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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

    hmmm 

    right then, back to how I first did it (sort of ... lost track now) seeing as how there is growing opinion Cc 0.9 is indeed applicable to It (to comply with I2 <= 1.45 Iz which for stated devices will be  if In <= Iz)

    10mm2 SWA 70DegC  In (BS88) = 63A   It = 60A  (from RefmD - but use 10% 'clause' to that table )  Cc = 0.9  Cd = 1.04    (all others unity or not applicable )

    Iz = (It * 1.1) * Cc * Cd   =  Iz = (60 * 1.1 ) * 0.9 * 1.04  =  61.7      (without the 10%  its  only 56.1 !  )

    Ib <= In <= Iz  =  32 <= 63  <= 61.7 (56.1)  [not satisfied

    It's a shame for less than a couple of amps  (one might imagine it would be fine in reality even if not regulatory compliant); of course  one could go scraping that extra couple of amps in Cs [soil conditions] perhaps ... or cajoling the manufacturer  :-)

    some solutions

    - increase cable csa  (cost increase, but copper underutilised perhaps  though poss. less power losses)

    - decrease fuse rating  (if feasible, no cost increase  but reduction/loss in selectivity - if any, or no issue if not important)

    - design so no overload possible (if feasible 0.9 prob goes away etc)

Reply
  • 0

    hmmm 

    right then, back to how I first did it (sort of ... lost track now) seeing as how there is growing opinion Cc 0.9 is indeed applicable to It (to comply with I2 <= 1.45 Iz which for stated devices will be  if In <= Iz)

    10mm2 SWA 70DegC  In (BS88) = 63A   It = 60A  (from RefmD - but use 10% 'clause' to that table )  Cc = 0.9  Cd = 1.04    (all others unity or not applicable )

    Iz = (It * 1.1) * Cc * Cd   =  Iz = (60 * 1.1 ) * 0.9 * 1.04  =  61.7      (without the 10%  its  only 56.1 !  )

    Ib <= In <= Iz  =  32 <= 63  <= 61.7 (56.1)  [not satisfied

    It's a shame for less than a couple of amps  (one might imagine it would be fine in reality even if not regulatory compliant); of course  one could go scraping that extra couple of amps in Cs [soil conditions] perhaps ... or cajoling the manufacturer  :-)

    some solutions

    - increase cable csa  (cost increase, but copper underutilised perhaps  though poss. less power losses)

    - decrease fuse rating  (if feasible, no cost increase  but reduction/loss in selectivity - if any, or no issue if not important)

    - design so no overload possible (if feasible 0.9 prob goes away etc)

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