UK Load in Amps on a mains single-phase circuit

[Carl Haworth]

I have a 230 Volt single-phase circuit with a 100 Amp cutout fuse and a 100 Amp MCB switch, so the supply is rated at 23 kVA.

Only about 13% of my total energy consumption is electricity. The remaining 87% is natural gas, supplying the following appliances (kW figures are net output power):-

22 kW boiler, 3 kW fan-flued convector heater. 8 Kw cooker.

After (if?) natural gas is turned off some time in the 2030s I can have 3-phase installed (at vast cost) to give me total kVA 55.15 at 230 Volts. As you can see, this offer is tied to reducing the supply capacity per phase to 80 Amps. So a connection from one phase (and the neutral) to my existing single-phase consumer unit would be rated at 18.40 kVA, leaving (I presume) 36.60 kVA for the 3-phase, once this is balanced (how is balancing achieved in this sort of situation, by the way?).

I will also, by then, need a powerful EV charging wallbox. An 11 kW one is for 3-phase only, as would be a 22 kW electric boiler (heat pump technology is not suitable for our home). If these two appliances consume 33 kVA, the 3-phase will be used to over 80% of its capacity, but it could be arranged that the two would never be on at the same time.

The 23 kVA single-phase circuit, with capacity reduced to 18.4 kVA, will need to carry extra loads:-

Cooker (12 kW maximum, 7 kW diversity), 3 kW convector heater.

I have done calculations of the peak consumption of the circuit as it stands now. I need to be confident that these are reasonably realistic, so that I can add the extra loads to the existing ones, and calculate the much higher peak consumptions with the extra loads added. I need to know if the total is likely to be within the reduced 18.40 kVA capacity of this circuit.

If this is relevant (please see below), the additional appliances mentioned for the single-phase, and both those for the 3-phase, have power factor 1.

As regards the existing load on the single-phase circuit, if a p.f. should be applied, it should be reasonable to use 0.8.

To work out first the load on the increased-load circuit as regards the its capacity/the capacity of the cutout fuse, I need to know if I should increase the kW of the total existing load by 1/0.8, so as to include apparent as well as real current, and get a figure in kVA. Or should use the total kW. At that point I would add the extra loads of the cooker and convector heater, as kVA and kW are identical.

Calculating Amps using kVA increases the load on the existing circuit by 25% if p.f. is 0.8, so, after adding the extra proposed loads, there is unsurprisingly considerably more "headroom" left in the circuit capacity if I calculate using kW.

I suspect that kW is the correct basis to use, because apparent current passes back into the grid after passing through the circuitry of an appliance. An appliance may draw, say. 10 Amps. If its p.f. is 0.8, 2 Amps are returned to the grid instantaneously as the 10 Amps are drawn. This is why a Watt/hour meter records using the voltage times the net current used up by loads, and so reads in kWh and not kVAh.

So I suspect that the cutout fuse draws the net current (full current less apparent current).

Which should I use for my calculation: kW or kVA? If the answer is kVA, then, in view of my comments just above, where have I misunderstood things?

I've tried to find an answer to this, but can't find any focussed guidance.

Can forum members enlighten me reliably, please?

Carl

 

[pc1966]

What matters to the supply is kVA, in fact more specifically the current. Some equipment draws more as volts go up (heater, etc) some less (switching power supplies with constant load).

Any big domestic loads have generally been heating which have a PF of 1 and it is more industrial where lots of AC motors are in use that you might assume 0.8 overall. I don't know off hand what large EV chargers are but I expect they are close to 1 by design.

However, if you are looking at the post-gas situation you really REALLY need to be looking at heat pumps of some form as they typically have a COP of around 3, so use around a third of the electric compared to the heat you use. To understand why please update your electrical plans to include to cost of the electric you are proposing to use, and then allow for 3-5% inflation (or possibly more) for the next 8 years....

 

[Carl Haworth]

What matters to the supply is kVA, in fact more specifically the current. Some equipment draws more as volts go up (heater, etc) some less (switching power supplies with constant load).

Any big domestic loads have generally been heating which have a PF of 1 and it is more industrial where lots of AC motors are in use that you might assume 0.8 overall. I don't know off hand what large EV chargers are but I expect they are close to 1 by design.

