What Size Rcd To Use ?

[regedit]

I know I need a 30mA RCD - but for the "in" Amp do I need 63A or 40A ? It will be in a split-load consumer unit. The protected side has 6 MCB's from 6A to 32A (cable is 1.5mm to 6mm). Main switch (for the whole CU) is 63A.

Thanks for any help.

[Crossy]

As you are aware an RCD has no overcurrent element so you just need to ensure it can carry and break its maximum expected load.

You should match it to the maximum current it would be expected to handle, either add up the loads / breakers on the protected side, or just match to your incoming breaker if it's smaller.

[electau]

If the main switch is an 63A MCB this will provide the short circuit and overload protection. The actual current rating of the RCD can be 63A or 40A. Generally protection on the line side is up to 100A. The max size of the protective device on the line side is usually indicated on the RCD.