Three phase wiring

[Theo]

While I was on the TLC website I was doing a thought experiment.

Twin and earth cable comes in a variety of sizes:

1, 1.5, 2.5, 4, 6, 10, 16 mm2.

But what if you want to wire three phase? TLC only have 1 and 1.5mm2 three core and earth. Doncaster Cables also have 2.5mm2 but not larger. CEF likewise. There's some three core in SWA but I assume that's primarily for external use?

I understand that using three phase you can get 3x the power compared to single phase - so a 32A radial would get you 7.3kW single phase or 22kW three phase.

But how should you wire that? For a 32A single phase radial the TLC calculator indicates it needs 6mm2. What cable do you use for three phase?

I suppose you could run a pair of 4/6/10mm2 T&E but that would seem 'surprising' - one of those pairs carries two phases instead of live and neutral, and anyone who mixes up a phase for neutral may be in for a shock. Plus the colour scheme is confusing (two brown and two blue, which phase is which again? You can sleeve, but what if you mix up the cables?)

You could use SWA, but that's fairly unwieldy and pricey for an internal installation.

So what's the 'proper' way to do it?

Thanks Theo

[newshound]

IME at the domestic / light industrial level 3 phase is usually wired in single core.

[Steve Walker]

And for heavier industrial use, you could use something like:

Although the biggest I have seen myself (while testing a gas turbine generator set) used singles - 3 pairs of 500mm2

[John J]

You could go to a "proper" supplier but are you really planning to install a three phase circuit in a domestic property or just shooting the breeze?

[Jeff Layman]

Somewhat coincidentally, and further to the thread which I started last week with the subject "At what low mains voltages do devices stop working?", there have been two 500kVA generators running locally while work was being done on a substation (I thought one was 300kVA as it was smaller than the other, but was informed by a guy from SSE that both were 500kVA). Both had 5 cables from them, four were yellow and seemed to be about 25mm diameter, the other was yellow with a green stripe and was somewhat less - maybe 20mm in diameter. A rather poor close-up photo of connections made to the overhead line (440V?) is shown here:

I think it is possible to just make out the colours, and assume the other ends hidden in the generator are also colour coded. Does the earth carry less current, or it it just there in case the neutral fails as a temporary backup?

I guess those 25 mm cables have about 300 - 350mm^2 wire in them, and are rated at 500 - 1000A.

Not far form me is a house with a Tesla parked in the drive. There is a shoe-box sized wall charger unit with a cable about 20 - 25 mm diameter going into it. I assume that's 3-phase.

[Andy Burns]

There are "lighter" 3-ph cables than SWA, where armour protection isn't needed (e.g. server rooms) e.g SY flex, or tuff sheathed

[ARW]

And YY

[charles]

does the premises have 3 phases incoming? If not, it might be quite pricey to have your supply upgraded.

[Brian Gaff (Sofa]

Which reminded me of something I'd not thought of for many years. For a while you could get insulated wire which had small grooves in the sleeping in various configurations. That would have been great today for identification of which wire was which in dark areas or for us blindies. I don't think its a thing any more, probably it compromised the strength of the sleeving or something in which case they could have made it a tiny tactile raised line instead? Be ideal for the colour blind electrician!

Brian

[Theo]

Interesting - is it common to use stranded rather than solid core? I was looking for solid core which might have been why I didn't find these.

Theo

[Theo]

The train of thought was that, as part of current works, I'm thinking of putting in a circuit for EV charging. I don't have an EV, nor do I have a space on the CU to fit it, but working on the 'materials are cheap, labour is expensive' principle (especially when you've made good on top of everything) it's only the cost of the cable to do it now. The CU replacement is likely to happen in the medium term (which would include wiring in this cable), which just leaves the (relatively limited) external wiring for the EVSE.

And then, scope creep being what it is, I was pondering what the uplift to three phase would be. I don't have a three phase supply, but it's 'just' another conductor to go from 6mm2 T&E to 6mm2 three core and earth. And then we'd be all set for a three phase EV charger - for example, charging multiple EVs at once. That would mean upgrading the supply, but longer term we might have to move it anyway and the uplift in costs to the DNO from single to three phase isn't *that* much.

But I couldn't find regular 'three core and earth' at suitable gauge, which is why I was wondering...

Given that I don't seem to be able to buy the suggested cables in less than

100m lengths, is running a pair of T&E with suitable sleeving a Really Really Bad idea? I suppose it doesn't matter until the point that 3 phase comes into the equation (just a regular single phase circuit up to that point), but if it came to that would it be up to code?

Until going 3 phase I could run the two T&E in parallel to reduce losses/heating, but I suppose that might be 'surprising' - would it be unacceptable?

Theo

[The Natural Philosopher]

I would have thought that running single cores in insulated or steel earthed conduit would be the normal way to go

[Andy Burns]

If you want maximum future proofing, put in cable that can handle connection to one or more current transformers for "smart" charge points.

[Robin]

TLC sell up to 6mm 3-core NYY-J by the meter

[ARW]

I believe 3PH chargers also need a neutral.

[ARW]

Yes but he also needs and earth and neutral:-)

[newshound]

That's simply because 3+e is made for two-way lighting

[Robin]

D'Oh!

(I'd thought "with the 18th an EV charger he might well need a local earth rod anyway so..." but then went full prat.)

[Theo]

I would go for the cat5e version, but that's for the external cable run (in SWA). I would terminate the cat5e inside the house, not run it all the way back to the CU.

But it's not suitable for 3 phase.

Theo

[Andrew]

Sounds more like a single phase charger. The chap near me who had a new supply to the house, requiring the road to be dug up, has a charger box on his house wall that looks to be about 2 to 3 foot high by 18 inches wide. I assumed he has had a 3phase supply connected to the house from the other side of the road (which being a slight curve, has all the street lamps).