Question about sub-main sizing

My TNC-S supply enters in the attached garage, where the tails are split to a CU (in the garage, feeding: the kitchen, cooker, utility and a few other bits) and a 60A switched fuse which feeds a 16mm2 sub-main. The sub-main is about 20m long and connects to an ancient fuse board that supplies the house radial and ring outlets. A second fuse board in the same location is connected to one of the MCBs in the CU via a 2.5mm2 cable and supplies power to most of the house lights. My plan *has* been to fit a new CU close to the garage and extend all the cables back to it, but that requires a lot of disruption. I'm now wondering whether I should simply replace the two wylex boards with a single new (RCBO) CU in their present position and feed it from the sub-main. From some quick checks the existing 16mm2 will be OK at 20m for volts drop if I protect it with a 50A MCB. However, from a quick look (more poking around required) the only earth connection seems to be the CPC in the 16mm2 T&E so that will need improving and the disruption involved won't be much less than extending all the circuits, although it will be less work. What do the relevant panel experts think?

Reply to
nothanks
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Yup, that sounds like a less disruptive option.

Since its a PME head end, the sub main will need to extend the main equipotential zone to the new CU. So its earth will not only need to meet the requirements of being a functional earth, but also a main bonding conductor between supply and CU. That means you need 10mm^2 of copper or equal or better amount of copper equivalent via some other means.

You could either run a separate main bonding conductor (10mm^2 green/yellow single), or upgrade the T&E to say a 3 core SWA, and parallel the armour with one of the cores.

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Reply to
John Rumm

... snipped Thanks John, that was what I thought. The existing 16mm2 T&E runs between joists (mostly just resting on the ceiling, rather than being clipped) from the garage to the fuse boards so it's going to be a (necessary) pain to lift carpets and boards to run the green/yellow, but it would be more of a pain to replace it with SWA

- why would I consider doing that?

I haven't been able to find the rules for sub-main protection. Is it permissible to run it from a 50A DP MCB in the garage CU, or does have to be from a separate DP fused/switch from a Henley block?

Reply to
nothanks

I've not got tables to hand but check your earth loop impedance against disconnection times for the 40A mcb

Reply to
Cynic

The sub-main earth loop impedance (20m of 10mm2 and 20m of 16mm2) should be about 60mOhm (37mOhm + 23mOhm) so about 0.4 ohms total (TN-C-S) - about 600A fault current. A type C MCB needs 10x rated current, so 500A for a 50A MCB. Unless I've boobed it should be OK.

Reply to
nothanks

You could use the T&E as a draw wire to pull in the single SWA that would give you both power, earthing, and EQ bonding.

No you can run from a MCB without any problem. Its commonly done.

Reply to
John Rumm

Yes, but it's not going to be simple to draw SWA through joist cut-outs. Why should I consider replacing the existing T&E with SWA rather than simply adding a 10mm2 or 16mm2 green/yellow?

Is SP permissible, or does it need to be DP?

Reply to
nothanks

You can do whichever is easiest and meets the required spec. Sometimes adding a wire can be more difficult that pulling in a new one. However much will depend on the detail.

MCBs are typically SP, but you will have DP switching at the destination CU, and also at the source CU via the main switches, so the switching on the circuit protection does not matter as such so long as it can break the circuit in the even of a fault.

Reply to
John Rumm

One problem with using an MCB is you may get little if any discrimination between that, and the downstream consumer unit MCBs in the event of a fault. So if something shorts out on a final circuit, you are quite likely to find the upstream

40/50A MCB trips, and you lose power on all the final circuits on that submain. A BS1361 HRC cartridge fuse used to be a better bet, but they have vanished (spare fuses still available, but new fuse holders are not).
Reply to
Andrew Gabriel

I was planning to use a 50A type C MCB to protect the sub-main and 16A type B RCBOs in the new CU - this should give discrimination. The reason for using 16A, rather than 32A, is that I have several radials and one ring with spurs. All the high current loads are fed from the other CU, in the garage.

Reply to
nothanks

Yup, I have noticed that CU MCB based fuse carries are much harder to find these days.

If you can size your sub main MCB a couple of ratings higher than the highest downstream one, or, fit a type C at the sub main head end, you might be able to mitigate the discrimination problems.

Having said that there are no guarantees, ISTR Adam mentioning a case where an appliance fault took out the main incomer fuse, leaving its plug fuse un-blown and the circuit MCB un-tripped!

Reply to
John Rumm

You might have boobed it - twice, if I am not mistaken!

  1. You are using 240V not 230V for your fault current. (Max Ze for a 50A C type is 0.37ohm and remember that is when the cables are at 40 deg)
  2. Your resistance calculations should be 16mm earth and 16mm live (unless you wish to chop out the 6mm cpc of the 16mm T&E and not bother using it)

Either way you are over the 0.37 ohms allowed.

It might be better to swap the head end with some thing like this

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Reply to
ARW

You are close John.

Two cases where this sort of thing has happened.

  1. 1970s bungalow. Tried to slacken off the cooker switch from the back box and the live cable fell out the terminal and shorted to earth. This took out the house fuse but left the 30A fuse wire intact. I'll put that down to an old fuse.

  1. Flats with a 16mm T&E submain supply to each flat. A fault[1] on the lighting circuit took out the 6A lighting MCB and the 63A head end MCB. In this case there was no 10mm extra earth cable from the head end to the flat as no supplementary bonding was needed (no gas and a plastic water supply) and it would be very similar to the OPs set up. The 80A BS1361 fuse in the suppliers cut out was OK.

[1] It was LE short in a new light fitting the tenant had fitted.
Reply to
ARW

I think I must be missing something here... where does the 20m of 10mm^2 come into this?

Looking at just the submain as it currently stands:

16mm^2 T&E has a round trip resistance live + CPC of 4.23 mOhms/meter [1], so that is a contribution of 85 mOhms.

If we are going with the standard estimate of 0.35 Ohms for the supply impedance (in the absence of a measurement), that gives a total Zs of

0.435 Ohms. So just inside the limit for a 50A Type C using the new cMin adjusted max ELI factors[2].

So a prospective fault current of 230 / 0.435 = 525A, adequate to trip in the instant part of the curve.

Now if you increase the CPC size to 16mm^2 with the addition of a 10mm^2 single, then the situation improves further?

[1]
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[2]
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Reply to
John Rumm

Of course the second boobie makes things better:-)

And I got the resistance of 16mm plus 16mm wrong in my calcs.

Reply to
ARW

Sadly, it was me that was missing something - I'd forgotten about the extg CPC being in parallel with the new single. I must be getting old.

Reply to
nothanks

That was the second point I meant to ask you about (I was tired yesterday as I had had a weekend off work).

That's more about diversity than discrimination.

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is worth a read if you have not already seen it.

Reply to
ARW

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Reply to
John Rumm

Ah, ok, sorry I though I had missed and extra cable somewhere rather than being the extra CPC/Bonding conduction for the submain

Reply to
John Rumm

:-)

Reply to
nothanks

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