IDENTIFY THE CAPACITY OF MY MAINS SUPPLY CUT-OUT FUSE (PLUS...)

[Carl Haworth]

About 15 years ago, after having had a modern consumer unit fitted, and prompted by our electrician, I asked my then energy supplier (Tonik) if our 1968 60 Amp cut-out fuse could be upgraded to 100 Amps, and what additional work/expense would be needed.

I did not know that my request had gone to my DNO until two men from the local team appeared one day to do the work!

Apparently, all that was needed was a new fuseholder, marked 100 A, and a new fuse (no charge!).

Our local substation dates from 1966, and appears largely unchanged. Many more houses have been attached to it since the first phase of our estate was built. However, I was told that nothing more than an "upgraded" fuse was needed. I pointed out the 40 A capacity marking on the nice old Sangamo meter, but the answer was that this was still OK for a maximum 100 Amps. (Really?)

Early this year, worried about the future and cost of natural gas (which we currently use for cooking, central heating and one wall-mounted gas convector heater), we considered installing a 15 kW electric boiler and a fast EV charger, both of which need a 3-phase supply, with single-phase electrical replacements for our gas cooker and convector heater.

So I asked the DNO about the possibility of changing our supply to three-phase with, the original single phase taken off one phase of the three-phase.

There was no problem in agreeing that this could be done (apart from the price - at that time around £3k, plus groundworks, "making good" etc etc and connection to the single-phase consumer unit, all being our responsibility - so probably easily doubling the £3k!).

However, instead of our present 23 kVA single phase, he told me that we would have a 55 kVA 3-phase supply. How much this would be reduced by taking a single-phase supply from it to supply our considerable needs at 230 V (including most notably an electric instead of gas cooker) was not explained.

I found that 55 kVA came out well below what the method of calculating the capacity of a 3-phase supply at 230 V per phase with three 100 Amp fuses showed, so I queried this.

DNO replied that the fuses would be 80, not 100 Amps and that the "100 A" fuseholder fitted to our present single-phase supply "probably contains an 80 Amp fuse, anyway".

I told the DNO that:-

1. I saw the fuse when it was fitted (that's true), and was sure that it is a 100 Amp (see below) ;

2. 55 kVA, at 415(?) Volts, with up to 18.4 kVA at 230 V taken off it, would only just allow the power level which we would need for an electric c.h. boiler and fast EV charger, plus single phase appliances which might well draw close to 18.4 kVA during peak use times.

I got no reply ! We decided put the idea of installing 3-phase on hold, pending the appearance of a hopefully less murky situation regarding the future of gas. But the now very high price of gas, and the possibility that it may fairly soon, at, say, around 70% overall efficiency here, start to make electricity competitive on price, have revived our interest in installing 3-phase.

Your comments on what we were told by the DNO would be much appreciated.

Please also be kind enough to explain how much of the proposed 55 kVA capacity of a 3-phase supply would be available taking into account a dependent 80 Amp single phase supply of which, for reasons already mentioned, we would expect to make fairly full peak use (7 kW cooker. 3.8 kW convector heater, plus all the usual rest).

I confess that I am no longer 100% certain about the capacity of the upgraded main fuse fitted, though I did see it and would surely have protested had it been an 80 Amp! What is the least expensive way of getting this checked, so the security wire is replaced and the DNO cannot accuse me of having tampered with it?

If is confirmed as 100 A, would it have been worthwhile paying for what may be useless "ammunition" when arguing with the DNO?!

 

[Lucien Nunes]

2. 55 kVA, at 415(?) Volts, with up to 18.4 kVA at 230 V taken off it, would only just allow the power level which we would need for an electric c.h. boiler and fast EV charger, plus single phase appliances which might well draw close to 18.4 kVA during peak use times.

I understand your concern but need to clarify this. 18.4kVA is 80A at 230V which means that with a limit of 80A on the cutout fuses, if the single-phase loads are all connected to one phase, the entire capacity of that phase is used up leaving no 3-phase supply capability at all. The other two phases would be standing idle.