However, if you are looking at the post-gas situation you really REALLY need to be looking at heat pumps of some form as they typically have a COP of around 3, so use around a third of the electric compared to the heat you use. To understand why please update your electrical plans to include to cost of the electric you are proposing to use, and then allow for 3-5% inflation (or possibly more) for the next 8 years....

I would say we have quite a lot of appliances which have electric motors, which are pretty inductive machines so have a p.f. less than 1. All but 4 of our electric lamps (bulbs and tubes) are a mixture of discharge and LED. Both these have p.f. of 05 to 0.6. So I doubt that our overall p.f. is 1. It may not be as low as 0.8, though. I deliberately erred on the pessimistic side to ensure, if apparent Amps do have to be included when calculating the likely peak load of the whole circuit (so I need to use kVA, not kW, for arrive at the Amps), that I am not under-estimating the current.

So I still need to know whether I should calculate current from kVA (assuming a general p.f. of under 1) or kWh, when doing arithmetic acrobatics to arrive at the likely peak consumption.

As regards your comments about central heating

Our system needs, particularly in cold weather, a water temperature of around 80. Our gas boiler easily achieves this. We have relations who have installed a very recent, and enormous, ground-source heat pump system. They have 7 acres, so masses of space for the ground loop. Unfortunately, without 3-phase electricity (they are deep in the country), although they have a 23 kVA single-phase supply (as we do), they were not able to fit a heat pump powerful enough (with an even bigger ground loop) to raise the c.h. water temperature to above 60, at best. This matters less to them than it would would to us, where we do not want to ruin a lovely garden by installing a ground loop. Their property is a big barn conversion, so they were abele to install underfloor heating throughout.

Underfloor heating is out of the question in our 50+ year old house, and we do not want the huge radiators which are necessary with water temperatures of 60 at best.

In fact, with an air-source heat pump we would be lucky to get 50 degrees, is seems. Air-source would be our only heat pump option, and is certainly not suitable. Ground-source is much more reliable in heat energy output, but you have to have the SPACE for it, and masses of disruption to install it.

So an electric boiler is our only realistic option, and we need 3-phase to run it. That is horribly expensive to install (though a lot less than a not-powerful-enough heat pump), and to configure so it also supplies our 1-phase circuits, and the running cost is eye-watering. However, if it's the only way to guarantee the radiator and DHW temperatures that we need (DHW can be heated direly by electricity of course - you do not have to run a cylinder heat exchanger for it), so be it.

 

[plugsandsparks]

Check out the latest heat pumps from Stiebel. I am wrestling with one at the moment that uses "Hot Gas recovery" - Its an extra heat exchanger around the main compressor. The temps coming off this beast are so high >100 degrees C that i am having some issues with over temp on the DHW tank and pumps !!
This is a GSHP but the concept is no different in ASHP.
I was quite frankly (and i am a cynic) amazed at where these sort of temp were coming from in heat pump tech but i can assure you they are real and makes electric heating for either DHW or CH largely redundant (albeit high initial outlay)

 

[snowhead]

Natural gas is NOT being turned off in the 2030's.
Sometime by or in the 30's the gas supply will have 20% Hydrogen mixed with it as most current appliances will work O.K .
Later, maybe 40's or 50's the whole network is likely to be 100% hydrogen, but that will require repiping of some of the network and modifying or replacing some appliances.

Trying to plan for something even in 10 years time is pointless.
Too much can change and new developments could have come online.

It may even be that they've managed to roll out the current gen 2 "Smart meters" into close to 100% households, but I would put too much hope on that happening.

 

[Carl Haworth]

Check out the latest heat pumps from Stiebel. I am wrestling with one at the moment that uses "Hot Gas recovery" - Its an extra heat exchanger around the main compressor. The temps coming off this beast are so high >100 degrees C that i am having some issues with over temp on the DHW tank and pumps !!
This is a GSHP but the concept is no different in ASHP.
I was quite frankly (and i am a cynic) amazed at where these sort of temp were coming from in heat pump tech but i can assure you they are real and makes electric heating for either DHW or CH largely redundant (albeit high initial outlay)

Thanks. That's very interesting. The Stiebel design sounds like waste heat recovery - along the lines of what happens in a condensing combustion boiler exhaust (and when you take your foot off the loud pedal on a car with "regenerative braking"!). "Every little (or lot?) helps".

