Hi,
4200W / 230V = 18.26A
https://www.electrical-installation.org/enw...a_of_conductors
Per reference method B1 (single circuit, solid stranded cable enclosed in conduit) able to handle up to 21A with 2.5mm² cable.

This 21A is assuming that quality cable (eg Mega Kabel, Caramay, or any of the MCMA cartel members) is being used.. 
If elcheapo or fake cables - you might be running a risk that it is undersized for the load

Also the length should be kept to reasonable length.. if the cables are too long then voltage drop will be a problem as well
Because if effective voltage drops, the amps go up
e.g. 4200W / 200V = 21A.
So there is about a 10% reserve capacity of the cable current capacity with 2.5mm² cables.. insofar as ST regulations go, officially they recommend minimum of 4mm² so that no need to worry or calculate so deeply.

What about this? Is this good enough?

Should be OK, they print their name and spec on the cable, there's an older SIRIM cert issued for MPC cable and it is not in the news for fake SIRIM certs  
There are cases before - https://www.sirim-qas.com.my/forgery-on-license-no-pc000218/

How to identify a sub-standard cable (per https://www.st.gov.my/en/contents/files/dow...ble_testing.pdf )
1) Check and verify MARKINGS on Cable – shall be as per standard requirement
2) Check and verify APPROVAL / CERTIFICATION Mark/Label
3) Measure conductor resistance ( need to have a proper meter)
4)  Measure size(s) • conductor • each strand ( and no of strands)
5) Check and verify copper content • normally by conductor resistance • weight the sample • Sometimes –by weighing the sample, with some experience, we can identify if the samples / weight of sample due to copper content or sheath of insulation

On the other hand w.r.t point no. 4 - there are also complaints on shopee that the cables are not as thick as they claim to be 
https://shopee.com.my/MPC-2.5MM²-PVC-CABLE...2276.3417950523

Let's say, touch wood, when the cable overheat, will the MCB/RCD trip? I scare it won't trip and cause fire. lol

Actually I did check the cable marking last time, but I found that, when I rub the cable the marking will get erased, is that normal? Will quality cable be erased too?

B Curve means the MCB trips between 3-5 times full load current. It is mainly used in residential applications where loads are resistive e.g. lighting fixtures, domestic appliances with low surge Levels.
C Curve means the MCB trips between 5-10 times full load current. It is used  in commercial?industrial applications where there is greater chances of higher short circuit currents e.g. mainly inductive loads , fluorescent lighting.

Let's say, touch wood, when the cable overheat, will the MCB/RCD trip? I scare it won't trip and cause fire. lol

Actually I did check the cable marking last time, but I found that, when I rub the cable the marking will get erased, is that normal? Will quality cable be erased too?

Let's say, touch wood, when the cable overheat, will the MCB/RCD trip? I scare it won't trip and cause fire. lol

Actually I did check the cable marking last time, but I found that, when I rub the cable the marking will get erased, is that normal? Will quality cable be erased too?

Challenge is how do we know the actual true max safe of fake cables

To answer the question..in diff POV
For upto spec cable, yes when fitted w appropriate MCB yes MCB will trip if the cable overloaded. Slow blow or instant trip depending on overload size. But anyhow this will work regardless slow blow or instant trip

For substandard cable..in the reverse scenario..if MCB sized accordingly to actual true max safe capacity of cable lousy spec then will protect as per above case.
If MCB was sized to untrue claimed spec per the fake labelling (which is normal procedure)..there is such a risk the cable melted before MCB slow blow happens (if the cable is really substandard aka fake shit).

Sis actually. 

1. What is the "MCB C20 WH"? is it different type of other MCB?
Standard C20 MCB.
WH denotes the one for WH circuit.

2. if I have 4 header, which mean I will need FOUR 10mA RCD?
Yes correct. WH 10mA RCD is per unit of WH. Each WH should be on its own dedicated circuit as one WH enough to use 15-17A nominal (WH is heavy appliance). Your 2.5mm cable only support 21A. And the 10mA RCD, only come (from branded mfg) up to 25A max current. In short, the standard cable sizing, the RCD current capacity (max 25A) all only suited for single unit WH as it is sensitive device (0.01A /10mA).
Whilst there are champions from china making 10mA RCD that can handle 40A I also dare not use.

3. The purpose of 10mA RCD is to trip when the heater leaking to our body? my current installation if leaking it won't trip?
Your current installation only trips at 300mA - by then, the human dieded 6 times over.
As the mains is 300mA, it allow 300mA leakage before it trip.
A human RIP at 50mA.
[attachmentid=10803044]

4. In short, I only need to have 10mA RCD in between the MCB and Header individually? Any other wiring need to be change/improve?
Yes, correct.
RCD is comparing input and output current difference. Input and output must = equal the same +/- RCD tolerance of say 10mA (0.01A).

Meaning, if WH input is 16A on L, at least 15.99A must return on WH output (N) else it has deemed to be lost somewhere.