What you would need to do is share the single-phase loads across all three phases. You might not be able to get them all equal, so might have to allow some wiggle-room. Say you reserve 35A of each phase of the 3-phase 80A supply, giving a maximum of 105A or 24kVA single phase with no more than 8kVA in any one lump. That leaves 45A or 31kVA for 3-phase equipment. Is this sufficient?

Of course, we have not considered diversity here, which might not apply to the 3-phase loads but will to the aggregated smaller single-phase ones. It is unfortunate that the benefits to be reaped from diversity are lessened when the many small loads are subdivided into three groups instead of all on one phase, but it's still valuable. You might be amazed what will run on an 80A 3-phase intake.

 

[MJPD29]

If you are drawing 80a per phase, which you wont, your electrician bill will be absolutley eye watering

 

[brianmoooore]

Gas currently costs just over 10p/kWhr, and is available at this price all day, every day.
There is the odd electricity dual tariff that can match this on the off peak night rate, but the corresponding rate for the majority of the day is at least four times the gas price, and increasing.

DNOs will not fit a main fuse that is larger than the max rating of the meter, and will remove larger ones without notice, or informing the customer, if they come across them in the course of other work. This policy has caused problems for more than one of my customers, where supplies that have been fault free for decades have suddenly started blowing main fuses at short intervals.

A 80A fuse does not happily conduct 80A indefinitely, and then blow if the current is increased to 81A. It will never blow at a little over 100A, and even at 200A will last over six minutes.

 

[Carl Haworth]

I understand your concern but need to clarify this. 18.4kVA is 80A at 230V which means that with a limit of 80A on the cutout fuses, if the single-phase loads are all connected to one phase, the entire capacity of that phase is used up leaving no 3-phase supply capability at all. The other two phases would be standing idle.

What you would need to do is share the single-phase loads across all three phases. You might not be able to get them all equal, so might have to allow some wiggle-room. Say you reserve 35A of each phase of the 3-phase 80A supply, giving a maximum of 105A or 24kVA single phase with no more than 8kVA in any one lump. That leaves 45A or 31kVA for 3-phase equipment. Is this sufficient?

Of course, we have not considered diversity here, which might not apply to the 3-phase loads but will to the aggregated smaller single-phase ones. It is unfortunate that the benefits to be reaped from diversity are lessened when the many small loads are subdivided into three groups instead of all on one phase, but it's still valuable. You might be amazed what will run on an 80A 3-phase intake.

Hello, Lucien - do you remember me as a "voice from the (recent) past on this forum"? If I remember correctly, yours was a very helpful voice, particularly as it finally goaded me into understanding why I should not use kW/h when I meant kW h (or kW.h or kW-h)!

Sloppy use of all language is unhelpful, and can be misleading. That obviously includes technical language, so I responded happily to your picking me up on that!

Thanks very much for going to the trouble to reply now.

I based my notion that the single-phase supply for the house would be from one of the three cores of a 3-phase supply on what I picked up on Google. You are obviously right that the s-p loads would have to be shared as 3 x 1 s-p. The DNO did say that "your electrician" would have to balance the s-p load between the 3-cores. I didn't appreciate the significance of this comment!

The DNO also referred to the maximum capacity of the s-p as 80A (18.4 kVA), without explaining how much out of the maximum 55 kVA available via the 3-p would be used up by relevant appliances. Does his "analysis" make any sense to you (it seems unlikely that the simply didn't know what he was talking about- but one can never be sure) ?

Do we really need 3-phase? Both a 15 kW (15 kVA?) boiler and an 11 kW (11 kVA?) EV charger exist in s-phase versions. Anything over, or, in the case of the boiler, seriously over, those ratings, is available only for 3-phase.

We would be abandoning gas, so other significant single-phase electrical loads would be:-

New: Cooker. An AEG one recommended by Which?, with an induction hob, is stated to need a 30 Amp breaker- obviously based on cooker diversity. Include this as (30 x 230) kVA = 69 kVA.

