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.