2 phase supply

[Why Not Indeed]

There was a programme on TV a year or so back about a new estate in wales where ALL the homes had 3 phase supplies - and these were small family homes.

Western Power Distribution consulted on making 3ph the default for new installs in 2020.

NGED Your Power Future - Superfast electricity consultation

yourpowerfuture.nationalgrid.co.uk

So presumably that's what they do now.

 

[Ragnar_AT]

Maybe the plan is to connect a new split-phase 230/460 V transformer to an existing single-phase medium voltage line? That would explain the „two phase“, even if not strictly technically correct.

 

[JohnW2]

This may all very well be the case but the supply does need to cope with the peaks.

Of course - but remember that a cable can 'cope' with currents considerably greater than its rating for short periods - that's the basis underlying the DNOs' traditional diversity approach.

If my understanding is correct the DNO only permits fusing to be twice the supply capacity so 16KW ~64A maximum fuse = 64*2/3 = 43A.

What's the " /3 " about?

So getting home from work, putting the car on charge, putting a load in the washing machine and starting to cook dinner ~60A or to put it another way 50% overload of the fuse and all of the allowance for all 3 properties. It smacks of a mistake to me.

As I've said, EV charging may well have totally moved the goalposts of the traditional approach. With the possible exception of electric space heating, we've never previously had to think about large loads which persist for 'long' periods of time.

Kind Regards, John

 

[ebee]

As I've said, EV charging may well have totally moved the goalposts of the traditional approach. With the possible exception of electric space heating, we've never previously had to think about large loads which persist for 'long' periods of time.

Yes agreed.

What's the " /3 " about?

Ditto

 

[SUNRAY]

... If my understanding is correct the DNO only permits fusing to be twice the supply capacity so 16KW ~64A maximum fuse = 64*2/3 = 43A...

...What's the " /3 " about?...

Kind Regards, John

16KW supply *2 ≅128A overload capacity
128A/3 ≅ 43A per property.

 

[JohnW2]

16KW supply *2 ≅128A overload capacity
128A/3 ≅ 43A per property.

Does that not imply that, at least in terms of 'traditional' (pre-EV charging) DNO thinking, there's probably something wrong with the 'rule' you postulated?

Kind Regards, John

 

[SUNRAY]

Does that not imply that, at least in terms of 'traditional' (pre-EV charging) DNO thinking, there's probably something wrong with the 'rule' you postulated?

Kind Regards, John

I what way?

 

[JohnW2]

I what way?

As I understand it, the 'traditional' DNO view would seem to have been that a "16 kW supply" would be adequate for about 6 houses (if they didn't have electrical space heating), each of which would have a cutout fuse of at least 60A, quite possible higher.

 

[SUNRAY]

One of the jobs I was on which turned out to be a blown cutout 30A fuse, DNO/supplier were on site and, referring to his on-line record, explained he couldn't increase rating as the existing fuse records already exceeded the 2* guidelines. Similar information to what I've heard elswhere too.

I haven't had oodles of direct experience in this aspect, if I have misunderstood it I offer my apologies.

As I understand it, the 'traditional' DNO view would seem to have been that a "16 kW supply" would be adequate for about 6 houses (if they didn't have electrical space heating), each of which would have a cutout fuse of at least 60A, quite possible higher.

So potentially nearly 1000% overload capability?

 

[JohnW2]

One of the jobs I was on which turned out to be a blown cutout 30A fuse,

Albeit I have very limited experience, I've never seen or heard of such a thing - 40A is the smallest cutout fuse I've ever heard of, and that only very rarely

..... explained he couldn't increase rating as the existing fuse records already exceeded the 2* guidelines.

I can't argue with your experiences, but I personally have never previously heard of thus "2* guideline"

So potentially nearly 1000% overload capability?

I'm not really sure what you mean by all your references to 'overload capability', because we don't know the CSA of the DNO supply cable.

However, yes, if they sized their cable according to their traditional view that the 'after-diversity'load was about 2.5 kW (about 10.9A) per house, then they were implicitly accepting that the peak instantaneous current might occasionally be at least, say, 6-8 times their cable's ('continuous') CCC for short periods - which I presume you would call "a 600 % - 800 % overload"?

.. but how often have you heard of a DNO supply cable melting?

Kind Regards, John

 

[SUNRAY]

Albeit I have very limited experience, I've never seen or heard of such a thing - 40A is the smallest cutout fuse I've ever heard of, and that only very rarely

20 or 30A are common for thing like garage supplies and street furniture, actually street furniture often has much smaller fusewire.

I can't argue with your experiences, but I personally have never previously heard of thus "2* guideline"

I'm not really sure what you mean by all your references to 'overload capability', because we don't know the CSA of the DNO supply cable.

I know the size of the cable in my street

However, yes, if they sized their cable according to their traditional view that the 'after-diversity'load was about 2.5 kW (about 10.9A) per house, then they were implicitly accepting that the peak instantaneous current might occasionally be at least, say, 6-8 times their cable's ('continuous') CCC for short periods - which I presume you would call "a 600 % - 800 % overload"?

It is only very recently that I've heard of the 2.5KW figure and that is from this forum but bear in mind the vast majority of my work is commercial, where a 100A supply will be expected to run at 100A 24/7, the same it true for a 500A or 5000A supply.

.. but how often have you heard of a DNO supply cable melting?

Kind Regards, John

Only once but it doesn't count as it was a result of roadworks and the power had been rerouted via a different cable.

 

[ebee]

Yes I think 2KW to 3KW is fairly common amongst DNOs (Ok was before all this lektrik car millarky) sepending a bit on the actuall DNO and I had assumed to number of properties involved and the likely diversity factors, with large numbers more likely to be appropriate than with small numbers more likely to lead to more excesses.
The DNOs will have years of experience prior EV etc.

 

[JohnW2]

20 or 30A are common for thing like garage supplies and street furniture, actually street furniture often has much smaller fusewire.

Oh, sure. I was talking about 'domestic properties ('dwellings')'

I know the size of the cable in my street

Maybe - but it hasn't been part of what you have told us about "x% overloads".

It is only very recently that I've heard of the 2.5KW figure and that is from this forum ....

I first heard about the "2kW - 3kW per dwelling" (for domestic properties without electric space heating) a very long time ago.

I've just done this with SSENs on-line 'demand estimating calculator'. It's for one domestic property with an electric cooker and and one electric shower, but no other 'significant loads other than the assumed 'baseline maximum demand' of 6.25 kVA per domestic property. As you can see, for a maximum (instantaneous) demand of 17.255 kVA, they design their network on the basis of it representing 3 kVA ('after diversity') load

but bear in mind the vast majority of my work is commercial, where a 100A supply will be expected to run at 100A 24/7,

That's obviously totally different, and irrelevant to participants in this forum. The 6:1 (or whatever) diversity factor DNOs work with relates specifically and only to domestic properties ('dwellings') - where most loads only draw current for a tiny fraction of a day,not "24/7".

Kind Regards, John