New: Replacement 3 kW output gas convector heater by electrical one of same output: 3 kVA

Sometimes used now: Direct electrical heating of DHW (not by the boiler wet system - that doesn't make sense): 3kW, so 3 kVA(?)

It looks possible for all these loads, plus the 3-p ones, to be on together at some times of the day/night. It might be possible to restrict the EV charger to the overnight hours when the boiler was off, but not always.

The above (new s-p loads) adds up to (69+3+3) 75 kVA. Considerably more with lights etc etc, washing machine, tumble dryer etc etc. The peak load could obviously exceed the capacity of our main fuse, even IF this is 100 Amps. Even more readily if it's 80 A.

Possibly an overall diversity calculation would show that the present supply would be adequate at peak times, but who could be trusted to produce a reliable one?

3-phase, even at 55 kVA, gives a more reassuring picture.

Yes, boiler at 15(?) kVA and charger at 11(?) are well within your suggested 31 kVA maximum load on this part of the proposed supply. That would leave, you say (and you clearly know what you are talking about), (3 x 35) Amps via (3 x 1) balanced single phase, giving a maximum of 24 kVA per phase.

This is not a neat solution, as you imply. It would need the s-p circuits to have three separate consumer units. Dividing these circuits on an existing installation up to achieve roughly the same maximum load per group would involve a lot of head-scratching - and "compromise", and might produce an intermittent overload of the 80 Amp fuse on one of the three phases. It's ideally something to be worked out when installing the mixed supply in a new environment.

Provided any overload was small enough and of short enough duration not to blow the phase fuse, would such imbalances affect the performance of the 3-p appliances (boiler and EV charger)?

Best wishes,

Carl

 

[snowhead]

It would need the s-p circuits to have three separate consumer units.

It would just need one 3 phase consumer unit for the whole dwelling with single phase loads split across the 3 phases.
Supplied from a 3 phase meter.

 

[Carl Haworth]

It would just need one 3 phase consumer unit for the whole dwelling with single phase loads split across the 3 phases.
Supplied from a 3 phase meter.

 

[Lucien Nunes]

Provided any overload was small enough and of short enough duration not to blow the phase fuse, would such imbalances affect the performance of the 3-p appliances (boiler and EV charger)?

No, not measurably.

 

[Carl Haworth]

That's useful to know. Thanks!

 

[Lucien Nunes]

That would leave (3 x 35) Amps via (3 x 1) balanced single phase, giving a maximum of 24 kVA per phase.

24kVA TOTAL, 8kVA per phase.

 

[Carl Haworth]

24kVA TOTAL, 8kVA per phase.

I see. Sorry! I misunderstood what you said. "24 kVA for single phase" doesn't mean 24 per phase. It's the total for the three (3 x 8 kVA).

You suggest leaving 45 A for the 3-phase section (229.63 V per phase, so 230). That is (45 x 230 x 3): 31,050 VA.

DNO said 55 kVA for the 3-phase (230 x 80 x3): 55,200 VA.

The two 3-P appliances proposed need a total of (15 + 11 Amps): 26 A, or 17,940 VA, so I need to allocate only 30 A, not 45 A, to the 3-P (20,700 kVA), still with a margin in hand.

That would leave (55,200-20,700); 34,500 VA for the S-P, meaning 16.67 A per core/group of S-P circuits - a 50 A total allocation to these circuits (34,500/230/3 cores/3 circuit groups); 16.67 A for each group.

Is that right??

If it is, then I would end up with:

3-.P section: 20,700 VA (30 Amps)
S-P section: 34,500 VA (3 x 50 Amps)
TOTAL: 55,200 VA (as stated by DNO)/(180 A. 180/3 = 60 A per phase)

If it this is right, it means that I would have for the S-P circuits 150 A total (3 circuit groups x 50 A each) (34,500 VA) instead of 1 circuit group x 80 A (18,4700 VA).

Looks too good to be true, but the high VA available to the S-P circuits would make it less difficult to divide these into 3 safe groups, and the 3-P allocation seems fine for what it envisaged.

