The above discussion almost but not quite addresses my own question. What is the minimum CSA I need if a sub-main of length < 1m is to be protected by my existing cut-out fuse - I assume to BS88 - and confirmed by the DNO to be rated at 80A?

Looking at the gG tables above, the 5s disconnection time for this fuse is achieved with a current of 430A. Presumably that is worst case, various manufacturer's data gives other results e.g. mersen give 300A.

The graphical method, looking at where the adiabatic curves cross the fuse characteristics, then shows for this "time to blow" I need somewhat under 10 sq mm conductors. Is that correct?

Also, can someone please explain where the figure for Zs of 0.27 came from in the original calcs above? The tables I have found e.g. these https://www.dungannonelectrical.co.uk/dun1-shop/pdf/loop-impedance.pdf show for 5 seconds a value of 0.425 ohms for the original 100A fuse and 0.525 ohms for my 80A case leading to rather different results. And why is a figure of 1 second then chosen (seemingly arbitrarily) to put in the adiabatic equation?

Hoping someone can enlighten me, I am more at home with electronics!

Proposed load is a heat pump drawing 23.3A abs max so should be fine on a 32A Type C RCBO, which will also provide earth leakage protection for the load as well as overload protection for the cable.

Short circuit protection is the remaining issue I wanted to sort out, it seems the 80A cut-out will be sufficient provided all 3 cores of the sub-main are 4.0.

Thanks for yr help.

a sub-main of length < 1m is to be protected by my existing cut-out fuse

:

NYY-J & T&E

How do you propose to connect this cable? Meters, REC style isolators and normal service connector (Henley) blocks have separate entries for each conductor - hence single core sheathed cables are normally used to avoid having basic insulation exposed.

There's also the issue of 434.3 (iv) - the DNO's fuse isn't part of the BS 7671 installation and isn't really under the customer's (or the customer's electrician's) control - i.e. the DNO are technically at liberty to change it for another type at some point in the future. So you need the DNO's agreement to demonstrate compliance with BS 7671. DNO policies vary, but many these days won't guarantee any particular performance of their cut-out fuse, but rather set conditions that if complied with will be satisfactory - typically that means 25mm² live tails and a 16mm² earth (some accept 16mm² for existing installs on 60A or 80A fuses) and a length limit (often 3m ish).

    - Andy.

According to a well-known book on EICR coding, a FI (ESQCR) occurs when the cross-sectional area of the meter tails between the meter and the consumer unit is below the 25mm minimum requirement.

I already have a 4-way unit with SPD and export meter, the new cable will be terminated within this enclosure on its load side busbars where there are spare ways.

Previously I added a zappi to the installation and it was inspected under a Building Notice without any reservations. The ticket is still open so it can be inspected again after the HP is installed.

The DNO visited last year following the HP application and confirmed that their fuse is 80A and they are therefore happy with the main 16 sq mm meter tails.  As these are routed via an unknown path through a 600mm stone wall I do not plan to disturb them. For this reason they will not upgrade the supply to 100A even if I were to want it, and if they were to do it unilaterally it would compromise their protection.

I cannot believe there is any requirement to upgrade meter tails to 25 sq mm retrospectively as it would affect literally millions of households.

On the load side of what sounds like a small CU, you have no issues anyway, as you presumably will fit the 32A RCBO you mentioned earlier,. From there carry on in 4mm cable of any type you like really.

Mike

I cannot believe there is any requirement to upgrade meter tails to 25 sq mm retrospectively

I didn't intend to suggest there was, merely that any new work (whether in a new installation or as an addition to an existing one) should comply with current requirements.

   - Andy.

The guidance suggests assigning an FI code to meter tails that are less than 25 mm. However, an FI code with ESQCR in brackets indicates a potential non-compliance with the ESQCR regulations and a possible hazard. Therefore, prior to assigning an FI code or any other code, the DNO and the relevant standards should be consulted. I initially misunderstood that you were referring to the size of the consumer meter tails and not the final circuit. When it comes to consumer meter tails, for up to 80A, use 16mm tails and for up to 100A, use 25mm tails (Domestic premises) New installations may require 25mm cables to meet the modern needs, especially for Electric Vehicles. 

Neither main meter tails nor a final circuit. Original Q was "What is the minimum CSA I need if a sub-main of length < 1m is to be protected by my existing cut-out fuse". It is to be teed off the load side busbars of item 5 below. From the above discussion it would seem that 3 x 4.0 sq mm is necessary but sufficient.

1. 80A DNO fuse
2. Meter
3. 600mm stone wall
4. REC isolator
5. 4-module  enclosure with SPD and multifunction meter
6. Main CU (via changeover switch to/from ESSS)

Chris Coffin said:

The graphical method, looking at where the adiabatic curves cross the fuse characteristics, then shows for this "time to blow" I need somewhat under 10 sq mm conductors. Is that correct?

Yes BUT depends on the actual prospective fault current at the origin. You may be able to lower the rating by using a non-adiabatic plot for the particular cable or wiring system arrangement.

Chris Coffin said:

Looking at the gG tables above, the 5s disconnection time for this fuse is achieved with a current of 430A. Presumably that is worst case, various manufacturer's data gives other results e.g. mersen give 300A.

Yes, it's based on the requirements of the standard, or "worst case seen from certain manufacturers" - meaning in the real world, things may get "over-engineered".

AJJewsbury said:

(whether in a new installation or as an addition to an existing one)

That's also an interesting discussion ... and I would say not always true of existing installations.

To cite an extreme example, if one was asked to provide an additional socket-outlet in premises with lighting protection system to BS 6651, and there were no SPDs, it would not be practicable to re-assess the whole installation against BS EN 62305 and provide SPDs, in order to simply fit a socket-outlet (regardless of whether the premises was of a type that would now require SPDs under 443.4.1). Luckily, we are talking about Construction Work, as defined by CDM Regulations, so the Client, or someone appointed by them as Principal Designer, would (for most of those types of premises) be involved in the discussion about what should and should not be done, and any relevant risk assessments, and the LPS/SPD issue would be one of ongoing review and planning for future works at an appropriate time.

AJJewsbury said:

(whether in a new installation or as an addition to an existing one)

That's also an interesting discussion ... and I would say not always true of existing installations.

To cite an extreme example, if one was asked to provide an additional socket-outlet in premises with lighting protection system to BS 6651, and there were no SPDs, it would not be practicable to re-assess the whole installation against BS EN 62305 and provide SPDs, in order to simply fit a socket-outlet (regardless of whether the premises was of a type that would now require SPDs under 443.4.1). Luckily, we are talking about Construction Work, as defined by CDM Regulations, so the Client, or someone appointed by them as Principal Designer, would (for most of those types of premises) be involved in the discussion about what should and should not be done, and any relevant risk assessments, and the LPS/SPD issue would be one of ongoing review and planning for future works at an appropriate time.