Estimating load requirements for a cabin in my garden

[d000hg]

From school physics I know the basics of current and amperage, and I'm aware of things like voltage drop, etc, but what I can't find is how to come to maximum amperage value for a log cabin I want to put in my garden. I spoke to an electrician who suggested it's as simple as "add up if everything was turned on at once" but when I used an online tool, it came to over 60A (without showing me the workings) for a 60m^2 single-storey structure... picture a 1-2 bed holiday cabin/flat... which seems crazy. And that's assuming non-electric heating too.

Of course in reality we don't turn everything on but for purposes of planning can anyone point me to an online guide? Are there standard practices a layman can use to come to the overall number relevant for fuses, cabling, planning documents, etc? Headroom on fuse above expected load, that sort of thing?

 

[EFLImpudence]

We call it diversity.

https://www.tlc-direct.co.uk/Book/6.2.3.htm

Search for more

 

[plugwash]

The thing is if you run a typical house's installation through those guidelines then you end up with a massive overestimate.

 

[d000hg]

Thanks. Apart from the 3-phase aspect I could follow that mostly - and "unity power factor"?

I did note this: "Distribution boards must not have diversity applied so that they can carry the total load connected to them." The distribution board in a residential setting, this is my consumer unit i.e. the fusebox?
Is that meaning simply that if my log cabin has its own consumer unit with circuits for lighting, sockets, oven, etc that I must use the sum of those circuit fuse values for the cabling to the cabin? So if I put 5 32A circuits in the cabin, the cabin supply has to be rated 160A(!) and actually then it goes against my nature to be generous/overestimate.

We're actually wondering about having a second meter put IN our house rather than get a whole second supply (pricey) but a normal house might only have a 80-100A supply from the line so we can't casually go rating the cabin at 60A because now (in theory) we could overload the supply into the house.

 

[EFLImpudence]

Is that meaning simply that if my log cabin has its own consumer unit with circuits for lighting, sockets, oven, etc that I must use the sum of those circuit fuse values for the cabling to the cabin? So if I put 5 32A circuits in the cabin, the cabin supply has to be rated 160A(!) and actually then it goes against my nature to be generous/overestimate.

No, The suppy fuse rating.

Incidentally, you might put in five 32A circuits - really? how big is it? - but probably two (or even one) would do, so...

We're actually wondering about having a second meter put IN our house rather than get a whole second supply (pricey) but a normal house might only have a 80-100A

Might be 60A. They might uprate it to 100A.

supply from the line so we can't casually go rating the cabin at 60A because now (in theory) we could overload the supply into the house.

I think we need to know just how big this cabin is. Is it another house?

 

[JohnW2]

I think we need to know just how big this cabin is. Is it another house?

It's not totally clear as to whether it is being said that this is what the OP's cabin is like, but ...

... a 60m^2 single-storey structure... picture a 1-2 bed holiday cabin/flat...

Kind Regards, John

 

[d000hg]

Incidentally, you might put in five 32A circuits - really? how big is it? - but probably two (or even one) would do, so...

Sure - I was just making an over the top example. In reality I'm certain 32A is loads in total but if you put in separate circuits for lighting, sockets and oven you could easily end up with a huge theoretical maximum and I'm nervous that could come back to cause problems regs-wise.

 

[JohnW2]

We call it diversity. https://www.tlc-direct.co.uk/Book/6.2.3.htm

There some major practical problems with most guidance on diversity (other than that relating to individual items of equipment or groups of items - e.g. 'cooking appliances') ....

Firstly, there is usually a lack of consideration of 'between-loads diversity'. For example, application of calculations such as in your link could/would result in ludicrously high 'after-diversity' figures for the total load on a domestic installation, particularly if there were an electric shower and large electric cooker (which, without anything else, could exceed the 'rating' of the supply and/or the CU). Those calculations 'assume' that, say, a shower, would be run 24/7!.

