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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
    psychicwarrior said:
    the I2 <= 1.45 Iz ...  (R433.1.203)

    Not seeing this 100 %.

    From Regulation 433.1.203, I see:

    In ≤ 0.9Iz (which is the line below)

    Whereas

    In ≤ 1.45Iz relates to 433.1.1 (iii). Usually, 433.1.1 is transposed into a fusing factor Cf and used in the calculation shown in Appendix 4, usually only for BS 3036 re-wireable fuses (see Regulation 433.1.201).

    The fusing factor is ignored (Cf=1) for devices listed in Regulation 433.1.201, but for re-wireable fuses to BS 3036,Cf is given as 0.725. If a calculation is necessary for any reason for devices other than BS 3036, values of I2 for common OCPDs are provided in Table 4.1 of the IET Electrical Installation Design Guide, and for formula to use that table also provided in the publication.

  • 0 in reply to gkenyon

    433.1.203 states compliance is achieved with 433.1.1 (iii)  (i.e. with In <=1.45Iz ) for devices listed BS88 BS61009 etc   if  In <= 0.91Iz ...   I think it is to take care of the 20DegC tabulation (for buried) as opposed to 30Deg with others ?

  • 0 in reply to psychicwarrior
    psychicwarrior said:
    433.1.203

    Definitely 433.1.201 (page 89 of BS 7671:2018, or page 95 of BS 7671:2018+A2:2022)

  • 0 in reply to gkenyon

    I have replied to your other posts which has seemingly created two threads -  to confirm,  does RefD of the tabulated values already take care of the Cc 0.9 factor as your [review of my calculation] reply appears to state please ?    i.e. so long a Ib <= In <= Iz  (32 <= 63 <=68 in the example) then all is well (no 0.9 required etc).

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  • 0 in reply to gkenyon

    I have replied to your other posts which has seemingly created two threads -  to confirm,  does RefD of the tabulated values already take care of the Cc 0.9 factor as your [review of my calculation] reply appears to state please ?    i.e. so long a Ib <= In <= Iz  (32 <= 63 <=68 in the example) then all is well (no 0.9 required etc).

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