Maximum Demand Query

[ukzerp]

We are trying to find someone to fit an EV charger. The most epensive part of this would be laying 20m+ of buried cable under block drive/concrete path/etc.
I am retired and have the tools and experience to do this groundwork but obviously want an electrician to do all the connecting up and testing. Finding someone willing to do "qualified electrician" part is proving much more difficult than I expected.
Anyway - I digress. While looking into this I have been paying more attention to our consumer unit and can see that for some reason that our modest 3 bed bungalow has far more than the usual number of circuits. For example, 2 x 32A ring main, 1 extra 32A kitchen ring main, 32A Hot tub, 32A Garden room, 20A garage, 45A electric shower and more.
Anyway, because of the shear number of MCBs doing the simple calc of 45+40% of the rest gives me a Maximum Demand value of over 120!
With just the two of us would an electrician be able to use another method of calculating the MD or are we going to have fun getting our DNO to allow our 7kW EV charger?
Given we still have a 60A main fuse it's a bit worrying.

 

[Murdoch]

Re running the cable - sure dig the trench and drop a duct in with a pull cord - do this and you won't get any issues down the line - BUT I do recommend you find a EV spark first

As for the maximum load, you do look like you could easily exceed the main fuse and it'll blow - reality says this is unlikely but maybe you need to apply to the DNO to upgrade your main fuse to 100A.

Hope this helps

 

[torbrowser]

Maximum demand is a calculation of appliances in use and there is an allowance for each type of appliance, not necessarily adding up all the appliances together, i use the guide in the IET on site guide.
But even using this requires additional common sense as you can easily go over your main fuse size with the allowances in the guide.
Looking at your loads i would say you are touching cloth with a 60A fuse, you will probably need a 100A fuse fitting before DNO will allow a EV charger.

Shouldn't be difficult to get a local sparky out to do your calculation for a reasonable fee

 

[Spoon]

Wouldn't blowing the main fuse depend on what EV the OP gets, as some monitor the power consumption from the incoming and then alter the charge power accordingly.

 

[binky]

Wouldn't blowing the main fuse depend on what EV the OP gets, as some monitor the power consumption from the incoming and then alter the charge power accordingly.

Most can be limited to avoid overloading and I think the new ones have some form of smart load control that allows DNO to limit power to suit the grid demand.

 

[ukzerp]

Thanks for the replies. 100A fuse upgrade hopefully shouldn't be an issue as the holder is plastic and the meter and consumer unit are rated at 100A.
We would be using (probably) one of the Octopus night time tariffs for charging, so the total load on the system in the early hours should be well within tolerance - even though my wife has her electric blanket permanently set on "Core of the Sun" setting!!

 

[binky]

Thanks for the replies. 100A fuse upgrade hopefully shouldn't be an issue as the holder is plastic and the meter and consumer unit are rated at 100A.
We would be using (probably) one of the Octopus night time tariffs for charging, so the total load on the system in the early hours should be well within tolerance - even though my wife has her electric blanket permanently set on "Core of the Sun" setting!!

Shouldn't be an issue even with a 60A main fuse under those conditions, but I would ask DNO for an upgrade just in case you need to charge during the day.

 

[SPECIAL LOCATION]

I have been paying more attention to our consumer unit and can see that for some reason that our modest 3 bed bungalow has far more than the usual number of circuits. For example, 2 x 32A ring main, 1 extra 32A kitchen ring main, 32A Hot tub, 32A Garden room, 20A garage, 45A electric shower and more.
Anyway, because of the shear number of MCBs doing the simple calc of 45+40% of the rest gives me a Maximum Demand value of over 120!

my observations are as follows:-

It is much better to have more circuits rather than fewer. But just adding an extra circuit will not automatically increase the max demand.
e.g a house with a single 5A lighting circuit, rewired and split onto two 6A circuit will not automatically have a higher load.
so using the simple MCB ratings to try and evaluate max has little or no value at all.

It is the loads that are connected to the circuits, and duration that loads or used for, and the number of those loads that are used simultaneously that has a significant impact upon max demand. e.g. your kitchen may well need a 32A ring, But I would bet your other two rings could quite happily work off a 20A (or possibly 16A) radial!

Unless you are using a lot of electric heaters plugged in, your average lounge/dining room/bedroom/conservatory etc will typically have quite a small load onto the circuit.

You are better off making a list of all of your higher power appliances and noting how long they are typically on for..
And then considering which of these higher power appliances are on simultaneously to try and calculate a more realistic figure.

Another electrician will not have any magic formula to calculate any values from, and will probably have less of an idea than you do about what your typically consumption is over an average month!

The property itself and the number of circuits is only one factor. as the number of people living at the property will affect how many appliances are in use. e.g. A family of 4 with two teenage children, moving into a property previously owned by a single elderly widow will have a considerable impact upon the max demand, even if the consumer unit and all the wiring is identical!

Other than measuring and logging your actual consumption over a typical period of time to see what is happening,
it will be a bit of guesswork, gut instinct and/or calculation from your expert knowledge of how your installation is actually used!!

 

[NozSpark]

The safe way to do this would be to get an EVSE installer to do the max demand calculations and send the ENA form off for approval before anything is installed... it's the hot tub that'll cause most of this issue IMHO

Secondly,,, get an EVSE with load curtailment built in, then it can be set so that the EVSE doesn't take the max demand over the fuse size,, the max setting is set to typically 5A lower than the fuse size

Locked to prevent being resurrected (again)

 

[GM at CA UK]

If it helps I just wrote an article about maximum demand measurement as an alternative to diversity calculations following a case study on a multi-occupancy building in Richmond, London (5 flats in a converted house of 50 years). It has a single phase 100A supply split into 6 and in a 2 week log the maximum current drawn was 48.83A, and that was only for 2 minutes during an evening. The article is in Energy Manager Magazine and can be read here https://issuu.com/abbeypublishing/docs/em_may_2023/22