Log Cabin Electrics

[Welshopen]

Hi all,

I have a log cabin built in the garden and am trying to get an electrician to come and wire up some electrics to the cabin. Who would have thought it was so hard to get people to respond to a job....maybe this is too small a piece of work?
Anyway.
I am slightly confused by the suggested approach by the 2 qualified electricians that have been round.
I had a shed before the cabin went up and this had a 2.5 T/E armored cable fed from a kitchen socket (8-10M long). We had an extension built and the electrician wired my shed up along with the new electrics for the extension. The shed only had 1 double socket.

The suggested approach for the cabin is to use this existing armored cable and fit a garage CU. Then from that have a radial circuit for the sockets that i need for my office. Around 4 double sockets to feed laptops and computers, monitors etc. Plus either a 2 KW heater or small air con unit. Lights will be Plugged in so no need for a separate circuit.
Both electricians advised that this would be OK as there would be a 20A breaker in the garage CU which would protect the cable to the house. Which then joins a 32A MCB ring circuit.

The reason why i am asking whether this sounds OK is i have read other forums where electricians have been frowning on this very setup.
What concerns me more is the fact that before my cabin went up i laid a patio/ slabs all around my house. Before doing this i laid proper conduit following regs to prepare for the electricians to run a SWA from the cabin back to the main house CU. Did i really dig that 20M trench for nothing?

Now i am not the certified electrician so who am i to question their proposal? well a concerned, not so daft home owner that's who!

Incidentally neither have come back to me for any availability or dates when this can be done, so I'm pondering if i should be seeking a 3rd opinion.

I would appreciate your advice.

Joe

 

[EFLImpudence]

Firstly, if you are in Wales (username?) , this will be notifiable to the Local Authority, unless you use a registered electrician, before you start the work .

 

[Welshopen]

No England, was watching the welshopen when I signed up.
I’m using electricians as stated above.
Thanks

 

[ericmark]

There are two limits to cables, one is to be within the volt drop and other to be within permitted earth loop impedance, the volt drop is normally the limiting factor, since it is volt drop supply to final destination you need to know the loop impedance at the incoming point and at the point where your taking the supply from before you can work out the total volt drop.

So incomer 0.35Ω and point where your connecting the cable 0.52Ω then should be able to reach shed and still be 0.69Ω so volt drop within the 6.9 volt allowed for lights.

However without the readings it just guess work, be it as prospective short circuit current or loop impedance you need readings at 20 meters right on the edge volt drop wise for 2.5mm² cable with 20 amp draw.

 

[oldbutnotdead]

If your electricians are properly qualified then whatever they install should be safe and compliant. But maybe not suitable for your needs.
..
Less than ideal having your garden office on the same rcd as the rfc in the house- long walk to reset it from the shed, potential for damp in the external wiring tripping your house rfc.

Better by far for the office to have its own supply, ideally not from the house cu, then your rcd protection can come from the garage cu.

 

[Welshopen]

Thanks.

You hit the nail on. The head. Big difference to it being safe and being safe and correct installation.
Shame they don’t get the regs to force good work!

 

[oldbutnotdead]

Well if you tell the people quoting for the job what you'd like (not in massive detail but the highlights) then they can price for it.
Did you take pics of your duct trench when it was open showing sand bed, secondary protection, marker tape and depth? If not then you can still use it for LAN cabling but your electrician may be reluctant to throw power cable down it.

 

[Welshopen]

Well if you tell the people quoting for the job what you'd like (not in massive detail but the highlights) then they can price for it.
Did you take pics of your duct trench when it was open showing sand bed, secondary protection, marker tape and depth? If not then you can still use it for LAN cabling but your electrician may be reluctant to throw power cable down it.

Yes I took pictures. Always do of anything I work on.
I’ll ask them to run it back to the main section in the house.

 

[Welshopen]

Can I ask. You said about the RCD protection would come from the garage CU if it’s on it’s own supply.
Does that mean the garage CU RCD would serve no purpose in the setup they have suggested, using the 32A ring as the supply? If so I don’t get why they have included this in the price at all.

 

[ericmark]

I wonder if it's the words used which have caused problems? And the person quoted not realising how the garage was supplied? It seems to me reading your post, that in the house there is a 32A ring final and at some point on that ring final there is a cable to garage which has a 20A MCB on one end which you now want to extend further to a log cabin? I would say no to that must come from consumer unit direct.

However if it is a 32A supply direct from CU to garage then why is it 2.5mm² that seems under sized?

If I can't get my head around exactly what you have, then anyone quoting may also make errors so can you starting at the consumer unit detail what you have?