The actual allocations could be varied within the pair of capacities - leaving the possibility of increasing the 3-P load (more powerful boiler and/or V charger?) at the expense of reducing the maximum S-P .load per group of circuits/phase core to bring this closer to present 80 A (or 100 A?) rather than 150 A.

However. Lucien, I stand fully prepared to be corrected - I fear that my result, although carefully checked, may indeed be found by you to be "too good to be true"!

Best wishes,

Carl

 

[Lucien Nunes]

TOTAL: 55,200 VA (as stated by DNO)/(180 A. 180/3 = 60 A per phase)

Actually 55,200 / 230 = 240A not 180A. Hence 80A 3-phase supply as per DNO.

That would leave (55,200-20,700); 34,500 VA for the S-P, meaning 16.67 A per core/group of S-P circuits - a 50 A total allocation to these circuits (34,500/230/3 cores/3 circuit groups); 16.67 A for each group.

By groups, I only meant assigning the loads into three groups for connection to the three phases. Not further subdividing them. (I am refraining from using the word balancing). So with 30A reserved for the 3-phase loads, there would be three groups of loads each of which could have the remaning 50A of each phase available.

If it this is right, it means that I would have for the S-P circuits 150 A total (3 circuit groups x 50 A each) (34,500 VA) instead of 1 circuit group x 80 A (18,4700 VA).

Exactly.

I mentioned avoiding the use of the word 'balancing' because it is not strictly necessary to balance these loads. It is helpful to do so in many cases for two reasons. One is to make the best use of diversity, so that none of the groups has an excessive tendency to exceed its diversified total under unfavourable usage conditions. The other is to attempt to maintain equal voltage drop on all conductors of the supply, to ensure that the voltage seen by 3-phase loads is as nearly symmetrical as possible. That would be of interest in a factory with lots of 3-phase induction motors, for example, which are at their most efficient when the line voltages are equal. If all the baseline single-phase load were allocated to one phase leading to it being fully loaded most of the time, while the others were only loaded intermittently, the voltage drops would often be lopsided and a very small percentage of efficiency thereby wasted. But some types of 3-phase loads, such as boilers and active rectifiers such as are used in car chargers, are not affected by voltage assymmetry in the same way, so that consideration has less impact in your application.

 

[Carl Haworth]

Actually 55,200 / 230 = 240A not 180A. Hence 80A 3-phase supply as per DNO.



By groups, I only meant assigning the loads into three groups for connection to the three phases. Not further subdividing them. (I am refraining from using the word balancing). So with 30A reserved for the 3-phase loads, there would be three groups of loads each of which could have the remaning 50A of each phase available.



Exactly.

I mentioned avoiding the use of the word 'balancing' because it is not strictly necessary to balance these loads. It is helpful to do so in many cases for two reasons. One is to make the best use of diversity, so that none of the groups has an excessive tendency to exceed its diversified total under unfavourable usage conditions. The other is to attempt to maintain equal voltage drop on all conductors of the supply, to ensure that the voltage seen by 3-phase loads is as nearly symmetrical as possible. That would be of interest in a factory with lots of 3-phase induction motors, for example, which are at their most efficient when the line voltages are equal. If all the baseline single-phase load were allocated to one phase leading to it being fully loaded most of the time, while the others were only loaded intermittently, the voltage drops would often be lopsided and a very small percentage of efficiency thereby wasted. But some types of 3-phase loads, such as boilers and active rectifiers such as are used in car chargers, are not affected by voltage assymmetry in the same way, so that consideration has less impact in your application.

 

[Carl Haworth]

Thank you very much, yet again, Lucien, for your response to my wondering if I'd got something wrong in my calculations, given that they showed such a "vast" capacity for the single-phase circuits (albeit divided into three groups).

And thank you also for your further enlightening on diversifying the peak loads possible on all the installation's circuits, so as to apportion these into groups which will each run reliably on c. 50 Amp supply limit (depending on load on 3-Phase). All that we know is that our peak loads currently produce no problems with what may be either a 100 A or an 80 A main fuse.

