device on the AC and DC sides of the inverter. In the absence of SPD or in the event of wrong selection or wrong design, the inverter can be critically damaged or the plant will not perform optimally.

Below mentioned points should be considered while selecting SPDs in your system:

  • Selection of DC SPDs for the DC side and AC SPDs for the AC side. In the event of wrong selection (DC SPD on the AC side or vice versa) inverter may be damaged critically.
  • Selection of correct DC voltage of the SPD (i.e. 600V or 1000V). The voltage should be selected considering Maximum Array Voltage (at the minimum expected cell temperature).
  • Selection of correct AC voltage of the SPD (i.e. 280V or 320V). This needs to be checked with the inverter manufacturer.
  • Separate SPDs for separate MPPTs of an inverter. Single SPD should not be used for different MPPTs of an inverter.
No AC SPD in ACDB. Risk to Inverter
AC SPD installed in DCDB box


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