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!

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!

I have this table for maximum measured values (80%) of maximum permitted.BS 1361 or 1362, 100 Amp, 5 seconds 0.27 Ohms

Thanks, I wonder why it is not the same.

Meanwhile I may have found the answer to my original Q. My copy of the 17th Ed OSG has in Table 2B on p101 the smallest protective conductor allowed off an 80A fuse to BS88 is 4.0 sq mm, so the smallest permitted T&E cable is 6.0.

Also the max allowable Zs is 0.22 ohms which I think is OK. By observation the voltage dip on a 50A load is <2.5V which is about 50 mohm, and the length of cable involved is negligible. (This squares with the supply cable which is about 20m of 35 sq mm from the street, so 25 mohm according to Table 6E2.)

220 mohm worst-case would give a PSCC over 1000A so a time to blow of 0.2 secs, and the graphical method shows this is actually OK for 4.0 sq mm cable as well as 6.0. And the real PSCC looks as though it might be >4x this w/c figure so off the graph.

Now to get rid of the column of "helpful" IET drivel which has inserted itself to the right of the discussion screen!

Hi Chris, I’ve never come across a 6mm T&E cable with a 4mm cpc as a standard cable. 6mm T&E cable has a 2.5mm cpc, and a 10mm T&E cable has a 4mm cpc.  

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