Before we had the new main fuse fitted, over 20 years ago, the installation was on a 1968 fuse box with a 60 A main fuse. There was no space for extra circuits:-

One fuseway for the front door chime transformer, 3 for lighting circuits (one of which supplied a wall-mounted heater in upstairs bathroom, and nothing else), one for an electric cooker (not used by us), one for the immersion heater/s, and two for ring circuits (each of which covered about half the floor area of both storeys of the house).

We had this replaced by a split consumer unit with twelve breaker slots and a RCCB for numbers 7 to 12.

This now has the following circuits, all with B breakers except 2:-

NOT VIA RCCB
1. Upstairs lighting, with inline RCD for bathroom) and forced ventilation (original circuit for only heater in bathroom - no longer in place).
2. Air conditioner (20D). Supplier of this system stipulated non-RCD-protected circuit.
3. Downstairs lighting (including outside, garage and workshop lights via RCD) plus door chime transformer
4. Downstairs lighting.
5. Downstairs lighting (original circuit)
6. Upstairs lighting (original circuit)

VIA RCCB
7. Ring
8. Ring
9. Immersion heaters (either/or)
10. Ring (original circuit)
11. Immersion heaters (original circuit)
12. Air compressor in my workshop (originally electric cooker circuit. The present appliance needs a circuit of its own with a 32B breaker because of high starting current (3.5 h.p. motor, so 20Dor lower would probably be more appropriate). But spike is largely avoided if compressor is started using the decompressor control.

The electrician who did most of this work advised uprating the main fuse from 60 to 100 A. We followed his advice, as much an anything to put the system into line with then-recent ones. It's conceivable that 60 would have remained OK.

Apportioning these circuits into three groups with roughly similar peak loads looks difficult!

ANOMALIES Which might be overcome when apportioning them into three groups of 4+5+5, say, rather than 4+4+4 (say):-

- One lighting circuit is for lighting plus door chime transformer which, for obvious reasons. it is undesirable to have on a shared circuit.

- If we dump gas completely (the daily standing charge becomes an increasing burden as consumption falls) we would need an extra 32B circuit for the cooker, whose wiring is in place in the ceiling void above the kitchen (with plenty of spare cable to drop down) and , at the other end, ready to connect to the present consumer unit.

Please don't feel obliged to respond to the immediately above. You will have better things to do! It's up to us to get it sorted out appropriately.

ASSYMMETRIC 3-PHASE VOLTAGES

Obviously one should aim to keep these as symmetric as possible, which is relatively straightforward when a complete installation circuit is being designed, but has to use hard-to-estimate-reliably data when the component circuits are established and working!

I suppose that a voltage drop on one phase relative to the other two (or on two relative to the other one) would simply reduce the wattage, and probably only slightly, of a non-induction-motor 3-phase appliance?

What is the effect of asymmetric voltage drop on an induction motor - just less torque, or reductions and increases which cause possibly fluctuating speed as well as loss of output power?

Very best wishes, and many thanks again for your time,

Carl

 

[Carl Haworth]

Gas currently costs just over 10p/kWhr, and is available at this price all day, every day.
There is the odd electricity dual tariff that can match this on the off peak night rate, but the corresponding rate for the majority of the day is at least four times the gas price, and increasing.

DNOs will not fit a main fuse that is larger than the max rating of the meter, and will remove larger ones without notice, or informing the customer, if they come across them in the course of other work. This policy has caused problems for more than one of my customers, where supplies that have been fault free for decades have suddenly started blowing main fuses at short intervals.

A 80A fuse does not happily conduct 80A indefinitely, and then blow if the current is increased to 81A. It will never blow at a little over 100A, and even at 200A will last over six minutes.

Thanks for that!

Electricity cost on my Octopus tariff is now, including daily standing charge, not much more than 2x similarly calculated price of electricity. I'm assuming that heat energy obtained by us from gas is about 75% of total heat energy supplied (we have oldish boiler at about 76%, gas cooker at considerably worse than that, and one gas convector heater which claims 80%).

