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!

My mistake, I was thinking it was one size smaller. Best answer is probably 4.0 singles in mini-trunking then as easier to terminate in tight corners than 10.0 T&E.

Ooh, I see TLC also sell 4.0 in NYY-J as well as triple flex to 3183Y, either might do even better.

Do you think the rest of the logic is OK?

If the load current is 50 A, then according to Table 4D1A for reference method B, the minimum conductor size is 10 mm2. However, you also need to comply with regulation 433.1.1, which states the conditions for the overload protection device and the conductor ratings. Alternatively, you may not need an overload protection device if the load or supply characteristics prevent overload currents, as per regulation 433.3.1 (ii)

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)

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)

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".

Whatever you want to call it, BS 7671 434.3 would apply wherever you have a conductor that's not protected by any of the installation's protective devices but relies instead on the distributor's devices.

   - Andy.

Implying what?

Chris Coffin said:

Implying what?

I would suspect that AJJewsbury  is considering specifically indent (iv) of Regulation 434.3.

The implications being, if you are relying on the DNO cutout:

1. You have to comply with the distributor's requirements (which may require a minimum csa) ... otherwise they may not agree "that such a device affords protection to the part of the installation between the origin and the main distribution point of the installation where further protection against fault current is provided"; and
2. The wiring must be selected and erected to minimize the risks of faults, fire and danger to persons.

All too difficult. Am now considering  supplying the HP from the existing main distribution board - which might overload my Victron Multiplus inverter in a power cut - or to avoid that, its AC 2 Out terminals which get disconnected on loss of mains. Which will mean in either case that ordinary 4.0 T&E can be used.

Graham's suspicion is indeed correct. The implication that if the DNO's generally ask for 25mm² but might sometimes tolerate 16mm², then 4mm² might be pushing things a bit... It's only up to the first protective device of course - after that your 4mm² will likely be fine. Could you not take it from an outgoing way in the 4-module enclosure (perhaps with a 'width upgrade') - in effect making it a 'non essential' DB?

   - Andy.

No, it is full (2 modules of SPD and 2 modules of Estron export/import meter. No space to expand it.

Every time I think I have got to the answer someone pulls another consideration out of the woodwork citing some new arcane aspect of the regs.

Equally I am not sure it is helpful for the OSG to say 3 x 4.0 is (just) OK on an 80 A fuse if it is subject to DNO approval anyway.

Using my professional judgement I would say the likelihood of <1 m of cable surface-mounted >1.2 m from the floor in a little-trafficked larder area getting damaged to the point of blowing the 80A fuse is very small, and the likelihood of a short circuit overheating the cable and not blowing the fuse even less. In any case it does not matter if the cable suffers thermal damage because the repair would be to replace the entire short length.

So I am grateful for everyone's contributions but equally I am happy it has led me to an entirely different solution to the underlying problem!

Equally I am not sure it is helpful for the OSG to say 3 x 4.0 is (just) OK on an 80 A fuse if it is subject to DNO approval anyway.

To be fair what the OSG says would be right in 99.9% of cases (well 100% of cases for circuits within the installation itself) - it's just because you're depending on a device that's outside of the BS 7671 installation and under someone else's control (which I suspect the OSG deals with explicitly elsewhere - possibly under the heading of meter tails, although as already discussed that's probably not a helpful description in this unusual case). The DNOs always want the right to change things - the rating or indeed type of fuse (or maybe even some kind of circuit breaker in the future, as is done in many other parts of the world), or reconfigure their network (e.g. feed your bit of road from a different substation as loads increase) which may well alter Ze - and they have a responsibility to ensure (AFARP) that their changes don't undermine safety. That's a lot easier to do if they know that everyone has (say) <3m of 16 or 25mm² - one set of worst-case calculations back at the office would cover everyone - life would get a lot harder for them if they had to account for odd bits of smaller conductors hidden all over the place. The DNOs have already changed the type of fuses they use - from BS 1361 to BS 88-3 - and their characteristics aren't entirely identical (even some of the ratings differ) - in a few edge cases what just scraped though with a BS 1361 fuse could be just a fail with a BS 88-3 one. It's in the DNO's interest to design-out the awkward cases before they happen.

Every time I think I have got to the answer someone pulls another consideration out of the woodwork citing some new arcane aspect of the regs.

It's not that new - the current form of words seems to go back at least as far as the beginning of the 17th Ed (2008) and the general gist probably goes back even further.

  - Andy.

Thanks. Perhaps instead of writing "citing ... new..." I should have written "newly citing..."

But as I no longer draft contract terms for a living perhaps I am excused for once.