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the 'laws' of Ib <= In <= Iz and I2 <= 1.45 Iz re: direct buried cable

psychicwarrior

Good day all

For this example: Ib 32A, OCPD 63A BS88 ,  10mm2 SWA @ 70DegC  0.5m direct buried (other stuff inc. soil conditions all at unity/not applicable/as tabled and so on)

Iz = It * (correction factors)

68.6 = 60 * 1.1 * 1.04

the Ib <= In <= Iz ...

32 <= 63 <= 68   [tick] 

the I2 <= 1.45 Iz ...  (R433.1.203)

In <= 0.9 Iz

63 <= 61.7  [cross]

In the example scenario, 10mm2 SWA wont do.  A jump up to 16mm2 will be required.

Q1) is this non-rigorous assessment correct ?

Q2) the cable cost increase is not negligible for the sake of measly ~2 Amps,  is something missed or is there a more rigorous approach (factors, experience, in reality etc) to be able to use 10mm2 and still comply with Regs, or is it done with ?

Regards Habs

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  • 0

    Dear PW

    I put in that bit because we were beginning to go down a dead end route in the use of BS7671. For example consider an electric motor as a load. Should I consider the starting current as Ib or some other value like the FLC? The answer to that depends on the number of starts per hour and their duration, which could both be considerable. Whilst this kind of problem has become easier with the use of electronic controls, it has not "gone away". This means that the blanket formula is not always right or useful.

  • 0 in reply to David Stone

    Appreciated and thank you - same  to all who continue to contribute on here.  There is always something to learn !

    Habs

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  • 0 in reply to David Stone

    Appreciated and thank you - same  to all who continue to contribute on here.  There is always something to learn !

    Habs

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