I can't help wondering, though, if the major contributor to the terrific performance of the heat pump system that you are installing is to the volume of the heat energy source (the ground). If you have a ground loop with a really huge surface area, the water arriving at the first heat exchanger contains more heat energy per litre than water from a loop with a smaller surface area. The temperature rise of the water from start to finish may be little different, but that's not the key issue. If you have a heat pump powerful enough to condense the heat energy out of a sufficiently large volume of water, the temperature of the gas in the vapour compression circuit will be higher than in a smaller/less powerful system, and so on. Waste heat recovery from the motor will make the pump effectively more powerful than its rating, but probably assists efficiency rather than making a large contribution to it.

Please do correct me if my above assumption is wrong.

The difficulty that my sister and co encountered when their system was being designed was the insufficient spare capacity in their 23 kVA single-phase electrical supply to drive a gshp which could have been much larger (they had built on a big plant room for it), and could have processed much larger volumes of water, compressing a much larger volume of gas in the VC circuit to a much higher pressure so as to transfer much more heat energy to the house loop. They would have been happy to pay more for a sufficiently powerful system, but could not improve on 23 kVA owing to the absence out in the country of a 3-phase supply from a nearby substation. As I have discovered, although the reality of 3-phase is a bit of a damp squib, in our case (and theirs, had it been available), it would increase a total kVA of 23 to 55.15 (to 69 if our DNO could be persuaded to leave us with a 100 A limit per phase!).

I agree that the hp concept is the same irrespective of the heat energy source, but ground temperatures even only a metre below the surface are far more stable over the changing seasons than air temperatures.

I fear that there risk being lots of very unhappy people, used to the water temperatures and power delivery capacities of combustion boilers, who will have fallen for the ashp sales pitch which will soon start circulating vigorously on the back of the government subsidy for installing this variant of hp technology.

 

[plugsandsparks]

Yes, you on the right lines. This is a 59KW GSHP, brine is the medium circulating in the ground loop. The difference in temperature brine in/out maybe only a degree or two, but all it needs to do is turn the Fgas inside the GSHP from liquid to gas so it can run through the heat exchanger and give up its heat and turn back to liquid. Admittedly the unit has a large compressor to push the Fgas around the heat exchanger and normally heat pumps are more efficient in heat mode than cooling mode as the heat generated by the compressor easily provides some extra heat transfer into the Fgas too aid liquid to gas transition. What Stiebel have done is more formalise this heat recovery into direct hot water to aid both DHW and CH and breaks the temp barrier that normally makes heat pumps most efficient at 35 degrees. The system i am working on is set to 50 degrees fixed value direct to buffer tanks which then heat the house, hot water tank, swimming pool and warm air heat exchanger for the swimming pool air handling unit, so it has alot to do and raising the temps available means i dont need electric immersion for Legionnaires and provides hot water at a high temp for mixing further down the line. As mentioned the issue i was wrestling with was because the hot water tank rose to 74 degrees which is too much for bog standard pumps.

 

[Carl Haworth]

Natural gas is NOT being turned off in the 2030's.
Sometime by or in the 30's the gas supply will have 20% Hydrogen mixed with it as most current appliances will work O.K .
Later, maybe 40's or 50's the whole network is likely to be 100% hydrogen, but that will require repiping of some of the network and modifying or replacing some appliances.

Trying to plan for something even in 10 years time is pointless.
Too much can change and new developments could have come online.

It may even be that they've managed to roll out the current gen 2 "Smart meters" into close to 100% households, but I would put too much hope on that happening.

You are right about planning so far in advance. I just want to see how the land lies IF methane is turned off.

I don't see how a gen 2 Smart meter would allow me to view and record current consumption peaks. Am I wrong?

You're not the first person to tell me that the big NG turn-off won't happen. My wife took the same line as soon as reports started appearing threatening this, and Boris did not help by assuring all the "worried well" that they would not be forced to change their gas boiler, but simply would not be able to buy a replacement after (I think) 2035.

