House to garage to garden room - general requirements

[davmol]

Hi all, I'm having a garden room installed in late April and it will require power. The initial suggestion from the garden room installer is to hook up the garden room to the external garage. The garage will require a full rewire and new CU, so I want to get an idea of what is involved before I get some quotes in this month.

Basic Info - garden room:
- The room will be 16ft x 12ft and used as a home office and gym. The supplier has stated the room will include LED spot lighting and 4x double sockets, which will run to a small CU, which they will then connect to the CU in the external garage, using a run of 4mm armoured cable.
- I won't have much hooked up in the room other than a laptop, monitor, stereo and small fridge, but I will need to consider heating and cooling, for which I'm considering either underfloor electric matt (c2.5kw max for this size room), or an all-in-one air-con unit, which appear to run to 3kw max output for the units I've seen.
- The run from the garden room to garage CU will be around 15m to back garage wall, then internally another 8mm to CU location, which I assume will be fine in T&E given it's inside. I have many large trees and heavy York stone paving slabs in the way, so burying the armoured cable is not an option, and it will need clipping along the adjacent timber fence that butts up to the garage (it's my fence, installed c2 years ago and in good condition).

Basic info - external garage:
- The garage is c15m from the house CU location and currently on a 16amp MCB / breaker, which runs to an old school metal fuse box in the garage. The run is external through metal conduit clipped to house wall for c10m, then buried for a short 3m run to garage, then up the wall inside (no idea what the cable size inside the conduit is at the moment).
- There are 4x50w LED strip lights, 2xexterior 50w LED spotlights and 5x double sockets on a ring main (T&E clipped along wall plate, with drops as required).
- I run a fridge freezer 24/7, and ad-hoc use a bunch of bench tools - grinder / bench sander / mitre saw / drill press etc. They never run at same time and I doubt any tools are more than 2kw max output. More importantly, given that I'm the only one that uses the garage and will also be the main user of the garden room space, I don't envisage a scenario where heavy loads will ever be used in both buildings at the same time.

What I would like to understand:
1 - Given the above, does it sound feasible to daisy chain from house to garage to garden room, as the garden room supplier has suggested?
2 - What would be a suitable cable size for the run from the house CU to the garage CU, and what might be a suitable amperage breaker at the house CU?
3 - What would be a suitable amperage breaker for the garden room (at the garage CU) and will the suggested 4mm armoured cable supplied and fitted by the garden room installer be ok for my needs?
4 - My neighbours have on occasion screwed / nailed items into my fence. Would the armoured cable resist a stray nail or screw, or should I double down with extra protection (conduit / trunking or similar)? I do intend to have it clipped to the lower rail, so they would have to "accidentally" go through 70mm of timber before reaching the cable, so hopefully a none issue, but want to cover the bases).

If there is anything else you think I should be asking of the electricians that come to quote for the garage rewire then please let me know.

Thanks in advance to those who managed to read it all, and double thanks to anyone that can assist

 

[blup]

1, Yes 2. 6mm minimum 40 amp mcb. 3. The same or less as 1. Ask your electrician. 4. Tell tne neighbours to be careful, a screw shouldn't penetrate but they - and your insurance - should be safe with a suitable rcd protecting the circuit. Think about the route, swa gently curves over distance but otherwise will need jb’s for angled turns, minimising these will save time and cost

 

[Taylortwocities]

should be safe with a suitable rcd protecting the circuit.

Armoured cable (SWA) does not need RCD-protection. Better to put the RCD at the garden room, or a fault at the garden room or on the cable would trip the house electrics.

 

[Murdochcat]

Sounds to me that you are going to do this .

Get a couple of sparks round to take a look and give you prices. Eyes on site trump random internet advice

The key to this is the maximum design load

 

[ericmark]

I won't have much hooked up in the room --- to run to 3kw max --- 15m to back garage ---- The garage is c15m from the house.

So looking at 16 amp over around 40 meters by time up/down wall etc included. It powers lighting, so maximum volt drop permitted 6.9 volt (3%) so 4 mm² would allow 41 meters, but that does not include any volt drop after the consumer unit, so it would seem you need 6 mm² to ensure within limits.

Note design current for circuit Ib is not the same as maximum current for the circuit, so it could be fed with a 32 amp MCB and have 16 amp MCB/RCBO feed the sockets, the SWA wants to go directly into the house CU, so there is no cable which needs RCD protection between the consumer units.

But why bother asking? It will need either a scheme member or the LABC likely the latter due to size of shed, likely needs planning permission, and they will ensure the wiring complies. Warning I have to pay £1000 a year rates on my garden room.

 

[davmol]

Sounds to me that you are going to do this .

Get a couple of sparks round to take a look and give you prices. Eyes on site trump random internet advice

The key to this is the maximum design load

I really can't be arsed with doing it myself

I will be getting people onsite but I don't know anyone and it's always a coin toss who turns up and what experience they have given the abundance of fake reviews all over the place. I just like to have a basic clue as to what's needed for my own understanding before I start ringing around

 

[Murdochcat]

^^ ask your friends and neighbours for recommendations

 

[davmol]

But why bother asking? It will need either a scheme member or the LABC likely the latter due to size of shed, likely needs planning permission, and they will ensure the wiring complies. Warning I have to pay £1000 a year rates on my garden room.

Ouch that must have stung! Fortunately mine is permitted development, and no LABC involvement required. Thanks for the info on the cable sizing

 

[Taylortwocities]

Ouch that must have stung! Fortunately mine is permitted development, and no LABC involvement required. Thanks for the info on the cable sizing

There will be LABC involvement as the electrical works are notifiable. A registered electrician will do this as part of the job.
Therefore you must make sure your electrician is a member of one of the recognised Competent Person Schemes.
Make sure they are listed HERE

 

[davmol]

There will be LABC involvement as the electrical works are notifiable. A registered electrician will do this as part of the job.
Therefore you must make sure your electrician is a member of one of the recognised Competent Person Schemes.
Make sure they are listed HERE

Yep the electrics I understand are notifiable, I was referring to the point labc was needed for the garden room itself, based on its size which it isn't.