Dedicated Circuit - Question About Consumer Units

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I’m looking for a bit of help from you electrical experts in the UK!
We’re busy with a big extension which is heading into the final stages. I’m taking advantage of this to get a dedicated circuit put in for my 500 series kit.

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Having shared this diagram (an old forum favourite!) with my builders, they were questioning the need to have a separate consumer unit when we’ll be installing a brand new one for the house, and those that meet current regs are, well, pretty good at isolating different circuits from each other.
Can anyone explain why (and if indeed I should) still need a second, separate consumer unit?

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Yes. A separate CU is required. This circuit is then totally separate from the other house wiring and associated noise.

DG…

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I would side with your Electrician. I cannot see the point of the additional CU…

But… YMMV… :crazy_face:

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The sparks who did mine was very much up to speed on doing these. He just nodded at the diagram and I got a new Henly/ CU on the separate cable running from the meter. Given the bother it was fitting things in place, I am sure he would have mentioned it if he thought leaving out the CU would have no impact.

Otoh, he argued that 10mm cable is ideal - as long as any links to other sockets are also in 10mm and the sockets are designed for it. Without those provisos, he reckoned we were better off with 2 runs of 6mm cable. As I understand it, this is far from being a universal view.

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You don’t need a seperate consumer unit for that setup, as you say a modern has isolation for each circuit… which is its point.
If you needed to have a separate supply in an outbuilding etc, you would likely consider a separate CU in that building…. and in that case you would likely separate the earths. The above diagram is loosely relying on any resistive drop between CUs to improve noise separation… which is going to be minimal… not least because the earths are joined.

The weak point in terms of noise isolation in circuits is the common neutral / earth. For low noise applications, you might consider a TT setup, which would separate or localise the earths (rather than using the supply Earth ). Here you might decide to use separate earths for different circuits, in this case you would clearly need seperate CUs. If you have a TNC based earth … you are limited in the improvements you can provide until you switch over to TT (though not all locations can use TT… it depends on ground soil impedance at your property)

I have gone TT, but currently using a single CU. TT earthing made a big difference on mains noise, but I use a rural overland supply, and that sounded better… for me, I believe significantly so.
I also use ring wiring, as that provides a lower impedance (better for hifi) compared to the same size radial wiring.

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As already said, you don’t need a separate consumer unit but from both my own and others trials it does sound better than sharing with the main house consumer unit.
Your electrician must have also questioned separate 10mm circuits to run pieces of hifi kit, not needed really but they sure do make the system sound better.

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There are also good reasons around having its own earth that goes back to the main earth block. The cost difference is also less than a small cable upgrade, so if you are having work done, it’s a no brainer in my view.

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Having tried various dedicated mains circuit arrangements, my experience is that those that use a separate consumer unit work very well, and those that just use a spare way on the main house CU are a waste of time.
This is in line with the advice I received from Naim, who also said that splitting the meter tails to a separate CU was the best way.

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Chris, intrigued as to how Naim really reached their conclusion; do you know?
A separate circuit is no doubt preferable, but much depends on perhaps locality (no heavy industry) and a relatively clean supply locally. IIUC a small(ish) number of domestic properties share a three phase, with only one going to each.
Historically mine has a three phase supply (long ago under floor heating existed [??] - isolated ) and two phases are in use, by coincidence in a long house, one phase does the kitchen appliances and the other includes a radial to two different music systems.
Old style Crabtree single sockets on a radial. I did all the wiring long ago and have the certificate.

What concerns me, is not the separate consumer unit, which is perhaps logical, rather the gauge of wiring. Any UK 13amp socket, single or double, is designed to accept two 2.5mm radial ring cables + subject to wiring spec restrictions, a spur. That totals 7.5mm.
If 10mm is used, a henley block is necessary in a daisy chain manner, to ensure each socket has the same cable size. If I was to do my dedicated circuit again today - read the original and now long forgotten, details two decades ago on the old forum and went for dedicated radial, without taking into account anything about cable size - so I would use 4mm radial to each 13amp socket, using quality unswitched sockets. I know one ex-sparky who was well aware of the quality of the old Crabtree sockets.

I rather wonder if the sparky who the op @GavinB has consulted, has used the question about two consumer units, rather than raise the issue of 10mm into a 13amp socket. After doing lots of electrical installations, 10mm into a socket is a tall order, which really requires rear entry and even then it is not easy.
I’m not disputing the sound gains, which may or not be noticeable - simply that asking a sparky to achieve the next to impossible cable wise will likely result in v mixed reactions.