Which? has joined the the chorus of reassurers with (unsurprisingly) their version of the Boris message.

None of the reassurers suggests up to when (after 2035) it will remain possible to run a gas boiler. However, as you say, probably (no more than that), first a mixture of H2 and NG will replace 100% methane/ethane, then, IF there is enough of it (a big if, I suspect), pure H2 - after changing the grid trunk pipelines with new ones made of a steel which does not react terminally to green H2. Oo, er!

For various reasons, the above seems a question more of if than when, so one can't be categoric about combustible gas for boilers if you look beyond 2035 (and if that is a reliable date!).

I hadn't swallowed the doom scenario hook, line and sinker, but I would prefer to find a way around our own dependence on NG, because it has become a political, as well as an environmental, weapon.

At the moment, dumping NG and switching to electricity doesn't solve this problem in the UK because its generation here is so dependent on NG. R-R's electricity generation project could much reduce that dependence. But even there, I can't help wondering if it is wise to proliferate nuclear reactors - on safety/military defence grounds. Also, they need planning permission to be built, and there is likely to be resistance in areas where there are the sort of worries I have just mentioned.

Dare I suggest that our bottom line for generating electricity might, in the end, have to be to switch back to coal (we've still got oodles of that filthy stuff)? It needn't be so filthy now, but it's the very opposite of green.

 

[Carl Haworth]

Yes, you on the right lines. This is a 59KW GSHP, brine is the medium circulating in the ground loop. The difference in temperature brine in/out maybe only a degree or two, but all it needs to do is turn the Fgas inside the GSHP from liquid to gas so it can run through the heat exchanger and give up its heat and turn back to liquid. Admittedly the unit has a large compressor to push the Fgas around the heat exchanger and normally heat pumps are more efficient in heat mode than cooling mode as the heat generated by the compressor easily provides some extra heat transfer into the Fgas too aid liquid to gas transition. What Stiebel have done is more formalise this heat recovery into direct hot water to aid both DHW and CH and breaks the temp barrier that normally makes heat pumps most efficient at 35 degrees. The system i am working on is set to 50 degrees fixed value direct to buffer tanks which then heat the house, hot water tank, swimming pool and warm air heat exchanger for the swimming pool air handling unit, so it has alot to do and raising the temps available means i dont need electric immersion for Legionnaires and provides hot water at a high temp for mixing further down the line. As mentioned the issue i was wrestling with was because the hot water tank rose to 74 degrees which is too much for bog standard pumps.

All very interesting.

I take it that the over-high water temperature affects the loop heating the buffer tanks, so this loop is at well over that temp. 74 degrees may be enough for radiators, unless they are sized for a lower temperature, but possibly too hot underfoot for underfloor loops (and might cook floor coverings)?

I don't understand about the buffer tanks. Are they to even out the normal ups and downs of the water temperature in ch etc loops?

My sister's system, as far as I understand its details, uses the water heated by the heat exchanger powered by the gas temperature in the VC circuit of the heat pump directly for the underfloor loops and the coil in the DHW tank. The temperature of this water being only around 60 degrees in her installation means that the unvented cylinder has to have two immersion heaters (one at the bottom, plus one to heat up water at the top, to top up during high electricity cost periods.

The need for direct electrical heating here meant, to her disappointment, that the whole house just missed the top energy band on its EPC.

Are the pumps that you refer to circulating pumps for the heat exchangers supplied by the buffer tanks (house loop/s etc)? If so, I'm surprised that they are not suitable for 74 degrees. I must be missing something, because the 25 year old Grundfoss pump which sends the water heated by our gas boiler round the CH. and DHW loops has to stand up to 82 degrees (when we have the boiler temp at max in cold weather). This 3-speed standard CH pump still works quietly, and well. What's different about the pumps which have been causing your problems?

