Electricity Supply

This is the supply as it enter our house.

Is it likely that we have a 3 phase supply but are only using one of the phases or is it probable that’s just an empty box with only 1 phase coming in?

Just asking for interest’s sake as we have no need for more than one phase currently (the labelled fuse is actually 60A) but day dreaming about a 3-phase car charger sometime in the future…

It will be a single phase coming in.

Unlikely unless a farm or property that had a business attached.

The cable at the very bottom of the photo looks like 100A single phase based on diameter…..

Can’t tell conclusively from the photo. Seems no reason for having a 3 phase cut out on a single phase cable.

Some old recs would use a 3 phase cut out at the end of a feeder even if it was only supplying single phase as it was a useful spot for testing and fault finding.

The easy way to tell would be to remove the two redundant fuses and measure voltage between them.

However, they are sealed and belong to the DSO so they are the only people who can pull them (or some others that they have authorised). There are risks to pulling those fuses if something fails.

Ring your DNO / DSO and ask.

As said, it looks like a 3 phase cut out with a single phase meter.

The cable could be anything from that photo – single, split-single, three – and the diameter of the cable will be influenced by the conductor cross-section as well as the number of phases.

To work it out conclusively simply cut that black cable at the bottom and count the wires inside.

Get a non-contact voltage detector (https://www.toolstation.com/tis-non-contact-voltage-detector/p38097) and check if there’s any voltage at the input side of the 2 redundant fuses at the cablehead. There’s usually sufficient separation/insulation to prevent cross reading from the next phase over.

Never seen a 3 phase cablehead used on a single phase set-up but you never know… might have been a Friday afternoon job for the sparkies 😂

How would household appliances work off 3 phase?

Why would a domestic supply have one?

One example is rural locations. Imagine two houses in the middle of nowhere. You can supply both off an 11kV 50kVA 3 phase transformer.

That will give about 67A per phase.

Either each house is limited to a single phase supply of 67A each and the transformer is unbalanced, or, if they each have a 3 phase cutout, they could have 33A on each of the three phases, totalling 100A.

All it needs is for the spark on the domestic side to split the load evenly across 3 phases.

Perhaps lights on one phase, upstairs power on another and downstairs on the other.

Question within a whatsapp group recently. Apparently if your house ever had economy 7 storage heaters you may have 3 phase installed. And people do have 3 phase installed domestically if they run a business on site, are very heavy users or as Onzadog above.

But in the uk it’s unusual domestically

How would household appliances work off 3 phase?

Yeah interesting question, electrics are genuinely like witchcraft to me but i believe key parts of Europe are 3 phase domestically.

[strong]B.A.Nana[/strong] wrote:

Yeah interesting question, electrics are genuinely like witchcraft to me but i believe key parts of Europe are 3 phase domestically

When I fitted a new kitchen in our flat in Germany I had to remove the old cooker. I was surprised to discover it was on a 3 phase connection. That was just a 1 bedroom 4th floor flat in a block.

Your final ring circuit (or spur) would just use one of the phases. You could have different circuits spread over different phases. You’d need a 3 phase distribution board though.

Each phase is like a single 230v supply, but the voltage waveform is out of phase with the other two..

J

One example is rural locations. Imagine two houses in the middle of nowhere. You can supply both off an 11kV 50kVA 3 phase transformer.

That will give about 67A per phase.

Onza – you don’t have as much ambition with diversity as me. I’d have half a dozen or more houses on that 50kVA tranny.

Our *house has a similar set up to the OP’s but our incoming feed is bigger and is definitely 3phase with only a single phase being used.

*converted industrial building

Onza – you don’t have as much ambition with diversity as me. I’d have half a dozen or more houses on that 50kVA tranny.

Funnily enough, I have exactly this problem currently. Both houses were 3 phase and working fine. Then a domestic spark turns up at one and says they shouldn’t have 3 phase and single phase should be good for 100A (not sure how off a 50kVA tranny).

So now they have a new cooker, electric shower and electric car charger all in one phase and a neighbour with flickering lights and an unbalanced supply to his heat pump.🤦🏼‍♀️

NB. These are not normal houses that you or I might live in.

Hmmm. Typically a group of houses takes around 12kW plus 2kW per house. Allow a little more per house for smaller groups of houses.
An individual house typically tips out at 15kW (say 65A @ 230V) but averages 400W.

That will change as heat pumps, EV charging and more solar generation become commonplace though.

As you say, I don’t think your houses are typical.

And yes a 50kW 3 phase tranny will be 75A or so. It’ll probably do a little more on thermal cycle.

Flicker is probably more impedance related and while that may be supply impedance, you may find your sparky may not have allowed enough cross-section for the length.

How would household appliances work off 3 phase?

Why would a domestic supply have one?

They don’t, in the cases of more than one phase they split the phases between different applications. Eg we have cottage in the Dales which has two phases, one goes to the distribution board for normal sockets / lighting circuits and the second phase goes to a separate board which supplies storage heaters.

Can’t remember what the third fuse box is for….

Twin phase supply by Ben Freeman[/url], on Flickr