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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

    Well all the stuff you assumed as unity for a start, and then there is the question of whether the load is on  all the time and how things heat and cool slowly. On the other hand the economics of the thermal losses in the cables over many years.

    It is a simplification, but not a bad one.

    Mke.

  • 0 in reply to mapj1

    thank you

    perhaps overload protection can be omitted if the 61A Iz is not ever  exceeded

    just for talking around the subject ... what's the view on omitting overload protection if the OCPDs at the end of a distrib circuit are <= Iz   e.g. in a board with no more spare ways;  would that be regulatory compliant and and acceptable reason for overload omission ?

    32A Ib (highest MCB perhaps) was used to lead on to selectivity with the 63A fuse -  likely to give  good selectivity (if not full) in the circumstances  (length of SWA etc).  with overload omitted then more options open up.

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  • 0 in reply to mapj1

    thank you

    perhaps overload protection can be omitted if the 61A Iz is not ever  exceeded

    just for talking around the subject ... what's the view on omitting overload protection if the OCPDs at the end of a distrib circuit are <= Iz   e.g. in a board with no more spare ways;  would that be regulatory compliant and and acceptable reason for overload omission ?

    32A Ib (highest MCB perhaps) was used to lead on to selectivity with the 63A fuse -  likely to give  good selectivity (if not full) in the circumstances  (length of SWA etc).  with overload omitted then more options open up.

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