Your comment about main fuse capacities does make me wonder how technically -wise (rather than just fitting-wise) the two men were who were sent by the DNO to uprate our main fuse all those years ago. I went to the trouble to point out the 40 Amp max marking on the meter (and the original fuse was a 60 Amp). They assured me that the meter was fine for 100 A. I doubt that we draw anything like this (though possibly now more than 60 A at times of peak load). The change was nearly 20 years ago and the meter still works fine.

Should someone more cautious/knowing from DNO spot this, wouldn't it make more sense for us to change the meter (which would have to be a Smart one)? Octopus fit these foc (we all know why foc!), if the fitting of an inadequate fuse was proposed?

 

[Lucien Nunes]

What is the effect of asymmetric voltage drop on an induction motor

Consider a 3-phase motor with three windings, each independently contributing a fraction of the torque needed to drive the rotor. The magnitude and phase angle of current in each winding depends on many factors but under ideal conditions all those factors will be equal and so will be the currents and the torque contributions of the three phases. If the voltage of one phase falls, the tendency for them all to equate will cause a shift in equilibrium that attempts to restore that voltage by transferring workload onto the others and increasing their currents. Because of the non-proportionality of some of the losses this results in higher total heat dissipation, as well as reduced overall torque capability and increased torsional vibration.

In the extreme case of one phase of the supply being completely lost, the motor will not only attempt to drive its mechanical load on the power available from the remaining two, but can also absorb power from them to regenerate the missing phase and attempt to energise any other loads connected to it. This normally results in heavy overcurrent and tripped control gear, although there are cases where a big motor on light load would carry on regardless and special precautions need to be taken to ensure the machine shuts down promptly. Conversely, the phenomenon can also be exploited as a means of converting single-phase supplies to 3-phase, using an 'idler' motor that does not drive a load, in conjunction with capacitors to compensate the reactive power flows.

 

[Carl Haworth]

Consider a 3-phase motor with three windings, each independently contributing a fraction of the torque needed to drive the rotor. The magnitude and phase angle of current in each winding depends on many factors but under ideal conditions all those factors will be equal and so will be the currents and the torque contributions of the three phases. If the voltage of one phase falls, the tendency for them all to equate will cause a shift in equilibrium that attempts to restore that voltage by transferring workload onto the others and increasing their currents. Because of the non-proportionality of some of the losses this results in higher total heat dissipation, as well as reduced overall torque capability and increased torsional vibration.

In the extreme case of one phase of the supply being completely lost, the motor will not only attempt to drive its mechanical load on the power available from the remaining two, but can also absorb power from them to regenerate the missing phase and attempt to energise any other loads connected to it. This normally results in heavy overcurrent and tripped control gear, although there are cases where a big motor on light load would carry on regardless and special precautions need to be taken to ensure the machine shuts down promptly. Conversely, the phenomenon can also be exploited as a means of converting single-phase supplies to 3-phase, using an 'idler' motor that does not drive a load, in conjunction with capacitors to compensate the reactive power flows

Consider a 3-phase motor with three windings, each independently contributing a fraction of the torque needed to drive the rotor. The magnitude and phase angle of current in each winding depends on many factors but under ideal conditions all those factors will be equal and so will be the currents and the torque contributions of the three phases. If the voltage of one phase falls, the tendency for them all to equate will cause a shift in equilibrium that attempts to restore that voltage by transferring workload onto the others and increasing their currents. Because of the non-proportionality of some of the losses this results in higher total heat dissipation, as well as reduced overall torque capability and increased torsional vibration.

In the extreme case of one phase of the supply being completely lost, the motor will not only attempt to drive its mechanical load on the power available from the remaining two, but can also absorb power from them to regenerate the missing phase and attempt to energise any other loads connected to it. This normally results in heavy overcurrent and tripped control gear, although there are cases where a big motor on light load would carry on regardless and special precautions need to be taken to ensure the machine shuts down promptly. Conversely, the phenomenon can also be exploited as a means of converting single-phase supplies to 3-phase, using an 'idler' motor that does not drive a load, in conjunction with capacitors to compensate the reactive power flows.

