Three-phase circuit breaker size calculation help

[ta-electrical]

Hi, i'm a little confused about the way to calculate the amp of the circuit breaker for a three-phase system.

If i have a power of 11.5 kW and voltage of 400V, what size circuit breaker is needed for three-phase system.

My calculation was 11500/(1.732x400) = 16.6 Amps per phase ... 16.6 x 3 = 49.8 Amps, so i need a 50 Amp breaker.

Please, tell me if i worked this out correctly, if not show me how it should be calculated and state the required circuit breaker size i need.

Thanks in advance.

 

[Sparky Ninja]

why did you multiply 16.6 x 3 again?

a three phase circuit breaker will have 3 x MCB's linked at the switch (most likely)

 

[ta-electrical]

I would have thought the 16.6 is in amps per phase so if there are a total of three phases you have to multiply it by three ? (This is what was confusing me)

There are not going to be individual circuit breakers, just the one.

 

[Sparky Ninja]

One of the biggest benefits of three phase is that we can lower the circuit demand of higher rated equipment.

 

[ta-electrical]

So, your saying a three-phase circuit breaker say 32 amp is actually 3x32amp MCBs fitted together?

 

[Sparky Ninja]

So, your saying a three-phase circuit breaker say 32 amp is actually 3x32amp MCBs fitted together?

A basic 3 phase MCB is 3 single MCB's, formed together with a link to ensure isolation of all Live conductors occurs. More specific, higher rated MCB's will be specially manufactured.

 

[ta-electrical]

Okay, i understand but, just to make sure your saying in reference to my calculation in my original post, a 20 Amp breaker would be enough?

 

[Sparky Ninja]

Correct,

11500w on single phase = 11500/230 = 50Amp
11500w on three phase = 11500/√3 x 400 = 16.60Amps, so a 20Amp MCB should suffice (assuming no inrush or anything to be considered).

 

[Knobhead]

Is this for a motor? If so you may have to consider a 32A type C and cable accordingly.

 

[ta-electrical]

It's an air compressor in a workshop, it wasn't a balanced load i.e. it required a neutral as well.
Origanally, i had used a 32 Amp type C but, it tripped instantly afterwards, i was told it requires a 50Amp breaker type C, which worked.
I'm not sure how 50Amp was calculated, the only way that made sense to me was to multiply by 3 (refer to original post), and this is where my confusion is.

Thanks for the replys, i hope you can clear my mind on this one.

 

[Sparky Ninja]

You sure it's power isn't stated as 11500w per phase?

 

[ta-electrical]

It is a very big compressor and it is very likely you may be right about it being per phase.
I will make sure to check it again to confirm, thanks.

 

[malcolmsanford]

ta I have to assume that as it's a large compressor it is star-delta connected, When you say you fitted a 32amp C breaker and it tripped have you connected it right ie star-delta, because if you haven't and connected to the wrong contactor so it is delta-star it would easily trip.

If you have connected it correctly is the star-delta timer an adjustable one, again if it is then you could be running on star start up to long, so look at that, or even look at a faulty timer.

Finally as widdler suggest is it 11.5kw per phase which in pit terminology make it a FBC ............... F big compressor.

 

[telectrix]

i can't see the motor being 11.5kw /phase. the fact that a 32A C is tripping leads me to believe that the motor is goosed.

 

[ta-electrical]

@malcolm, yes it is star-delta and i would assume it is connected correctly otherwise the 50Amp breaker would not have worked either. I haven't confirmed whether it is 11kW per phase but, will do so soon as well as checking the timer.

@telectrix, sorry can you elaborate on what you meant when you said 'goosed'? Thanks.