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Minimum size of CPC

bakey1959

When using the Adiabatic equation, only to calculate the minimum size CPC, I am thinking that using the fault currents given in Appendix 3 of BS7671 rather than that calculated (UO/ZS) would give a more realistic outcome.

For example: A D6 circuit breaker is protecting a circuit where the calculated fault current is 328A. The table associated with Fig 3A6 in Appendix 3 of BS7671 shows that an overcurrent of 120A will operate the 6A breaker in 0.4s. Using these values and a K value of 115 results in a minimum CPC of 0.66mm2 (1.0mm2), whereas if you use the 328A the calculation results in a minimum CPC of 1.80mm2 (2.5mm2).

I cannot see a problem with using the lower value, but would appreciate confirmation and guidance from others.

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    I guess you could consider if the cable is running at a temperature less than 70c, in which case the k value could be more than 115. If the formula is plotted it provides a straight line with k at about 163 at 0C and 115 at 70C. Run at a conservative 50C would give k at about 129.

    The question of minimum csa of cpc to meet thermal constraints is a very common one on the 2396 Design course. There would be no expectation to adjust the k value, but I guess the various methods indicate some calculations in installation work are what Mapj referred to as measuring with a micrometer and cutting with an axe!

    Using the 5900 figure that gives a csa of about 0.59mm2. As a matter of interest, you would be marked correct in the 2396 Design course using the simple method and the 1.38KA and a fixed time to disconnection of 0.1s, which would lead to a minimum csa of  around 3.8mm2. 


  • 0 in reply to lyledunn

    It is rare to see a 1mm2 circuit protected by a 6A breaker running at anything like 70C unless the ambient is unusually hot. Possible in parts of the middle east  - so if 1,1mm seems a squeeze remember the assumption about cable core temp before fault is probably most pessimistic.

    Mike.

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  • 0 in reply to lyledunn

    It is rare to see a 1mm2 circuit protected by a 6A breaker running at anything like 70C unless the ambient is unusually hot. Possible in parts of the middle east  - so if 1,1mm seems a squeeze remember the assumption about cable core temp before fault is probably most pessimistic.

    Mike.

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