 

[Carl Haworth]

Dear Lucien,

I'm afraid that I got my figures badly wrong yesterday for the A drawn by the two 3-p appliances. It did look "too good to be true", and I now see that it was.

That was because I said 26A total for the 15 kW boiler and 11 kW charger. Nonsense! 26 is 15 +11. These are the kVA of the two appliances!

It is all too clear that I didn't know how to calculate the current drawn by an appliance of known wattage on UK 3-phase.

How to do this?

Obviously I should not use line to neutral voltage, because this is for S-P

Do I use UK 3-P line to line voltage quoted as 230 x 1.732: 398.36 V?

If so, the proposed 15kW boiler at pf1 draws 37.65A

(Wrong, I suspect!

Why? Because calculator from kW to Amps available via Google gives, for this wattage, and 400V line to line, 21.76A.

Isn't line to line voltage (single phase voltage x 1.732), which gives almost the same value: 398.36V (presumably rounded up by calculator to 400?).

Is the calculator right, and the A is 21.76?

If Yes, can you say how it arrives at this? (No indication given of how calculator works it out. Must be a complex calculation.)

Same calculator gives 15.96A for 11kW charger (also at pf1).

If these values are correct, my 3-P needs 38A (round figures), so say 40A, meaning (at 230V x 3) 27,600VA.

That would leave 40A for S-P, meaning 120A total, so (40 x 230 x 3) VA total: also 27,600VA.

Grand total VA is 55,200, as propsed by DNO (3 x 80 x 230).

Any total amperage at or above 80 should be fine for S-P. As you say, the devil is in the aggregating there to produce 3 reasonably similar and representative loads.

Cheers!

Carl

 

[Lucien Nunes]

You can consider a balanced 3-phase load to be three single-phase loads of 1/3 the power. In the case of electric heating that is often a true representation of what is inside.
So 15+11=26kVA consumes
26000 / 3 / 230 = 38A

You still have 42A available or 126A for the single-phase stuff. Actually is the boiler only 15kVA?

For convenience, one can still calculate this way even for loads that don't have a neutral and operate at 400V connected between the lines. In this case, the calculation ought to be
kVA / sqrt(3) / 400 but the answer for the line current is the same. This is intuitively logical because the line current depends on the power consumed, not how the individual coils are connected within the machine.

The reason for the equivalence is that the loads or coils, when delta-connected at 400 volts between pairs of lines, pass a current of kVA / 3 / 400 i.e. the load can still be thought of as divided into three equal ones. But because of the delta cinnection, the line current is now the vector sum of a pair of coil currents which is higher by a factor of sqrt (3).

In normal 3-phase working, one does not routinely say or think 'per phase'. Circuit current always represents that in one conductor, regardless of the number of conductors. TBH it grates when people unaccustomed to 3-phase systems say 'I've got a pump that takes 5A per phase' or similar, or ask whether that makes 15A total. After all, one does not declare the line and neutral currents separately in a 2-pole device, a 100A DP main switch is not rated at 200A on account of both the line and neutral being good for 100.

But there is nothing sneaky about looking under the hood of a 3-phase device and finding it's just three 230V devices in one box.

 

[Carl Haworth]

You can consider a balanced 3-phase load to be three single-phase loads of 1/3 the power. In the case of electric heating that is often a true representation of what is inside.
So 15+11=26kVA consumes
26000 / 3 / 230 = 38A

You still have 42A available or 126A for the single-phase stuff. Actually is the boiler only 15kVA?

For convenience, one can still calculate this way even for loads that don't have a neutral and operate at 400V connected between the lines. In this case, the calculation ought to be
kVA / sqrt(3) / 400 but the answer for the line current is the same. This is intuitively logical because the line current depends on the power consumed, not how the individual coils are connected within the machine.

