Circuit Breakers

[telectrix]

i have always found it far easier to use tables to obtain the rating of the cable according to the installation method. and instead of multiplying by a derating factor e.g. for ambient temp, divide to get the reduced ccc of a particular cable.

 

[Richard Burns]

Start from the beginning again.
In your example you have a 6kW load that will take (6000/230)=26A.
Therefore the design current Ib is 26A.
Therefore the next highest rating of circuit breaker is 32A so this is the In selected.

You then need to have a cable that can handle the current that could be drawn.
The design of circuit breakers and the tabulated current ratings of the various cables are coordinated such that a cable that has a current carrying capacity of 32A (according to the installation method listed in the relevant table) is protected by a 32A circuit breaker under all conditions of overload.

However the tables do not account for all factors that may affect the cables capacity to carry current.
The tables state that the ambient temperature is assumed to be 30°C and that the installation method is as stated in the table.

The limits are there to ensure the cable does not overheat, so if the current carrying capacity is 32A then the cable can take 32A on a continuous basis without overheating. This is the Iz.
However if the cable is buried in 100mm of insulation then the heat cannot leave the cable so easily and it would get hotter than expected, so if it were carrying 32A it could overheat.
Similarly if the ambient temperature is 35°C then it is harder to transfer the heat of the cable to this hotter air and so the cable could overheat again.

To take this into account the current carrying capacity of the cable is reduced by a correction factor e.g. the cable can take less current than the tabulated value; so because of the higher temperature it may only be able to take 29A.
This would then prevent this cable from being used with a 32A breaker, however a cable with a greater csa which may be tabulated as being able to carry, say, 35A would have the current carrying capacity reduced to 32A which would be OK.
Therefore you would be looking in the tables for a cable csa that is capable of carrying 35A (the It) because this value is going to be reduced by the ambient temperature to 32A (the Iz).

In your case the tabulated value that you are looking for is 43.7A because the ambient temperature and the insulation will reduce this value down to 32A (or more) which is the limit for the circuit breaker required.

The way the regulations calculate this value is to apply the correction factors as divisors to the In so that you get a higher value of current carrying capacity to look up in the tables, but this tabulated value is only to allow you to look up a cable that is suitable once the CCC has been reduced, it has no bearing on the actual current in the circuit.

 

[telectrix]

Richard. you have missed your vocation. as you explain things so well, you should have become a tutor.

 

[Pat H]

Thanks Richard a very clear explanation. I often find myself explaining it (not as well) as its easy for it to "appear" as if the correction factors are increasing the current in the cable rather than indicating a cable CSA size that will be ok with those correction factors at that current.

 

[DominicB]

Thanks guys for for your help that all makes perfect sense now, very much appreciated!