log cabin - Electrical Advise

[a18041967]

I'm in the process of installing a summer house which will require electricity, I'm not a qualified electrician but my plan is to purchase and install the hardware and then pay for an electrician to check and certify my work. I have a few questions that I'd like some advice on before proceeding.

I have a garage that has an SWA supply from the main house RCD and this has two circuits one for lighting the other for power, the plan is to feed the summer house from the garage, the circuit will be as follows:

(House RCD -> SWA -> Garage RCD -> Power Circuit) -> SWA 20 metres -> External Power Socket -> Log Cabin

Bold - Existing items

Questions:
SWA cable run, this will be approximately 20m in length, should I use 2.5mm or is 4mm SWA due to cable length? This will be secured to the fence which has concrete posts, not shiplap fencing, I know this can be a bit of a grey area but will this cause problems?

The garage already has a SWA spur of the house mains to an RCD with two circuit breakers, one for lighting and one for power. My current thinking is to connect the new SWA to the existing power circuit in the garage. Can the SWA feed to the log cabin be connected to the power circuit or will a new circuit breaker be required in the garage RCD and the SWA directly connected?

Will a new RCD need fitting inside the log cabin similar to the one fitted in the garage, power requirements will be two dual 3-pin sockets and basic lighting, maybe 12v lights?

I plan to purchase and run cables myself, and then have the connections made in to the existing circuits and certified by a qualified electrician?

[Someone-Else]

I would suggest you get an electrician to do the job BEFORE you buy or do anything.
That way you can ask him what to do.

If (on any forum) a person says yes do this this and this, and the electrician says no, you can't do that, you will have wasted money, but if you do what "your" electrician says it must be right.

[kellys_eye]

Treat the new system as an 'extension' and you won't need any certification - simply stated you could run a 16A 'caravan cable' (although you'd use SWA instead) from the garage to a power point or unit with a couple of additional breakers if you felt the need.

Rather depends on what you intend to connect to it but for low power lighting (LED) and a socket for ?? mobile phone, laptop or even small heater (<2kW) you don't have to have anything too complicated. 2.5mm would be adequate for such an arrangement.

I'd fit an SWA junction box at either end. In the garage I'd connect a lead to a 13A plug (this makes it an 'extension') and at the log cabin end wire the SWA junction box via another cable directly into a 13A socket and fit a switched, fused (5A) spur to the lighting circuit.

Of course, if you want something more permanent and with future potential for expansion then wiring directly into the house CU and fitting a CU in the cabin would be required but cost considerably more.

[a18041967]

Thanks for your suggestion, I'm not sure if you've seen another post but my thinking has been very similar to the wiring in our caravan. I'd already decided to install an external power post.

I've asked a local electrician to provide a quote and it'll be interesting to see what his solution will be and how similar it'll be to the extension approach. Having inspected the suppliers demo log cabin their solution is extension with no circuit breakers, I'd include images but not sure how to upload them.

[ericmark]

I am going for the moment forget the legal requirements, but instead consider electrical requirements, so first you need some readings and facts.

1) Line - neutral impedance at consumer unit (potential short circuit current) can be used instead.
2) Line - neutral impedance at where you are adding the extra bits, from this you could work out volt drop.

We know from calculations once we know incoming impedance, what impedance the final circuit has to be, we also know how much of that allowance has already been used, so we can calculate what size cable will be required to keep within the limits.

But until you have some readings, we can only guess, however often we can guess close enough to take a chance, so we think it's going to be well within limits so we fit then test, if we however think it may be on the edge then we calculate.

Now you should already have some readings, be it the original installation certificate or a electrical installation condition report, there are two limits, one the earth loop impedance and other line - neutral both limits depend on the overload protective device.

If you post some readings then maybe some one will say how much extra cable and what size, but without readings it is rather pointless, we are just guessing.