3 phase breaker sizing

[Mike2007]

I have a load of 12kw on three phase supply, 20m of 10mm 4 core cable. What would the protective device be considering its already RCD covered?
Basiclly I am asking, when buying 3 phase breaker a"16 amp" 3 phase mcb would be 16amp per phase, not the unit as a whole??
Cheers

 

[holmslaw]

..

 

[flameport]

a load of 12kw on three phase supply
...
20m of 10mm 4 core cable
...
a"16 amp" 3 phase mcb

How have you arrived at those three items - as none of them appear to go together in any way?

 

[Mike2007]

a load of 12kw on three phase supply
...
20m of 10mm 4 core cable
...
a"16 amp" 3 phase mcb

How have you arrived at those three items - as none of them appear to go together in any way?

Flameport, yes my oversight. I think that might be 10amp breaker @ 4kw per phase? Nominal voltage of 415vac. Please set me straight.

 

[Paul_C]

I think that might be 10amp breaker

I = P / (1.73 x E)

So 12,000 / (1.73 x 415) = 16.71A approx. per phase, assuming the load is balanced.

A 20A breaker would be more appropriate.

 

[Mike2007]

I think that might be 10amp breaker

I = P / (1.73 x E)

So 12,000 / (1.73 x 415) = 16.71A approx. per phase, assuming the load is balanced.

A 20A breaker would be more appropriate.

What is the 1.73 factor Paul? My assumption was 12,000/415 =28.91A
This is getting more awkward each time I look at it? :/

 

[EFLImpudence]

Square root of 3.

 

[Paul_C]

What is the 1.73 factor Paul? My assumption was 12,000/415 =28.91A

That would be the current if it was a 12kW load with a simple 2-wire connection across 415V. But we're talking about a 3-phase load, which is where the 1.73 factor comes in - It's the square root of 3.

 

[Mike2007]

Forgot to ask, what instance for designing does a 4 pole breaker need to be used over a 3 pole breaker?
Thanks

 

[ban-all-sheds]

Are you sure you should be doing 3-phase work?

Looking at your body of posts you seem to be engaged in something substantial, and seem to be one of those people who thinks he can muddle through by asking questions about the things he realises he doesn't know.

Which is a very dangerous approach.

 

[flameport]

Forgot to ask, what instance for designing does a 4 pole breaker need to be used over a 3 pole breaker?

When there is a neutral which needs to be isolated with the three phases. Which is somewhat unlikely.

Still don't see why you have got 10mm cable.