Obviously, provided the water in the DHW heating loop is well over the cut-out temp of the cylinder thermostat, the stored DHW will achieve the set temperature. Certainly we have no trouble keeping our DHW at 60 degrees when the loop temperature is over 80. The loop just heats the DHW faster when it is around this level than in seasons when the loop is at more like 70 degrees (but there is then less work for the loop water to do to achieve a cylinder temp of around 60, so there's probably not much difference between the two loop temperatures and the seasons to which they relate and the recovery speed of the DHW temp).

What can you do about the water temp in the buffer tanks, if it is too high? Is the ground loop too big (it may be very difficult now to shorten)? Can the speed of the heat pump be regulated? If neither of those solutions is possible/satisfactory, can you change the heat pump for one suitably less powerful?

MOVING ON

Although my wife won't hear of digging up even the lawn of our back garden, I'm very interested by your findings as regards the performance of a Stiebel gshp (or, presumably, ashp?) system which uses waste heat from the pump compartment to boost the temp of the loop heating the buffer tanks (or, presumably, the CH and DHW loops direct).

I don't know the make of hp in my sister's system, but it may predate the design improvement that you highlight. The system itself was designed and supplied by Ice Energy. Their contribution was apparently very impressive (not just the sales pitch!), but many problems were encountered owing to mismatch between the initial bland assurances of their builder, and the ignorance, which was revealed later, of his plumber. This was all sorted out satisfactorily in the end, but Ice Energy had to come several times to "hold the plumber's hand".

I believe that Ice Energy gave a 10-year warranty on the design and components, so the fact that this apparently very competent and helpful firm has subsequently ceased trading must be a potential worry for my sister , and is disquieting news for people looking for an honest and competent system designer/components supplier I wonder if, as can happen, an over-generous warranty policy plus too many post-installation problems. made it impossible for IE to continue trading profitably.

Your reports and comments mean that I would be interested to know approximate surface area of the pit which would have to be dug to over a metre deep to accommodate a ground loop for a gshp powerful enough for our own home. Our "heat only" gas boiler provides the heat energy for radiator and DHW heating. It is rated at 28 kW. The manufacturer claims 80% efficiency (it's a fan-flued non-condensing boiler), but independent sources suggest more like 76%. So the maximum heat output is probably around 22 kW. A local "engineer" who services this boiler (when it needs it, which is not often) considers that it is over-sized, but our experience in very cold weather is that it is about right. It may be a bit over-powerful, but that is preferable to having one which won't get the radiators hot enough in very cold weather!

Assuming that about 22 kW is the heat energy requirement of our home (ch and DHW), can you give me an ideal of the length and width of the pit that would be needed for a gshp ground loop connected to the type of motor-heat-recovery hp that you have told me about?

We are in the Trent valley, about 20 miles north of Newark. We are on flattish ground, about 75 metres above sea level. Our subsoil is clay, and we have an obviously variable, but usually quite high, ground-water table. So I would say that our ground is good, maybe very good, for heat pickup during most seasons by a gshp ground loop.

If the pit (excluding the trench between the back of our long garage, where the pump would have to live, and the ground loop proper) is well within the area of our back lawn, my wife might reconsider her opposition to digging up our lawn. However, she also points out that (a) we may well be able to continue using gas in the from of methane/ethane, methane+H2 and even eventually H2 only, for another 20+ years, and (b) even if we felt it best to eliminate our dependence on gas sooner rather than later, the cost of an electric boiler, and the 3-phase supply to accommodate its load, would be a minnow in comparison with the cost of installing a gshp system. The running cost of the latter would be far lower than that of an electric boiler, and even that of running our present gas boiler, but it would take decades to recover from such savings the difference in installation costs.

 

[UNG]

While you are concentrating on the heating output and trying to achieve something similar to what you have now in the future you have to bear in mind a lot of the newer technologies rely on an increased level of property insulation to deliver the most effective results
I was always a bit sceptical of the passivhaus building methods until I got involved in a project that a local vet was building, he is very into the eco concept and built a new surgery to passivhaus spec with heat recovery, ASHP, solar panels and masses of insulation, the end result is very effective and costs very little to run but it does involve more cost. One of this guy's other projects was renovating his own house Low Energy Building Consultancy | Kingsleypassivhaus | Frodsham - https://www.kingsleypassivehouse.com/ it might seem a bit extreme but with the ever rising energy costs it has to be an option