Three Phase question

[John D v2.0]

I think that 'taxing' is probably somewhat of an understatement! I don't think a DNO would be happy (i.e. would not give permission for such an arrangement - if they were asked, as they ought to be!).

Kind Regards, John

Maybe I'm missing something then, I thought if you have a 100A supply then as long as your diversified load is less then that then you're in the clear?
Failing that you can share with next door, now that your electricity supplier really would be upset about!

 

[Lectrician]

Fuse size is not the same as your agreed capacity.

 

[JohnW2]

DBX18 can be connected to a single phase supply, but NOT a standard domestic install.

Is that from personal experience? The text of the brochure says "Requires three phase supply (not DEX12).", the spec on p23 mentions only 3-phase for the DBX18, and the graphs/tables on pages 25 and 26 show only the DBX12 in the single=phase bits.

Its around 78amps on a single phase, 26amp per phase if on separate phases. This is no good for a typical home.

Quite so, and as for John D (v2)'s suggestion of 2 x 11 kW ones ......

Kind Regards, John

 

[John D v2.0]

How do i find find my agreed capacity?
Admittedly the old fuse wire, even the ukpn guy didn't know its rating. When he changed it he put in a 60A because of the tails but he said I could get a 100A if I wanted. He even said their cable has hardly any meat either, but they have their own rules for the paper insulated cable.

 

[JohnW2]

Maybe I'm missing something then, I thought if you have a 100A supply then as long as your diversified load is less then that then you're in the clear?

No. Apart from anything else, don't forget that the DNO's estimate of the average house's "after diversity maximum demand" is about 10A! If a significant number of homes created a demand anything like the DNO fuse rating, the DNO (and its consumers, who would be expecting a significant number of volts!) would be in deep trouble!

Kind Regards, John

 

[John D v2.0]

Haha I'm assuming none would leave such a load on all day! The 50 pound a day bill would be scary enough! That would certainly average out at under 10a even if a family of 6 had decent showers every day!

 

[JohnW2]

How do i find find my agreed capacity?

Good question. However, DNOs seek to avoid excessive loads by requiring consumers to seek their permission before installing high-demand loads - such as more than one shower, welders and other large machinery etc. etc.

Kind Regards, John

 

[Lectrician]

Is that from personal experience? The text of the brochure says "Requires three phase supply (not DEX12).", the spec on p23 mentions only 3-phase for the DBX18, and the graphs/tables on pages 25 and 26 show only the DBX12 in the single=phase bits.
Quite so, and as for John D (v2)'s suggestion of 2 x 11 kW ones ......

Kind Regards, John

I had made an assumption, it seems from the install manual that it is only suitable for a TP supply (no neutral).

 

[JohnW2]

Haha I'm assuming none would leave such a load on all day! The 50 pound a day bill would be scary enough! That would certainly average out at under 10a even if a family of 6 had decent showers every day!

Agreed, but if two or more consumers on the same main happened to run 2 x 11 kW (about 96A) loads simultaneously, the system would collapse ...

... think of a TN-C-S installation with a Ze of, say, 0.3Ω (hence also a L-N loop impedance of 0.3Ω) at the house closest to the transformer. Each 96A load applied by houses on that main would result in a 28.8V voltage drop at that first house - so a VD of about 86V if there were, say, three of them (and even more VD for the more downstream consumers).

Kind Regards, John

 

[JohnW2]

I had made an assumption, it seems from the install manual that it is only suitable for a TP supply (no neutral).

That is, indeed, what the brochure seems to be saying.

Kind Regards, John

 

[John D v2.0]

Seems like ukpn assume Max demand of 15kva basically 62A for domestic single phase unless otherwise requested.

Agreed, but if two or more consumers on the same main happened to run 2 x 11 kW (about 96A) loads simultaneously, the system would collapse ...

... think of a TN-C-S installation with a Ze of, say, 0.3Ω (hence also a L-N loop impedance of 0.3Ω) at the house closest to the transformer. Each 96A load applied by houses on that main would result in a 28.8V voltage drop at that first house - so a VD of about 86V if there were, say, three of them (and even more VD for the more downstream consumers).

Kind Regards, John

You're assuming most of the impedence is in the street cable, I don't know myself, but I am suspecting that cable is thicker than the drop to each house. And we're getting dangerously close to the cooking Christmas dinner argument!
Also if the closest house is on 0.3 ohms there's not much left for anyone else, but that's beside the point.

