Cable size; MCB Amps, Watts

[Cashboy]

I google searched for MCB to cable size (mm) and got so many different figures that I am confused now.

 

Can I assume that the standard voltage in Thailand is 200 volt?

 

Then I see this Table 4D5 of BS7671:

 

 

I am running all my cables (loose cables in plastic conduit in walls and inside the roof of a new building.

 

All I need to know is what MCBs I require for the following cables cross section inside the plastic conduit tube.

 

1.0mm

1.5mm

2.5mm

4mm

6mm

10mm

16mm

 

On the basis that Thai electricity is 200 Volt

And on the basis of knowing that Watts = Amps x Volts

 

Then for an Steibel Electric Shower of 4,500 watts with 200 volt = 22.5 Amp

I thought I would need 4mm cable for that but the instructions in the box say 2.5mm or 4.0mm.

 

Edited October 6, 2020 by Cashboy
added

[Crossy]

Thailand is nominally 220V (230V in Bangkok - MEA area).

 

Your 4,500W shower will draw about 20.5A which would be OK on 2.5mm², better on 4mm² which leaves some spare capacity for future upgrade.

 

Generally, one would wire lighting in 1.5mm² (1mm² is a bit delicate mechanically) on a 5A or 10A breaker, outlet circuits (radials, no ring-finals in Thailand) in 2.5mm² on 20A breaker.

 

Higher power circuits sized according to the load, generally, unless you have some really big aircons that would be water heaters, up to 6kW on 4mm² and 30A breaker, 8kW in 6mm² on 40A. Sub mains wire according to load, I'd be using 6mm² or 10mm² generally.

 

To directly answer your question, breakers for THW in conduit, fill ratio of <50%, volt drop of long runs not considered :-

 

1.0mm - 10A

1.5mm - 16A 

2.5mm - 20A

4mm - 30A

6mm - 40A

10mm - 60A

16mm - 80A

 

The free-air ratings of the above cables are somewhat higher.

 

If you have runs over about 15-20m you may need to consider volt-drop, particularly on pumps, aircon and other appliances with large start-up surges (induction motors).

 

BS7671 is way too deep for basic installation, TLC have a good on-line guide https://www.tlc-direct.co.uk/Book/1.1.htm which is a much easier read although I would consider AS/NZ3000 (Aussie / Kiwi standard) as a better fit for Thai conditions and wiring requirements.

 

Note that some Thai practices, particularly how MEN (PME in UK speak) is implemented, seem a bit alien to one used to the UK way.

 

You may find this PEA document useful when talking to your sparks Groundwire Mk2 book-Manual.pdf

 

And this diagram from the above document (with translations)

 

 

Don't forget to include earth-leakage protection (RCDs or RCBOs) and you would do well to include surge protection devices (SPDs) as lighting is common and most supplies are overhead.

 

 

 

[Cashboy]

I had a look at the "PEA manual that you suggested that I show the installer".

 

I am actually doing all the wiring myself in this house/office.

 

When I looked in the "PEA manual" I cam accross this:

 

When I attempted to google translate it; left column seems to be Amps and Right seems to be cross section of cable.

 

4 mm cable seems to be good for a 40 Amp load

 

What really surprises me is that 16mm seems to be good for 200 Amp load.

In the UK, I was under the impression that 16mm was only good for 80 Amp (i.e. mains cartridge fuse is maximum 80 Amp).

So why are the Thais says the mains supply should have a 50 Amp MCB.

Is it the PET meter that is the weak point as opposed to the cable?

 

I know this is wrong but while the building work has been going on, the site manager connected just 2.5mm cables directly from the PET meter to a 4 plug socket.

They do this because most houses in the village have a 5 (15) only and trips the house breaker if you switch on a couple of power tools.

This seems to be common practice on sites I have visited.

We have been using that for the last two years (no RCB or MCBs) and running 250 Amp DC welder, power tools, 3 HP mixer and those cables have never burned.

 

 

 

 

 

Edited October 7, 2020 by Cashboy

[Crossy]

1 hour ago, Cashboy said:

When I looked in the "PEA manual" I cam accross this:

 

That's the minimum size of the ground cable.

 

[Cashboy]

1 hour ago, Crossy said:

 

That's the minimum size of the ground cable.

 

So for the ground cable (earth I assume) ; 10mm cable is enough?

[Crossy]

6 minutes ago, Cashboy said:

So for the ground cable (earth I assume) ; 10mm cable is enough?

 

10mm would be "to code".

 

BUT

 

With TNC-S (MEN) there could be fairly large diverted neutral currents which are NOT controlled by a breaker. Personally I'd be using 16mm.

 

EDIT Sorry about the mixed terminology, I'm a Brit but deal with documents written in American all day.