Realistically or not - I could run as a test, three different cables size cables from 4mm/6mm/10mm without any great difficulty; save for the fact that it would be a physical challenge to create the same cable size to each and every one of ten sockets. For every installation, ymmv, here the result achieved, heard by two dealers and two staff (quirky issue - different story - great Naim service) suggest that the Naim active system I enjoy, performs stunning well with a small cable size radial circuit

There is perhaps is no definitive answer for the best solution - do the best (electrical) installation that the situation permits and enjoy the music.

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Yes so am I … as its effect will hugely depend on many many factors, such CU tail lengths, type of CU, tail cable diameter and CU quality. So will depend from implementation to implementation. Wylex, Hager and Schneider CUs are considered by many to be the best/premium.
It doesn’t really align with standard practice for low noise setups specifically as I understand it. having family members in that business… ie including recording studios. It’s the TT earth isolation that is pivotal, but not all houses can enjoy the luxury of a TT setup, though if you can I strongly recommend it for clean low noise mains… assuming you share your substation.

Indeed, as it will depend on the environment, installation type and location … so the answer to the best solution is ‘it depends’

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I’m having a solar system installed that will be able to power my house in the event of a power cut (Tesla Powerwall 3 and Gateway 2). I understand it will need a separate earth, since it has to disconnect from the grid if there is a power outage. Would that separate earth amount to a TT connection? Currently we have a TN-C-S connection.

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Dont forget many local devices in your house will dirty it up again, such as fridges, hair dryers, USB power adaptors, swimming pools, Jacuzzis etc

An MK double unswitched socket will take 10mm, although it’s a bit tricky. A deeper back box also helps here. Many on here have done that, including me. No issue with regs. You might then use a power block from one of these sockets. You could consider 6mm if you choose as many on here have also done, and people will argue both ways if you really want to go there :wink:

I believe Hager dropped somewhat when it was taken over a while back

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Your electrician will advise - the key thing is that the earth must be completely separate for the TNC-S and TT - and connected devices must be physically separate with a minimum space between them (can’t remember what it is) to avoid accidental electrocution.

Now in an isolation setup of the switch between grid and offline power includes the earth then you should be fine with a TT setup for the backup power. If it doesn’t then you might need to convert your main supply to TT as well or you still rely on the TNC-S combined neutral earth by your utility provider - which will be bonded on yourside of the power station or other distribution point.

I am having a new PV/Inverter/battery system installed, (GiveEnergy components), however the occurrence of power cuts in our village has massively improved - we are typically one or two a year at most - where as before they upgraded the overhead lines it was about one a month if not more… so I am going for a manual switchover isolator which will be just for live and neutral - but then we are already TT which might make it simpler. Because we live in an older property with over head feed we were advised to avoid an AllinOne battery inverter just because of the complexity of wiring involved in such a setup, New properties will probably be more effective here.

The key thing with TT - is that the utility earth is NOT used and is disconnected - and you use your own electrodes that your electrician will install and certify. They must past a ground impedance test.

You can also use such a setup as an RF sink but you need a group of electrodes (a centre electrode with a radius of electrodes) - with the length of each radius at quarter/three quarters wavelength multiples of the RF of interest. The length must be the total earth path wiring length between device to RF sink and the radial electrode. Most mains safety earth systems will not sink RF. I do this for other applications- but is over the top for hifi :slight_smile: Very low level RF currents can easily capacitively couple to ground and be good enough for audio systems.

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Thanks, I’ll speak with the installer to see if I can move to an all TT connection since they have to put in a local earth anyway.

I nearly went for a GivEnergy system (not All in One). But the more capable inverter in the Tesla Powerwall 3 will enable me to export more. The house backup is for me a nice to have. But it got me thinking about going full TT.

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A separate unit won’t add much cost so why not just do it. It’s really important to take the earth from the dedicated CU back to the meter, rather than piggybacking off the house unit.

It may not be ‘needed’ but who cares. Do it and have peace of mind.

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Yes in many rural setups there is a limit to how much you can export to the grid :slight_smile: - so we have gone for a relatively modest 5kW hybrid inverter (AC out) and 7.5kW (DC in) with a 9.5 kWh battery with 4.9kW of PV initially. I wanted expansion capacity for more panels for some of our flat roofs in future - so I have 2.5kW DC headroom… and there will be panel string plugs provided for expansion - and as I said above it will support total grid isolation in case of power cuts - but via a manual isolator/changeover switch.

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Yikes! That’s a whole can of worms :flushed:. I’ll have to see how I can get a good enough local earth.

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My system will have almost 7kW PV. The Tesla also charges quicker and has higher output. It is quite a bit more expensive though.

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Yes I have an aversion to anything associated with Musk.

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