Could someone help me calculate maximum demand for my house?

[thomp1983]

Hi all, I'm applying to my dno to have my cutout fuse upgraded to 80a ahead of having am evse installed and they want to know the maximum demand.

Both consumer units contain all the following circuits and I've listed the mcb or rcbo size too.

Cooker circuit b40
Downstairs sockets b32
Garage sockets b16
Upstairs lights/garage lights b6
Shower b40
Upstairs sockets b32
Kitchen sockets b32
Air conditioning b16
Pv/solar 32a rcbo

That's all the currently connected circuits and the evse will be added on a 40a rcbo.

I know you apply diversity to the circuits due to not everything being in full use all the time but I don't know how to do all the maths.

Is there any other information required to calculate maximum demand such as number of outlets/sockets on each circuit etc.?

Thanks,
Chris.

 

[Murdochcat]

60A will be fine

 

[ericmark]

I good question, in my own house I have solar, and as a result I have the solar software showing me what I am using.

There is a particular time of the day which is critical, 00:30 to 05:30 when I have off-peak so I run dish-washer, washing machine, tumble drier, and charge batteries. and one can see I have peaked to 4643 kW = 20 amps, using the shower is likely the largest load, bit misleading as shows 5.905 kW but the shower uses more like 8 kW and it is the fact some things show direct and some go to a server so there is a slight time shift, which results in a spot error.

Smart meter used ½ hour blocks so also not that accurate, so the blip over 16 hours shows 0.308 kWh which since in kWh not kW shows very little.

So in real terms, an EV will use 7 kW = 31 amps approx, so the question is what will be used at the same time, with something like Octopus Go your off-peak is 00:30 to 05:30 and the question is what else will be used at that time?

So if we assume by 00:30 you will be in bed, for me there are only three items I could use in those hours, dish-washer, washing machine, tumble drier, unlikely I will take a shower then or cook a meal. So I can't set the time to switch on, I have to set a delay, in either ½ hour or 1 hour slots, depending on how far in advance I am setting them, each has some form of heating, but in the main heating water for a short time, so the chance that two heating elements will turn on together is slim, but possible, but fact is I have never blown the 60 amp DNO fuse, and looking at use over the years since solar and off-peak, I have not seen the software show anywhere near 13.8 kW = 60 amps even when charging my batteries at 4 kW, so your EV will take around 13 amp more than my batteries, so it seems unlikely it will ever exceed 60 amps.

I can understand the DNO asking on a new build, but on existing they should realise the 60 amp existing fuse has never ruptured so adding another 31 amps at night with an 80 fuse, it would be highly unlikely that the 80 amps will every be exceeded.

Theory with two electric showers and a battery I could exceed 60 amps, but I never have.

 

[thomp1983]

The calculator on tradecalcs.co.uk shows a max demand of 139a so I've supplied that figure to national grid and I'll see what they come back with, so far they emailed this morning asking for a photo of the cut out.

I understand that a 60a fuse would be fine as the evse ct clamp will reduce the load if it sees it getting close but if I can have it upgraded to 80a or 100a so I can avoid that situation then I will, I want to be able to use the full capacity of the evse if and when I need to ideally.

Thanks,
Chris

 

[blup]

Technically 60A but upgrade to 100A as the demand for electricity is only going to increase.

 

[flameport]

The calculator on tradecalcs.co.uk shows a max demand of 139a

 

[thomp1983]

Are you suggesting it's wrong John?

 

[flameport]

Rather obviously, if the max demand was 139 amps, the main fuse would have been blown out long ago.

 

[thomp1983]

I get what your saying John, that max demand does include the new load from the new charger which is 40a as diversity isn't applied to it.

I also can't find what diversity rating would be applied to our solar install so if you assume that's 0 too then that would be 72a that diversity can't be applied to.

I understand nothing is ran at max all the time, especially as we're a 2 adult 1 child house but I had to supply a figure and the only calculator that was reasonably quick to use without going around adding up all the light bulb wattages and socket outlets was the one I mentioned above.

 

[flameport]

I also can't find what diversity rating would be applied to our solar install

You won't, as no such thing exists.

There are a selection of items regarding diversity in the On Site Guide, Appendix A, that states assumptions such as 66% for lighting circuits, 100% for the first and 40% for other socket outlets and so on.
Before it was in the OSG, all of that appeared in the wiring regulations. It has been used extensively in all kinds of training materials, books, courses, online calculators and the rest.
It hasn't been revised in any meaningful way since it first appeared, which was in 1934. Yes, 92 years ago.
There is zero evidence to suggest it was ever valid, or where the information originally came from. Highly likely it was made up figures by someone or other and shoved in there so that electricians could have something to use.

The nonsense in another table about lighting outlets being assumed to be 100W each has recently been slightly amended to suggest that LED and CFL lamps will use less, but the bulk of that table hasn't changed since it was created in 1981 - and once again, no evidence to suggest it was ever valid or appropriate.

Using these 'calculators' based on junk created nearly a century ago just ensures that actual demand is massively overstated.

the new load from the new charger which is 40a as diversity isn't applied to it.

EVSEs are 32 amps maximum, and in most cases will have load monitoring so that the EV load current will be reduced when other parts of the installation are over a set threshold.
They are often installed on 40A MCBs / RCBOs , but that is to avoid operating a 32A device at it's full load current for hours at a time which can cause overheating.

