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

[Lucien Nunes]

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.

I tested it and did not get this problem. There might have been a bug which they have since fixed, or some input values were not correctly refreshed when you entered them in your browser. For a given power it gives me the same current for 400V line-line and 230V line-neutral, whereas the 21.77A you got indicates it had taken the 398V as line-neutral.

an alloy such as nichrome, with a highly positive coefficient of resistance, will draw a starting current much higher than the operating current.

It's not that high. There are various alloys with different values of α and I am struggling to get agreement between various sources on the WWW as to the tempco of alloys normally used for heating elements, I think some of the data I am seeing is simply wrong. I am not near my library to check in a grown up text book but will revisit this. Anyway, the cold current of a normal heating element that runs below red heat isn't much higher than its running current. The only common example of a true resistive load with a high inrush is the tunsgten lamp; tungsten has a higher tempco and the operating temperature might be 10 times higher than a heater, so the effect coud be two orders of magnitude greater.

the winding form, especially if spiral, may well be inductive

At 50Hz you can ignore the inductance, no matter how the element is constructed it will be swamped by the resistance. The self-inductance of a wirewound resistance becomes important at radio frequencies, while at the other end of the scale, the physical disposition of conductors working at 50Hz becomes important on very low impedance circuits such as busbars carrying thousands of amps.

Given that the appliances in question (even the charger?) are predominantly non-inductive,

I haven't analysed the behaviour of EV chargers, but I expect they follow that of large SMPSUs with active power-factor correction. In the bad old days, early line-fed switchers simply rectified the mains and smoothed it, resulting in a rather wonky current waveform with poor distortion power factor that was somewhat capacitive. These days, the general idea is to use a DC-DC converter driven by the PFC control that converts the instantaneous AC voltage to the line side DC bus voltage, so that the instantaneous current can be actively modulated into a reasonable sinusoid in phase with the voltage, resulting in good pf both in terms of displacement and distortion. I need to research what the real-world performance is, but top of head would expect EV charging to be no worse than 0.8 and probably much better.

 

[Carl Haworth]

FOR LUCIEN NUNES

What is

I tested it and did not get this problem. There might have been a bug which they have since fixed, or some input values were not correctly refreshed when you entered them in your browser. For a given power it gives me the same current for 400V line-line and 230V line-neutral, whereas the 21.77A you got indicates it had taken the 398V as line-neutral.



It's not that high. There are various alloys with different values of α and I am struggling to get agreement between various sources on the WWW as to the tempco of alloys normally used for heating elements, I think some of the data I am seeing is simply wrong. I am not near my library to check in a grown up text book but will revisit this. Anyway, the cold current of a normal heating element that runs below red heat isn't much higher than its running current. The only common example of a true resistive load with a high inrush is the tunsgten lamp; tungsten has a higher tempco and the operating temperature might be 10 times higher than a heater, so the effect coud be two orders of magnitude greater.



At 50Hz you can ignore the inductance, no matter how the element is constructed it will be swamped by the resistance. The self-inductance of a wirewound resistance becomes important at radio frequencies, while at the other end of the scale, the physical disposition of conductors working at 50Hz becomes important on very low impedance circuits such as busbars carrying thousands of amps.



I haven't analysed the behaviour of EV chargers, but I expect they follow that of large SMPSUs with active power-factor correction. In the bad old days, early line-fed switchers simply rectified the mains and smoothed it, resulting in a rather wonky current waveform with poor distortion power factor that was somewhat capacitive. These days, the general idea is to use a DC-DC converter driven by the PFC control that converts the instantaneous AC voltage to the line side DC bus voltage, so that the instantaneous current can be actively modulated into a reasonable sinusoid in phase with the voltage, resulting in good pf both in terms of displacement and distortion. I need to research what the real-world performance is, but top of head would expect EV charging to be no worse than 0.8 and probably much better.

Dear Lucien,

Many thanks!

398V line to neutral. Wow! I've tried it again and got the same results as before. Yes, I refresh after each attempt.

I'll stick with your calculation method!

My comments on nichrome resistances were based on those of a grumpy electrician called in at frequent intervals to re-set the main breaker at a camp site. Apparently the breaker frequently tripped in the early morning when "everyone plugged in his electric kettle". The visits ought to have been expensive for the site owner! (Perhaps the same electrician did the whole installation, and was visiting foc under a warranty - hence grumpiness?) I wonder if an uprated breaker has since been fitted, or if the owner has learned how to do it himself.

Must have been the result of tiny residual peak loads from simply TOO MANY KETTLES!

Yes, the old tungsten/gas bulbs are/were a great (collectively) offender. The mighty starting currents are very brief indeed. and each one is miniscule on the large scale, but big when compared with the running current. The total effect on the grid may have been significant.

This problem should have greatly diminished by now, but is replaced by the one generated by the very high pf of the "eco-friendly" LED bulbs!

Thanks also for the important point about the inductance of wirewound resistances. Obviously they should not mean a poor pf for an electrical boiler (etc).

Your reassuring point about recent EV chargers is also noted.

