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It is not necessary to know Iz if method use is5.1 and 5.2

JohnE
Basic cable calculation
Checking Iz 
From BS7671

Method is used in items 5.1 and 5.2    Ib or In / Correction factors
 
It should be noted that the value of (It) appearing against the chosen cross-sectional area is not (Iz).
It is not necessary to know Iz where the size of conductor is chosen by this method.

But it is possible to get incorrect coordination with cable calculations that seem acceptable?


Ib   28A
In 32A
L 40m
Ca 0.94
Cg 0.7
 
Cable T&E
Clipped direct
 
Calculation
 
It ≥ Ib ÷ correction factors so:    It ≥  28 ÷ 0.94 x 0.7 =  42.5A
 
Table 4D2A Method C  6mm2  cable - 46A   It = 46
 
Volt drop 8.17V
 
So 6mm cabe looks OK
 
But if I do check Iz
 
Iz = It x Correction factors     46 x 0.94 x 0.7 = 30.26A
 
Ib ≤  In ≤ Iz      28 ≤ 32 ≤ 30.26     
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  • 0

    Thank you.

    For Ib
    Where the overcurrent device is intended to afford fault current protection only, In can be greater than Iz and I2 can be greater than 1.45 Iz. The protective device must be selected for compliance with I²t ≤ k²S²
    Form methods 5.1 and 5.2 
    It is not necessary to know Iz where the size of conductor is chosen by this method.
    So that suggests Iz is not needed for Ib or In ? I presume I'm reading that wrong
Reply
  • 0

    Thank you.

    For Ib
    Where the overcurrent device is intended to afford fault current protection only, In can be greater than Iz and I2 can be greater than 1.45 Iz. The protective device must be selected for compliance with I²t ≤ k²S²
    Form methods 5.1 and 5.2 
    It is not necessary to know Iz where the size of conductor is chosen by this method.
    So that suggests Iz is not needed for Ib or In ? I presume I'm reading that wrong
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