The 25mA and 10mA is the trigger point when the MCB tripped to protect people from electrocution. So, the 10mA is recommended to be installed at water heater by TNB.

The water heater circuit breaker is additional to the house MCB.

This post has been edited by Zot: Dec 28 2023, 12:07 PM

Here is the guideline from TNB

You can see where the 10mA breaker is.

You have been running your current water heater which is rated 3.5kW – 5.5kW for so many years already and your wire probably a 4mm² wire. I think it should be just fine to just use the same wire.
The wire can carry 32A roughly if it is inside tubing or directly nailed on or inside the wall cement wall. Your 4.2kW water heater will max at 18.3A @230V. I would consider it as continuous load current. So, I would derate wire requirement. 1.25 x 18.3 = 22.875A which i still within the wire load capability.

You need to install a 100mA RCD (not MCB as mentioned by ceo684. This is for safety against electrocution. You do not want any current more than 10mA run through your body if accidentally touch wire or if the water heater got leak current. You can put it on wall just before the 2 pole switch inside single breaker casing if you do not have space in main DB box.

Just for the water heater for safety. If use 10mA for whole house then you probably will experience unnecessary tripping

Opps... I thought I typed 10mA .... did not check before posting.

This is for the water heater.

This post has been edited by Zot: Dec 29 2023, 02:22 PM