Calculating main switch size

[bluie3]

My friend, a plumber is at a job with a 63A 3 phase main switch, and the board has a few 20a circuits, a few 10a, and a few 32a. Hes replacing the 20a double pole breaker from the old water heater to a 3 phase 40a breaker, to suit the new heater.

What is the formula/calculation to determine the size of the main switch, when you only know the size of the breakers? I know how to do to max demand when I know the number of points inside, but how do you calculate the main switch rating when only knowing the sizes of the breakers?

 

[M-Ty]

I don't think you can calculate it without knowing the types of loads on the circuit breakers. Equal to the supplier's fuses is probably the best anyone can do without the necessary information.

 

[dmxtothemax]

Tell us more about the loads ?
What are they ?
Are they continuous ?

 

[LeeH]

Firstly, tell the wet pants to stick to push fit pipes.

And secondly the board it’s self has a maximum allowable rating and probably already has a suitable disconnector fitted by the ELECTRICIAN who DESIGNED, fitted and TESTED the installation.

Are you an electrician?

 

[Spoon]

Hi @bluie3 . Welcome to the forum. How's it going in Australia?
Your post is a bit confusing.
Regarding the main isolator switch: As above, this will have a max size for the dist board and secondly it has to be equal or bigger than the supply.

Hes replacing the 20a double pole breaker from the old water heater to a 3 phase 40a breaker, to suit the new heater.

Is the existing cable suitable for the the change in breaker size?

Is the plumber going to test the circuit afterwards? I don't mean a 'bang test'

 

[Spoon]

Yes, @Zdb it is a bit optimistic that the cable will be rated for 40A. That's why I asked..

 

[bluie3]

I should have also stated that I am a sparky, but ive always worked for the huge companies where all the selection is done, and we just get the materials and are told to install.
I will be running the new circuit, with 6mm cable, but as I didnt do the house install, and ive never been there, I dont know whats inside until I get there and have a look. All Ive seen is a photo of the board. Just didnt know if theres a calc to use from just the breaker sizes

 

[The Ghost]

How about looking at the manufacturers requirements for cable and MCB requirements?