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

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.

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.

psychicwarrior said:

the I2 <= 1.45 Iz ...  (R433.1.203)

Not seeing this 100 %.

From Regulation 433.1.203, I see:

I_n ≤ 0.9I_z (which is the line below)

Whereas

I_n ≤ 1.45I_z 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 (C_f=1) for devices listed in Regulation 433.1.201, but for re-wireable fuses to BS 3036,C_f 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.

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.

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 ?

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:

433.1.203

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

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).

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.