Good day fellow forumers!

I've hired a contractor to do renovation for my parents' 2 storeys old house.
I've asked them to totally knock down everything inside the master bedroom toilet.
And I've asked them to change new wiring for the (instant) water heater shower unit too.

So, just now I went to check the progress and I found that the contractor used 2.5mm for the live wire,
but only 1.5mm for the neutral and ground wires. And they tiled it up already. Is it dangerous?
Should I ask them to knock down the new tiles and redo the neutral/ground wires with 2.5mm ones?

My parents' house is on 3 phase power. And there is a sub DB upstairs powering 3 bedrooms.
The said downsized wires are upstairs in the master bedroom toilet.

Thank you experienced sifus for your inputs!!

usually no need to brake tiles. just pull the wires through the pvc hose
should use 4mm2 cables. (my mistake. should not be 2.5)

https://www.st.gov.my/en/contents/publicati...r%20Systems.pdf

you may read the guide cover to cover.
some points
- use double pole 20A water heater switch
- as much as possible, the LN wires should not have joints.
- earth cable should not have joints.
- do not use 3pin plug to connect water heater
- leave a loop of wire before entering water heater (so water wont go inside)

Damn, my electrician put 3 pin plug for the water heater when we did whole house reno.