TWO DIFFERENT QUESTIONS

COLLECTIVE QUESTION 1

- The resistances of a boiler may be connected in Delta. The current toes and froes exclusively through the three phase cores of the supply. That means that the boiler itself \(but perhaps not its control system etc) has no connection to Neutral.

That, to my innocent/ignorant eyes, implies imbalance between lines and neutral when the boiler (and/or the charger?) is working, suggesting that one can't use RDCs of any sort at any points in the circuit - not even RCBOs for the single phase circuits in the consumer unit.

Ah, but do the circulating currents cancel each other out to match their zero V in the Neutral (doesn't this do the same thing at the point of generation, but with star winding)?

- sqrt (3) = square root of 3? (Must be: that's that handy multiple 1.732)

QUESTION 2

I've read (in connection with installing an ASHP) that you mustn't attach a load exceeding 14kW to a single breaker on a single-phase consumer unit. That means a single circuit load limit of 60 Amps. Is that true even if the supply limit is 100A?

If it is correct, it means that you need 3-P for any heat pump wattage higher than 60 (this sort of restriction applies to boilers and EV chargers, as well - where one would expect to install only one of each).

But what if you need, say 20 kW/87A from two ASHP's piggy-backed, each on its own 45A breaker? You have 100 Amps supply capacity and have retained your gas supply for cooking, so you can manage with 13A (2,990 VA) for all the rest of your installation (no EV charger, obviously). It's probably an unrealistic situation, because even two ASHPs piggy-backed probably won't achieve even 60 degrees for your rad circuits in winter without a booster tank, which is usually electrically heated.

But you could retain your gas boiler, or fit a new one, as that worthy lot at the Energy Saving Trust suggest, and use that to heat up a booster tank (and do your DHW).

At least at present, and counting in electrical heating for an ASAP (or GSHP) booster tank, gas costs no more than energy obtained from a total HP setup to heat your house and water. So it should be a valid booster fuel. But I suspect that gas will effectively match electricity in price by the end of this decade.

You don't need anything like such high temperatures for air heating if you can rip out your old system and fit UFH downstairs and either this, or jumbo radiators, upstairs. But in how many older properties is this feasible/acceptable/affordable? You'd probably still need an electrically-heated booster tank for winter extremes, and you'd have to have enough spare electrical capacity to heat your DHW directly by electricity as well as to supply all the rest of your circuits. Or "hybridise" (fine weasel word!) with gas.

Sorry if the immediately above digresses. If it doesn't interest you please don't bother to respond to my musings.

All the best,

Carl

 

[Lucien Nunes]

That, to my innocent/ignorant eyes, implies imbalance between lines and neutral when the boiler (and/or the charger?) is working, suggesting that one can't use RDCs of any sort at any points in the circuit - not even RCBOs for the single phase circuits in the consumer unit.

All the live conductors (= line(s) and neutral) of a circuit always pass through the RCD regardless of how many of them there are. All current that 'goes' via one conductor must 'return' via some or all of the others (Kirchhoff) therefore the RCD will function correctly. It will see an imbalance only in the event of leakage to earth, which is always bad.

 

[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.

Suppose DNO re-confirms (after my last enquiry a year ago) that we can still have 3-phase, but with a limit of 80A per phase (55.20kVA), and that we get this done.

We would notionally allocate the 3-phase part of our installation to two appliances (18kVA boiler and 11 kVA EV charger. That represents 42A, so we would add 3A extra, making the notional total 3-P load 45A (31.05kVA).

The remaining 24.15kVA (3 x 8.05kVA, so 3 x 35A) would provide three S-P supplies for our current installation. If this has an 80A limit (not the 100A I was promised when the main fuse was upgraded some 20 years ago!) it has a capacity of 18.4kVA.

Via a new 3-P consumer unit, we would connect the three phases of the supply to a 3-P output for the two proposed 3-P appliances and three groups of MCBs. We would divide up our existing S-P circuits and aggregate them into three groups, each of which would have to be within the 35A capacity of each phase. As the fantastically helpful Lucien Nunes has pointed out, it may be difficult to apportion a total peak load which is (obviously) comfortably within the capacity of an 80A(?) main fuse into three groups where the peak load does not exceed 35A in any one of them

Provided that last hurdle can be overcome, the above should comfortably cover our present S-P loads and the two new proposed 3-P appliances, plus additional S-P load due to having possibly dumped mains gas completely (mainly an electric cooker).

You mention DNO appearing and, apparently without customers' consent (which they apparently don't need) or forewarning, replacing say a 80A main fuse by a 60A. The 80Ahad been fitted to cover modern needs. 60A has insufficient capacity, so it blows at high consumption peaks.

Is the DNO not required to assess the customer's peak load, which might quite reasonably be over 60A, and discuss with the customer what steps he could take for a 60A main fuse to run unproblematically?

I am thinking here of our own plans. Every time there is a new planning application for any substantial development, the local authority planning officers usually recommend granting it. If the planning sub-committee has to to debate it (only when objections have been received), very rarely do the institutions responsible for the infrastructure in the area concerned (eg, gas, electricity, water, drainage, highways etc etc) object to it. The excuse often heard is "we must not stand in the way of building new houses which are so badly needed".