So something like house consumer unit RCD protected B32 MCB feeding 4mm² 10 meters long to garage and from garage consumer unit B20 MCB to log cabin with 2.5mm² cabled 20 meters long. From that one can work things out, in the case given 3.84 volt drop to garage, and 7.2 volt drop garage to log cabin total 11.04 volt drop which is OK for power but not for lighting. But to work out it needs to be correct, so once you have said exactly how fed it can be worked out, if you don't know how much cable then line - neutral loop impedance can be used, so on my example with 0.35Ω at house consumer unit would expect 0.47Ω at garage and 0.83Ω at log cabin. Of course 0.35Ω = 657A prospective short circuit current, so can be either reading it can still be used.

Remember if trench is 20 meters then likely 1 meter either side up wall to CU so cable 22 meters long. In real terms the volt drop will not be as much as calculated as working on max draw, and when using 20A in log cabin you unlikely to be also used 12A in garage, but electricians should either work to regulations or agree with you to use a lower figure, which should be listed in the installation certificate.

 

[Welshopen]

I wonder if it's the words used which have caused problems? And the person quoted not realising how the garage was supplied? It seems to me reading your post, that in the house there is a 32A ring final and at some point on that ring final there is a cable to garage which has a 20A MCB on one end which you now want to extend further to a log cabin? I would say no to that must come from consumer unit direct.

However if it is a 32A supply direct from CU to garage then why is it 2.5mm² that seems under sized?

If I can't get my head around exactly what you have, then anyone quoting may also make errors so can you starting at the consumer unit detail what you have?

So something like house consumer unit RCD protected B32 MCB feeding 4mm² 10 meters long to garage and from garage consumer unit B20 MCB to log cabin with 2.5mm² cabled 20 meters long. From that one can work things out, in the case given 3.84 volt drop to garage, and 7.2 volt drop garage to log cabin total 11.04 volt drop which is OK for power but not for lighting. But to work out it needs to be correct, so once you have said exactly how fed it can be worked out, if you don't know how much cable then line - neutral loop impedance can be used, so on my example with 0.35Ω at house consumer unit would expect 0.47Ω at garage and 0.83Ω at log cabin. Of course 0.35Ω = 657A prospective short circuit current, so can be either reading it can still be used.

Remember if trench is 20 meters then likely 1 meter either side up wall to CU so cable 22 meters long. In real terms the volt drop will not be as much as calculated as working on max draw, and when using 20A in log cabin you unlikely to be also used 12A in garage, but electricians should either work to regulations or agree with you to use a lower figure, which should be listed in the installation certificate.

Ok so I have attached an image of my CU in my house.
The B32 MCB labelled kitchen sockets is the ring that feeds the sockets in the kitchen. We had an extension which is the kitchen and the electrician ran the sockets in the extension on this B32 MCB.
He ran a spur off one of the double sockets(not fused just joined in an outside junction box) and ran a feed to my then shed. That feed is the SWA 2.5mm mentioned here.
That shed is gone and now sits a small 3M x 4M cabin. The cabin is 3M from the junction box.
So what has been suggest to me is connect that 2.5mm SWA to a garage CU. That garage CU would have 1 x B20 MCB and a 40A RCD. The 40A RCD is an assumption based on me looking at garage CUs online.
No light circuit a radial circuit connecting 6 sockets.

Hope that helps.

Joe

 

[flameport]

So what has been suggest to me is connect that 2.5mm SWA to a garage CU. That garage CU would have 1 x B20 MCB and a 40A RCD.

Permitted, but it's a mediocre design.
The RCD isn't required - you already have one in the house.
Cabin on the same circuit as the kitchen may not be desirable either, depending on what loads you intend to use in there.

Far better to have a new cable from the house to this cabin, which can then be sized appropriately for the load and not be dependant on some other unrelated circuit.

 

[EFLImpudence]

Not ideal but it all seems acceptable to me - obviously as long as all the tested measurements are within the limits and not made unacceptable by the distance.

The 20A MCB will protect the cable from overload.
The relevant figure on RCDs is the 30mA. The 40A is just the current it can handle, so as long as it is more than the 20A is irrelevant.
The second RCD is not required but will do no harm.

 

[Welshopen]

Thanks all!

Kitchen load varies. It’s the washing machine that seems like it’s on forever(young kids).
Dishwasher always late at night. Kettle, again throughout the day.
Not much else. Random uses of toasters, wisks etc but very rare.

I guess the RCD is part of the CU anyway. At least from what I see.

Ok you have made me feel more at ease about their solution.
Just need them reply now...

 

[Welshopen]

Is there something that can measure what power the cabin is using? Like a module that sits in the CU?