 

[JohnW2]

Seems like ukpn assume Max demand of 15kva basically 62A for domestic single phase unless otherwise requested.

If you say so, but I'm sure that "maximum demand" (per property) figure assumes a high degree of 'diversity' - i.e. I feel sure that the system would not cope if a substantial proportion of consumers supplied by one main all drew that 'maximum' current simultaneously.

You're assuming most of the impedence is in the street cable, I don't know myself, but I am suspecting that cable is thicker than the drop to each house.

Yes, for simplicity, I was making that assumption. However, I would suspect that, certainly in many urban situations, and particularly for those more 'downstream' consumers, the 'drop' to their house probably accounts for a pretty small proportion of the total loop impedance.

And we're getting dangerously close to the cooking Christmas dinner argument!

Well, it's perhaps heading the way, but is really nowhere near the ballpark of high-powered water heaters. Don't forget that, within each household, diversity means that a "full out" cooker probably doesn't represent more than about 20A - not quite the same as 78A or 96A! Mind you, I suspect that in no-gas villages like mine, supply voltages probably do take a bit of a hit on Christmas Day - I had intended to (but forgot to) take some measurements this last Christmas!

...if the closest house is on 0.3 ohms there's not much left for anyone else, but that's beside the point.

Again true - I just pulled a simple number out of the air for illustration. However, you can substitute any numbers you like and the general principle of what I was saying still remains.

Kind Regards, John

 

[SimonH2]

Interesting - does anyone else wonder about having "bare wire" immersion heater elements ?
Looking at the specs, even the most powerful unit will only give 8l/min - less than 2 gal/min - which is not going to be stunning at filling a bath. If it could be connected to single phase, it would be pulling 117A (according to the spec sheet) and the DNO would certainly have something to say even if you didn't blow the main fuse.

If electric has to be the source, I'd have suggested sticking with a cylinder for storage. People will warn you about standing losses, but if you are sensible then that's not an issue. Lag the cylinder and pipework well and losses will be low, if it's inside the heated part of the house then any "losses" aren't actually lost as they'll contribute to the heating - so only losses for part of summer when there's no heating needed.
Once you've got storage, you can size the heating to what the supply can provide - and also use off-peak lecky as with all electric heating it's quite likely that it'll pay to have it.

Personally I'd be considering a thermal store (or heat bank). Do it right and you can use it to store off-peak heat, and run both the heating and hot water from it. While there's a bit more to it than an unvented cylinder, you get to avoid the problems of storing large quantities of hot water under pressure You can either buy (at great expense) an off the shelf system, or with a bit of thought you can make your own from standard copper cylinders, some pluming, and a heat exchanger.

On a side note, some years ago I recall we had a consultant in at work. He lived in a small village at the end of a long local distribution line. All the houses had night storage heating - and they had "problems" due to the massive change in load between heating on (low voltage) and heating off. In particular, they had problems with the voltage spike in the early hours when the heating loads switched off.
Your neighbours are likely to notice you switching on/off a huge heater !

 

[JohnW2]

If electric has to be the source, I'd have suggested sticking with a cylinder for storage. People will warn you about standing losses, but if you are sensible then that's not an issue. Lag the cylinder and pipework well and losses will be low, if it's inside the heated part of the house then any "losses" aren't actually lost as they'll contribute to the heating - so only losses for part of summer when there's no heating needed. Once you've got storage, you can size the heating to what the supply can provide - and also use off-peak lecky as with all electric heating it's quite likely that it'll pay to have it.

Indeed. I have a well-lagged 140 litre hot water cylinder which is normally heated only by off-peak (E7) electricity. When there are just two of us here, that is plenty, and there is still hot water available at midnight the following night (just before the immersion comes on). For when the house is more full, we have a second identical cylinder (not normally heated) which supplies the first cylinder, and the pair of them (both heated) is usually adequate when several people are living here.

Kind Regards, John

 

[flameport]

Hot water cylinder heat losses are typically between 1 and 2 kWh per day for a modern insulated cylinder.
Some of the newest types can be less than 1 kWh. Older ones will be more, but unless it's an ancient totally uninsulated copper thing, the losses and the cost per year are usually insignificant.
Specified losses also assume it will be hot for the entire 24 hour period, and that the loss to the air is wasted - in reality the cylinder may not be heated all the time and in the winter any losses will contribute to the heating of the building.

The majority of hot water heat loss is through the pipes, which is why they should all be insulated. They rarely are.