So the application is passed, and the developer is not required to pay for any serious infrastructure uprating.

The infrastructure is not uprated, or only at the edges/in token (=ineffective) ways

Such factors, as well as the age of our 1966 local electricity sub-station, may explain our own DNO now ruling that local 100A supplies should be downrated to 80 Amps if an increase current draw is needed via installation of 3-phase. (As the local supply gets increasingly over-loaded, that could also be applied to existing single-phase supplies.)

If we get 55.20kVA (3 x 80A main fuse) installed and need 90% (of its capacity (49.68kVA) for our installation, all should be well. But what it the DNO calls to "check our installation", tut-tuts over the 80A fuses and, whatever we do, or threaten to do, replaces those fuses with 60A ones, we will be in trouble. We need almost all the capacity of the 55.20KVA supply, and will not be able to run some of our installation if this is reduced by 25% to 41.40kVA

Please don't think that we are asking you to advise us, but it would be pointless asking the DNO.

I just wonder how seriously we should take the apparent dilemma in which now feel ourselves to be. Should we really be worried, or, in your experience, do you think that the DNO would not not dare to interfere substantially with the supply of a given household's established, and perfectly reasonable, energy needs?

 

[snowhead]

So the application is passed, and the developer is not required to pay for any serious infrastructure uprating.

Yes they are required to pay for upgrades to the existing network if it's required, check the DNO websites and see how much they have to pay.
The Planning approval does not guarantee that capacity will be available, that's for the developer to establish prior to planning applications.
It could be that the development would not go ahead, so no point in even going for planning, due to the high costs of upgrades.


Your 55 kVA 3-phase supply will be part of a contract to supply, they won't just reduce you ability to make use of that by changing fuses.

 

[Carl Haworth]

All the live conductors (= line(s) and neutral) of a circuit always pass through the RCD regardless of how many of them there are. All current that 'goes' via one conductor must 'return' via some or all of the others (Kirchhoff) therefore the RCD will function correctly. It will see an imbalance only in the event of leakage to earth, which is always bad.

Thanks indeed, Lucien,

RE THE ABOVE

If each coil has one of the three phase connected to it, and a delta configuration has only three supply connection points, then isn't it true that no neutral connection is needed for the coils to work?

So how is the current that they draw returned via neutral, so as not to create a current imbalance between any of the phase conductors and the common neutral?

DEALING WITH THE DNO

Would you mind seeing if you could give us some of your always very helpful advice on this one?

My wife saw the original account provided by Brian Moore (about which which I had forgotten), and, understandably, expressed doubts about making ourselves 100% dependent on our electrical supply by having a very expensive job done to install 3-Phase.

I have asked Brian to elucidate the situations that he described, where his local DNO arrived, apparently without notice or consultation, and fitted reduced-amperage main fuses in some of his customers' supplies.

These supplies had presumably, like our present S-P, been uprated previously to cover the much greater electrical loads imposed by equipment etc which was not contemplated when the original installations were made.

The effect was that some of the new lower-amperage main fuses keep blowing, Brian said.

Brian hasn't answered, but I'm now worried, too, by what he said. When I contacted our own DNO a year ago, and he said we could have 3-phase, but with 80, not 100, Amp fuses (he cast doubt on whether our own uprated main fuse is really a 100A), he sounded "take it, or leave it"- no discussion, no interest in whether 55.2kVA would be sufficient for us, etc.

I now estimated that we would need close to the 55.2kVA, which, at least a year ago, was on offer.

If local housing growth continues as it was doing up to 2019/20, and if there is "no money" to upgrade seriously our local substation, that may well explain why our DNO refused in 01/2022 to countenance 69kVA (3 x 100A).

By now they might be refusing to countenance a 3-P upgrade by us to 55.2kVA, and offer us only 41.4kVA (60A x 3) - I haven't checked.

Without a suitable electrician to do accurate(!) calculations for us, I feel pretty sure that we will need quite close to all of 55.2kVA. I'm sure that we would not be able to run on a 25% lower kVA (41.4).

From what Brian reports, DNOs appear to be a law unto themselves, and can reduce supply capacities radically without informing, let alone consulting, their customers.

What is your reaction to all this? My wife thinks now that we should drop the plan for 3-P, at least if this makes us so very dependent on its being 55.2kVA. as seems to be the case. She says, which is very sensible, that we need two separate fuel sources. HP would almost certainly not give us enough heat energy to reduce our electrical load, and is bulky and clumsy indoors with its need for a big booster tank. So she thinks that we should stay with gas for heating and cooking, and hope that the 80/20 plan works out and that the price of this fuel will not in reality, as I feel is quite likely, increase enough over the next 5 or so years as to make its cost in usable heat energy very close, or even equal, to that of gas.

What are your views here? Is it really the case that DNOs can behave like "little Hitlers", and create big problems for electricity users without any discussion beforehand of how a household might manage with a substantially reduced supply capacity?

Before doing this, if you would be kind enough.to do it, you may wish to cast your eyes over what I wrote to Brian after my wife had read his earlier post. Its not very far up the present thread.