The reason for the equivalence is that the loads or coils, when delta-connected at 400 volts between pairs of lines, pass a current of kVA / 3 / 400 i.e. the load can still be thought of as divided into three equal ones. But because of the delta cinnection, the line current is now the vector sum of a pair of coil currents which is higher by a factor of sqrt (3).

In normal 3-phase working, one does not routinely say or think 'per phase'. Circuit current always represents that in one conductor, regardless of the number of conductors. TBH it grates when people unaccustomed to 3-phase systems say 'I've got a pump that takes 5A per phase' or similar, or ask whether that makes 15A total. After all, one does not declare the line and neutral currents separately in a 2-pole device, a 100A DP main switch is not rated at 200A on account of both the line and neutral being good for 100.

But there is nothing sneaky about looking under the hood of a 3-phase device and finding it's just three 230V devices in one box.

Dear Lucien,

Many thanks again!

Ah, it was OK (I later got cold feet, and decided that I had been wrong!).

It's OK only if one knows the kVA of an appliance, or if it is obvious that this must be the same as, or very close to, the kWh rating.

Given that the appliances in question (even the charger?) are predominantly non-inductive, the kVA should be either identical, or very close, to the kWh. The boiler's element, like that of an electric kettle or immersion heater etc. wound using an alloy such as nichrome, with a highly positive coefficient of resistance, will draw a starting current much higher than the operating current. This will very quickly drop as the winding heats up. so the appliance ends up running at a kVA more or less (the winding form, especially if spiral, may well be inductive) identical to the quoted kWh.

So the current (A) calculation is:-

USING kWh AS kVA IF ONLY kW IS GIVEN, AND IF THIS IS OBVIOUSLY VERY CLOSE TO kVA

kVA (of appliance)/3 (phases)/230 (V per phase)

So the two appliances together (15,000VA+11,000VA) work out, as you say. at :-
26,000/3/230 = 37.68A,
38A rounded.

The same calculation can't be done with the 3-P phase to phase line voltage at 398.36V (230 x 1.732) because this does not use (one phase V x 3), so gives a very different result (65.27A!!).

The correct calculation leaves, as you say, (42A x 3 single phases) 128A total for the S-P installation.

The 42 could sensibly be rounded to 45, leaving (35A x 3: 105A total) for the S-P circuits

There can be give on one side and take on the other, or vice-versa.

BOILER: IS 15,000 W SUFFICIENT (Your implied question)?

Our 1997 non-condensing gas boiler generates, in Wh. about (28,000 x 0.76): 21,280 of heat energy. This includes DHW heating. This boiler is probably a bit over-sized, though not to the monstrous extent of some of the combi boilers installed in more in recent years.

So, with DHW being heated directly at 3,000 Wh by one of two S-P immersion heaters rather than by a boiler circuit ,15,000 Wh may be enough for this house, which is much better insulated now than it was in 1997. Coincidentally (and importantly!), it appears that any electrical boiler with an output higher than 15 kW has to be floor-standing. We can't accommodate this - only a wall-hung one.

The heat requirement would have to be worked out on the basis of a proper energy survey of our house.

We could accommodate, an 18,000 Wh boiler:

3-P: (18,000 + 11,000)\ 230\3 = 42A. Say 45A.

That leaves 3 x 35A (105A) for S-P. Should be fine. Given the circuit/splitting/diversity problems it would be sensible not to reduce S-P total below 100A.

THE AMPS-FROM-kWH CALCULATOR THAT I FOUND VIA GOOGLE

This is at

Kilowatts to amps (A) conversion calculator

Kilowatts (kW) to amps (A) conversion calculator.

www.rapidtables.com

fUsing 26 kWh, this calculates (I) 21.77 Amps with voltage entered as 3-phase line to line 398V, and (ii) 37.68 A for 3-phase 230V line to neutral.

No wonder I became unsure!

This calculator looks serious, though calculation methods are not displayed.

What do you make of the current levels that it calculates? They're clearly not right - at least for UK 3-phase. (ii) is closer than (i), which is way out.

Many thanks again, and best wishes,

Carl