I also can't find what diversity rating would be applied to our solar install

Solar is a generation source which in terms of a grid supply will reduce the demand. At the same time it allows an installation to use more current than would normally be available from the grid which has implications for the rating of consumer units and the devices in them.


Some other things to consider:
Long ago, a typical modest house had a 3 way fusebox.
One 5A lighting circuit for all of the lights in the house.
One 30A ring circuit for all of the socket outlets.
The third circuit was 30A for a cooker.

Then that house was rewired, and a shiny new consumer unit was installed with many more circuits.
Now there are two lighting circuits of 6A each for upstairs and downstairs
and three 32A rings for downstairs, upstairs and the kitchen.
The cooker circuit is still there, now 32A.

Using any of these calculators or the slop in the OSG on the old and new installations above will basically double the total demand in the 'new' installation.
However the number of lights, outlets and other items has not changed at all.

 

[ericmark]

which has implications for the rating of consumer units and the devices in them

That is very true, I have a 60 amp DNO fuse, and solar can add 22 amps to that, although not for too long, so can be considered as 20 amps, so I could upgrade to an 80 amp fuse, and the consumer unit would still be within its 100 amp rating, but if I want a 100 amp DNO fuse, then it would require some other overload device to ensure the consumer unit could not be asked to handle over its 100 amp rating.

In real terms it is highly unlikely I will ever use over 100 amps, in fact unlikely I will use over 60 amps. But I have seen many a consumer unit fitted with 63 amp RCDs which supply over a total of 63 amps worth of MCBs, which is OK when on a 60 amp DNO fuse, but not if that fuse is upgraded to 100 amps.

As to if the person who upgrades the fuse actually looks for that I don't know. Theory the should, but not sure what happens in practice?

As the owner/occupier I can ensure both showers not used at the same time, if I had lodgers or rented out part of my house, I would loose that control, and would need to consider how to stop it.

In the main we are well aware our inverter is only 5 kW, so I try to ensure during peak times I do not draw over 5 kW. I was talking to my son on Thursday about batteries, and solar, and he was saying how he favoured the tesla Powerwall, as it has a large inverter build in which could supply the 7 kW the car can draw, since I don't have an EV, that is not a problem for me.

I looked it up Tesla Powerwall 3 "On-grid maximum continuous discharge power is 11.04 kW to 11.5 kW (depending on the regional configuration; up to 15.4 kW off-grid or peak under specific conditions)." so that's 48 amps. Even with a 60 amp DNO fuse that takes it over the 100 amp rating of the consumer unit. The Tesla Powerwall 2 is 5 kW same as the inverter I am using.

My son leases his EV from Octopus, and it is a complete package, so he pays less for off-peak to me, even when we are both using Octopus, and he had to let Octopus fit the EV charging socket, and they kept it completely independent to rest of house, even when there was room in the brand new consumer unit, their installer has to do as he is told, so could not use the existing installation. The same with me when I got solar panels etc, I had to use an installer who could issue the paperwork required, and let them do all of the install.

Both my son are electricians, well I suppose with our degrees we are electrical engineers, in my case retired, in his case still on the up, just had his interview to become charted, but neither of us can work on many of the electrical installations in our own homes, due to Part P building regulations to DIY becomes too expensive to be worth while. Due to fees charged by LABC.

So at the end of the day, your unlikely to be able to DIY the installation without silly fees and hoops to jump through, so just leave it all for those you are paying to do the work to sort out.

 

[Gridcap]

Chris, I'd go back to National Grid with a more realistic figure before they assess it. The 139 A is mostly breaker ratings added together: the 40 A charger circuit and the 32 A solar circuit at full value, with diversified house load on top.
Two things bring it down. The solar shouldn't be in an import figure at all, because when the panels are generating the house draws less through the cut-out, not more. And the charger draws what it's set to, about 32 A for a 7 kW unit, not the rating of its breaker. ericmark's point about the consumer unit rating is a separate check for your electrician.
For the rest of the house, your solar app gives you real numbers. Take your highest evening peak, plus a margin, as apps usually show averages, then add 32 A for the car, and you have a maximum demand you can explain. The ENA guidance for EV chargers and heat pumps also lets a load limiting device that monitors the whole house be counted, so tell them the charger has a CT clamp and what limit it's set to.
A figure well over 100 A on a single-phase supply may make them look at a bigger job than a fuse change, so it's worth correcting now.
Alan P.

 

[JohnD]

The calculator on tradecalcs.co.uk shows a max demand of 139a so I've supplied that figure to national grid and I'll see what they come back with, so far they emailed this morning asking for a photo of the cut out.

I understand that a 60a fuse would be fine as the evse ct clamp will reduce the load if it sees it getting close but if I can have it upgraded to 80a or 100a so I can avoid that situation then I will, I want to be able to use the full capacity of the evse if and when I need to ideally.

Thanks,
Chris

139A is highly improbable. The calculation is not correct.

I too use energy monitors. I occasionally top 6kW if I am boiling a kettle at the same time the tumbledrier or some other appliance is runnjng.

Since washing machines, dishwashers and ovens only take full load when heating up, which might be around ten minutes per cycle, it is rare to use much more as they don't all heat up at the same time. Your lighting is a negligible load, most of the things you plug into your sockets are trivial.

An immersion heater might run continuously for an hour or two.

Your Solar is not a load.