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

psychicwarrior said:

Iz = It * (correction factors)

Yes,

I_z = I_tC_gC_aC_sC_dC_iC_c

but surely this includes the "buried cables" factor Cc, ... we don't see 0.9 in your calculation at that stage ... BUT I think you are using Ref Method D (buried in the ground) because we have 60 A for 10 Sq mm in Table 4D4A which already takes into account C_c , for the tabulated current-carrying capacity, so you can ignore BOTH C_c AND the line for Reg 433.1.203.

So, provided your other correction factors you assumed are OK (which you don't list separately), so I can't check.

Iz = It * (correction factors)

68.6 = 60 * 1.1 * 1.04

note: 60 already takes into account Cc, so I can ignore Reg 433.1.203

the Ib <= In <= Iz ...

32 <= 63 <= 68   [tick] 

the I2 <= 1.45 Iz ...  (if necessary, usually only for BS 3036, see R433.1.1(iii) and R433.1.201)

In <= 0.9 Iz (ignore because I used Ref Method D)

63 <= 61.7  [cross] (R433.1.203 need not be applied. remove, see above as we used Ref Method D)

So, PROVIDED your other correction factors are OK, looks like the 10 sq mm woulddo the trick.

The problem with Appendix 4 is you can't then go back and directly calculate I_z to use in the Regs, from the value of I_t you look up in the Tables in Appendix 4. I understand this comes up in relation to questions submitted to the IET's Helpline.

The latest EIDG and GN 6, with the following statement to try and help, but I think the overall issue is that, after reading all of the introductory material to Appendix 4, one perhaps doesn't know where one is up to in the design process, to be honest:

It only equals I_z in cases where all C factors are 1 (or do not need to be taken into account). Where any of the C factors are less than 1, I_t is necessarily greater than I_z.

It is important to understand the relationship between I_t and I_z when comparing the calculation methods shown in Appendix 4 with the relevant normative regulations in Chapters 43, 53 and 55, and in Section 712 so that C factors are, in error, neither cancelled, nor applied twice.

And so, in the original calculation, after selecting Ref Method D, the correction factor C_c was [inadvertently] applied twice by back-tracking to Reg 444.1.203 after it had already been taken into account in Column for Ref Method D in Table 4D4A.

hello Graham ermmm :-)  is all that effectively backing up your earlier reply that the 0.9 factor (for the 20Deg basis) is already taken care of in RefD of the  tabulated values ?

just to confirm , as Ive looked at your other reply, that the 'Ib of 32 <= In of 63 <= Iz of 68' confirms that (as you mention too presuming all other factors are correct)  10mm2 would be fine ?  

Thank you

psychicwarrior said:

s all that effectively backing up your earlier reply that the 0.9 factor (for the 20Deg basis) is already taken care of in RefD of the  tabulated values ?

Yes ... but would also be valid in other cases of correction factor where we might be tempted to go back to the Regs after getting a value from the cable tables in Appendix 4, e.g. factor for thermal insulation when these are already taken into account such as Methods 100#, 101#,102# and 103# in Table 4D5.

psychicwarrior said:

s all that effectively backing up your earlier reply that the 0.9 factor (for the 20Deg basis) is already taken care of in RefD of the  tabulated values ?

Yes ... but would also be valid in other cases of correction factor where we might be tempted to go back to the Regs after getting a value from the cable tables in Appendix 4, e.g. factor for thermal insulation when these are already taken into account such as Methods 100#, 101#,102# and 103# in Table 4D5.