Circuit Breaker Sizing

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cshaopin

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How do I size a residual current circuit breaker?

I have an incoming power supply of 400VAC 5KW 50Hz 3 phase. I use transformer to step down to 230VAC.

Its to power up fluorescent lights (total 360W), Floodlights(2kW), Air-Cons, Electric Chain Hoise ...etc. A total not above 5kW.

How do I select a proper RCCB at the incoming supply? Should I total up the currents draw by all the electrical components, then size the RCCB accordingly? or roughly by using I = P/V = 5000/230 = 21.74Amps, meaning that I can choose the next higher range of RCCB available in the market, for example, a 40A RCCB?
 
Firstly follow the relevant rules/regulations for the country that you are in, sounds like it may be the UK?
If the instalation is offshore, then in general the regs of the nation who own the facility would be a guide, but may not be obligatory.

The RCBO must be able to handle the maximum expected current, this may be the total of all connected loads, it may be less, depending on what if any diverseity is allowable.

Provided that the subcircuits are correctly protected by correctly rated fuses or MCBs then the RCBO can be oversized.
Even for a fairly small instalation like this, is it wise to use an RCBO to protect the whole instalation?
Use of a transformer to obtain to 230 volts from 400 is also unusual, most 400 volt supplies are 400 volts phase to phase, and would give the required 230 volts from any phase to neutral.
It is course possible that you have a non standard supply, either 400 volt delta, or 400/690 volts Y in which case a transformer will be required.
 
Broadgage, thanks for your reply.
Yes the installation is offshore. I'm aware that the RCBO must be able to handle the max expected current. It can be oversized because it's function is to detect any imbalanced phase current caused by leakage.

If you're saying by connecting any phase to the neutral will give the required 230V, lets say I have 3 live wire with 1 neutral, how should I arrange those lightings, floodlights and all the electrical components so that I can have a balanced load on all 3 live wires?

For example lets say live wire L1 L2 L3 and neutral wire is N1.

I have fluorescent lights E1 to E5 each 36W. Floodlights E6 to E8 each 400W. AirCon 1.25HP. How do I distribute to 3 live wires to achieve a balance load? How is it calculated?
 
Presuming that you have a UK type 3 phase, 4 wire supply, then phase to phase will be 400 volts nominal and phase to neutral will be 230 volts nominal.

Single phase loads such as you describe should ideally be balanced between the three phases,if however this is small addition to a large existing installation, then it should be ok to put then all one one phase.

If this installation is under UK control, though offshore, then the 17th edition of the UK wireing regs, should be consulted.
If the installation is anything to do with oil or gas, as many offshore installations are, then additional stringent requirements may apply, and appropiate advice be taken.
 
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