Secondly, it all rather falls on its face when (as often will be the case) many of the loads are plugged in to sockets circuits, since one often does not any clear idea as to what will be plugged in (hence what the 'X, Y and Z' figures would be for calculations per your link).

I think that, in practice, the only way to make a sensible (and realistic) estimate of the total load on an installation (or 'sub-installation') is to apply common sense, together with whatever knowledge (and/or estimates) is available in relation to what loads will be active - and when and for how long.

Kind Regards, John

 

[EFLImpudence]

We call it diversity. https://www.tlc-direct.co.uk/Book/6.2.3.htm
Search for more

 

[JohnW2]

... Search for more

Yes, you wrote that but, as I implied, I'm not sure that any diversity guidance one might find could really address the sort of issues I mentioned in a blanket manner - since it is always going to 'depend' (on individual circumstances and 'judgement/estimation/common sense).

Showers would appear to present a particular problem since, unless one is bold and ignores them completely (since 'they are never on for long'), they have the potential to have a major impact on 'whole installation' load calculations. However, at the other extreme, one estimated 'the average load per 24 hours' for a shower, one would get a very small (and maybe 'inappropriate') figure.

Kind Regards, John

 

[ban-all-sheds]

Of course in reality we don't turn everything on but for purposes of planning can anyone point me to an online guide? Are there standard practices a layman can use to come to the overall number relevant for fuses, cabling, planning documents, etc? Headroom on fuse above expected load, that sort of thing?

Why isn't your electrician working it out?

 

[Simon35]

Why isn't your electrician working it out?

You know why, he or she will be on holiday. Skiing probably.....

 

[flameport]

In reality I'm certain 32A is loads in total but if you put in separate circuits for lighting, sockets and oven you could easily end up with a huge theoretical maximum and I'm nervous that could come back to cause problems regs-wise.

Theoretical is the the important part.

If you had a house with a single 32A circuit for all of the sockets, the maximum load is 32A.

If that same house was rewired with a 32A circuit for upstairs, another 32A circuit for downstairs, a third 32A circuit for the kitchen and a fourth for a futility room, the theoretical maximum is now 128A.
However in reality the appliances and other equipment in the house is exactly the same and the load hasn't changed at all.

 

[JohnW2]

If you had a house with a single 32A circuit for all of the sockets, the maximum load is 32A. ... If that same house was rewired with a 32A circuit for upstairs, another 32A circuit for downstairs, a third 32A circuit for the kitchen and a fourth for a futility room, the theoretical maximum is now 128A. ... However in reality the appliances and other equipment in the house is exactly the same and the load hasn't changed at all.

Indeed so.

An 'interesting' feature of the diversity guidance that EFLI linked to (which apparently comes from the "Electricians Guide") is that it appears to be implying that, for example, if the largest load on a domestic 32A sockets circuit is 13A, then (without any qualification) it is acceptable to plug in a total load of 60.5A into that circuit [13 + (0.4 x 47.5) = 32, and 13+47.5 = 60.5]. Comments on a postcard?

Kind Regards, John

 

[d000hg]

Theoretical is the the important part.

If you had a house with a single 32A circuit for all of the sockets, the maximum load is 32A.

If that same house was rewired with a 32A circuit for upstairs, another 32A circuit for downstairs, a third 32A circuit for the kitchen and a fourth for a futility room, the theoretical maximum is now 128A.
However in reality the appliances and other equipment in the house is exactly the same and the load hasn't changed at all.

Yes exactly. I'd thought there would be standard rules - or at least guidelines - for this so if you need to get the work signed off, you can be confident it'll pass. Now it seems like this isn't the case at all!

The limiting factor in my case - we either want to simply wire this off our existing meter or have a second meter off our existing supply - is (I think!) whether the power grid will say this is too high a theoretical load. e.g. house+cabin totals more than 100A. It's not clear where I can do diversity - which seems to be somewhat about common sense - and where the maximum theoretical values have to be added up.