Estimating load requirements for a cabin in my garden

[JohnW2]

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!

As has been said, it would not really be possible to produce blanket 'rules', or even guidelines, since individual situations and requirements vary so much.

Given that, as a consequence of the above, opinions/judgements/interpretations will vary between individuals, the way to ensure that work (any electrical work) will 'pass' ('get signed off') is to involve the person who will be doing the 'signing off' at an early stage. If an electrician is responsible for a design, he/she will obviously 'pass it'!

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.

Having "a second meter off your existing supply" would make no difference - the issue is whether the capacity of the total load would be adequate for your new leads. If the present supply were deemed to be inadequate, a new supply for the cabin would be very expensive to install, and would also lead to your having to pay two 'Standing Charges' etc.

The chances are that there will be no problem, the main issue being to ensure that the cable supplying the cabin (from your house) is adequate in size. Could you give us some idea of what electrical things you plan to have in the cabin?

Kind Regards, John

 

[d000hg]

The chances are that there will be no problem, the main issue being to ensure that the cable supplying the cabin (from your house) is adequate in size. Could you give us some idea of what electrical things you plan to have in the cabin?

Sure, at least roughly. As I say it's approx 10x6m, mostly open-plan. LED lighting throughout which I guess is a single Amp, and a bunch of sockets for low-draw things (lamps, laptops, hoover).
A kettle, couple of toasters, microwave, washing machine and dryer (which we should assume may all be used at once). A normal family size fridge-freezer.
No dishwasher planned.
Electric oven*

*Our big question is heating and cooking. We prefer to have bottled gas in which case we'd have electric oven, gas hob and a combi for hot water and heating. But since we haven't costed/planned this yet we'd probably like both contingencies covered. I would expect we'd probably get a small gas bottle for a gas hob in any case but having an electric water heater and a few electric heaters (the cabin is very well insulated) clearly will add quite a few Amps.

 

[d000hg]

The other issue is getting supply from our house (whether on a separate meter or not). The distance is approx 30m of cabling needed so I asked an electrician about voltage drop and cabling requirements. The issue he pointed out, which I'd not considered, was more about physically attaching the cable in the house - that we'd need a thick armoured cable to reduce voltage drop and this would be difficult to connect to the consumer unit purely due to its bulk and stiffness.

 

[JohnW2]

The other issue is getting supply from our house (whether on a separate meter or not). The distance is approx 30m of cabling needed so I asked an electrician about voltage drop and cabling requirements. The issue he pointed out, which I'd not considered, was more about physically attaching the cable in the house - that we'd need a thick armoured cable to reduce voltage drop and this would be difficult to connect to the consumer unit purely due to its bulk and stiffness.

If the 'electrician' was suggesting that this was a problem, then I think you would probably be best advised to look for a different one who knows what he's doing!

Yes, connecting armoured cable, particularly if 'thick' directly to a CU can be difficult, particularly if it is a plastic CU. The standard way of handling this is to take the armoured cable to some sort of 'box' (often soon after the cable enters the house) and to join it there to 'ordinary' (non-armoured) cable for the rest of the run to the CU.

Kind Regards, John

 

[JohnW2]

Sure, at least roughly. As I say it's approx 10x6m, mostly open-plan. LED lighting throughout which I guess is a single Amp, and a bunch of sockets for low-draw things (lamps, laptops, hoover).

That is obviouysly all pretty trivial.

A kettle, couple of toasters, microwave, washing machine and dryer (which we should assume may all be used at once). A normal family size fridge-freezer.

That's no more than one will often see being run of one 32A kitchen sockets circuit.

Electric oven*

If it were literally just an oven, that would probably not significantly (if at all - many have 13A plugs) exceed 13A.

*Our big question is heating and cooking. We prefer to have bottled gas in which case we'd have electric oven, gas hob and a combi for hot water and heating. But since we haven't costed/planned this yet we'd probably like both contingencies covered. I would expect we'd probably get a small gas bottle for a gas hob in any case but having an electric water heater and a few electric heaters (the cabin is very well insulated) clearly will add quite a few Amps.

"...an electric water heater and a few electric heaters" (and possibly an electric hob), if you had them, could, as you imply, make a dramatic difference to electricity requirements. You really need to decide about that, and discuss the implications with your electrician (and maybe even the DNO), before you attempt to plan the electrical installation.