Lost somewhere mean could be leaking through human.
So when the return not enough (<15.99) say only 11A return. The 4.99A cannot be lost, it mean it leak. As 4.99A far exceed the 0.01A allowed by 10mA RCD it will immediately trip.

MCB only consider single wire (L) amount of current passing through (short circuit prevention).
For a C20, meaning the speed limit is 20A.
Slight overload at 23A will trip the SLOW thermal overload after say 20 min.
Major overload at 100A will immediately trip the magnetic overload.

5. between I'm quite confusing are ELCB & RCCB the same? can you correct me if the naming in the image below wrong?
The difference is how and where they measure.
ELCB only detect earth current rise >0 leak (L to PE) leak on that appliance's earth. If it leak via anywhere else (L-N, or L-other PE eg wet floor, L-human) it will not bother. Hence in short I just say it protect that appliance only.

RCD because it constantly compare L input =N output + allowed tolerance (like 10.00A in on L, >9.99A must return as allowed tolerance is 0.01A for normal operation things like wire resistance that do NOT constitute a fault condition). It does not care where it leak through, RCD always compare in and out current so if it leak through any method (L-N, L-PE, L-other PE, L-human) it always trip.

Note: There is something called RCBO which do both function of MCB+RCD in one.
6. Appreciate if you can share the image of "MCB C20 WH" and 10mA RCD
MCB C20 is any usual C20 (20A MCB). WH is just designation for the WH circuit.

10mA RCD (must be used in conjunction together with C20 MCB) ie. add this along with MCB C20
ABB= https://www.lazada.com.my/products/abb-f202...i2IABn&search=1
Hager= https://www.lazada.com.my/products/hager-cc...l5n2dU&search=1
Schneider = https://www.lazada.com.my/products/schneide...8t5PsK&search=1

10mA RCBO = RCD+MCB in one package pao kah liao 2 function in one
Schneider RCBO https://www.lazada.com.my/products/schneide...PVCaV4&search=1
The drawback is, officially, their MCB is a C25 equivalent so you need to use with 4mm cable. As 2.5mm cable only good up to 21A rating.


First one from left  is MCB 3P (3 pole) C63 (63A)
Second one is 4P RCD but this is 300mA. Only used in commercial/industrial where they use heavy motors. Not safe for residential and its against regulation, as the human has dieded 6x before this trips. (RCD/RCCB/RCCD = same function with diff name like zebra crossing=pedestrian crossing)
Third one is MCB 1P C10 (10A)

You will then ask, why do people still put the 300mA in residential?
Just to save the few bucks for more profit/lower tender cost

Holy.. I just bought a 3phase rccb and the seller gave me Hager 63A 4 pole 300ma RCCB. Should i tell them i want the smaller amp one? I already installed it in my client house just 2 days ago. -___- Why even the sellers dont know or they dont care? lol.

Thanks.
I've another issue, I noticed that, when I switch on the light, my Fan Live cable will have minor current, the test pen light up, but not strong. Do you know what might be causing it? That wireman told me earth cable not deep enough, but I don't believe. lol

For 4P, 300mA cheap ma coz insensitive.
They sell the 300mA because still valid use for industry.
Not all RCD created equal

Should use the 30mA then up to par with current residential standard. 4P 30mA 63A usually cost almost double of the 4P 300ma.

Personally I'd still try to swap out to 30mA, safety first.

Aiyooo my hse too also using 300ma. Looks like these  so called certified electriciant don’t know what they are doing.

Your wires looks so thin. What size you buy? I use minimum 2mm. I'm sure you loop the wires.

Looks scary.. like gonna melt at any time...

For 4P, 300mA cheap ma coz insensitive.
They sell the 300mA because still valid use for industry.
Not all RCD created equal

Should use the 30mA then up to par with current residential standard. 4P 30mA 63A usually cost almost double of the 4P 300ma.

Personally I'd still try to swap out to 30mA, safety first.

Whoa. Double the price? 😱 whoaaa. Expensive oh.. need check with my supplier see how much.

wires run together in the same trunking.

it can induce a potential in nearby wires.

so there is voltage in them even without the switch is on. but if you test the current by putting the voltage to some or any work the voltage would immediately drop to zero.

pretty much normal and not a concern. nothing to do with earth or neutral

Sorry, I don't understand what u said. x faham what's the voltage & current in ur sentence. Haha
But can u tell me, when i switch on the light, the test pen have a very weak light(fan Live cable on the ceiling) is that normal or something wrong with the installation?

Is this smart meter that I can use myTNB app to monitor my house energy usage?

Sorry, I don't understand what u said. x faham what's the voltage & current in ur sentence. Haha
But can u tell me, when i switch on the light, the test pen have a very weak light(fan Live cable on the ceiling) is that normal or something wrong with the installation?

Yes, correct, RCBO 20A 10ma.