Kind Regards, John

 

[Taylortwocities]

this would be difficult to connect to the consumer unit purely due to its bulk and stiffness.

Or don't connect it to the consumer unit. There are other, better ways. I agree with John, your 'electrician' is short of a bit of understanding and needs replacement.

 

[EFLImpudence]

Also, the supply to the cabin need not be, and would be better not, connected to the original CU but to the tails.

If you have gas heating and hob then I would say there is very little to be concerned about.
It would be no different than converting a normal house into two flats.

I don't know why you seem concerned about voltage drop as a suitably sized cable would be chosen to negate it.

 

[d000hg]

If the 'electrician' was suggesting that this was a problem, then I think you would probably be best advised to look for a different one who knows what he's doing!

Yes, connecting armoured cable, particularly if 'thick' directly to a CU can be difficult, particularly if it is a plastic CU. The standard way of handling this is to take the armoured cable to some sort of 'box' (often soon after the cable enters the house) and to join it there to 'ordinary' (non-armoured) cable for the rest of the run to the CU.

Kind Regards, John

As I understood it, he was saying the actual cores would be the issue as they would be thick meaty lumps of copper. But what you say makes sense - the thick cores are to reduce resistance and voltage drop (right?) over a longish cable run, but clearly the last metre or a few meters could be different without affecting it non-trivially. The aspect of our house has the CU on the opposite side of the house to the garden but it then sounds like we could run the beefy armoured cable under the floor in a straight-ish line to the room with the CU, then have a box and a more 'friendly' cable to avoid difficulty bending and attaching.

Many thanks.

 

[flameport]

As I understood it, he was saying the actual cores would be the issue as they would be thick meaty lumps of copper.

Get someone else.

30m is pretty much nothing in terms of distance - probably only looking at 10 or 16mm² cable.
Even if it was significantly larger, that just means it connects to equipment with larger size terminals at each end. Nothing unusual or difficult about it.

but clearly the last metre or a few meters could be different without affecting it non-trivially.

Cables are the same size throughout the length of the circuit.
In some instances it may be desirable to attach armoured cable to a separate enclosure and continue from there in singles or some other kind, but the size of the individual cores is the same.

Either way, this cable is a minor, trivial part of the construction.

 

[JohnW2]

As I understood it, he was saying the actual cores would be the issue as they would be thick meaty lumps of copper. But what you say makes sense - the thick cores are to reduce resistance and voltage drop (right?) over a longish cable run, but clearly the last metre or a few meters could be different without affecting it non-trivially.

If it were merely a question of voltage drop, then I suppose you are theoretically right, although in practice one would normally use cable of the same size (equally meaty lumps of copper) throughout the entire length of the run (both armoured and non-armoured). The entire length of cable (including "the last metre or a few metres" has to be 'meaty enough' to carry the required current (in terms of 'current carrying capacity', not just voltage drop).

As has been said/implied, if the 'electrician' you have talked to is thinking of connecting this cable to an existing CU (rather than supply it directly 'from the meter'), then that is another reason why you should be looking for a proper electrician.

Kind Regards, John

 

[ban-all-sheds]

And regarding an electrician, what exactly do you mean by "signing off":

so if you need to get the work signed off, you can be confident it'll pass.

?

 

[d000hg]

I'm glad I asked He is well-recommended and seems to know his stuff but how can a consumer tell... anyway that aspect seems something of a non-problem.

Our wiring is quite old (think the house was re-wired 20-30 years ago) and messy where the supply and consumer unit are (all kinds of extra bits) so am I right in thinking that if we take this cable into our house (whether a new meter or just an additional circuit) we might find the whole thing needs replacing? That wouldn't be the worst thing in the world as it probably is due to be replaced anyway but I was curious how it works... if someone does work like this are they responsible for checking the CU is up to current standards? Or could they effectively "T off" so my existing mess is left untouched and a nice modern connection put in for the new cable/circuit?

 

[ban-all-sheds]

Given that question, and the preceding ones, and these:

I spoke to an electrician who..

...I asked an electrician about voltage drop and cabling requirements...

I wonder if you are actually using an electrician to do any of the electrical work.

When you applied for Building Regulations approval what did you say, or by default allow them to assume, would be the way that you would ensure compliance with Part P?