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Hi guys,

In very early planning of a possible garden room, to be built alongside my garage. I need to know the art of the possible around getting supply into the new room so looking for advice. I will of course get the pro's in to do the job, but just looking at what I can do or can't do with the limitations of my existing setup.

I live in a 9 year old house with detached garage. The house consumer unit has a 16A MCB for "Garage" and a SWA cable runs out under my garden/patio into the garage. The garage has its own mini consumer unit, with MCB's for 6A Lights and 16A Sockets. I have photos of the consumer units/MCB's if needed.

I really dont want to have to dig up my patio or driveway so hoping there is a solution via the existing SWA cable and feed new SWA from the garage along a short couple of metres to the new room. The garden room will be fully insulated, rubber roof, cedar clad, bi-fold type variety. It will be 4m x 3m and will have its own consumer unit. It will contain the following:

- 2kw electrical heater
- 3 x double sockets to power a laptop, monitor and exercise bike
- 6 internal LED spotlights and 3 external LED spotlights

What options do I have please? If needs be I could upgrade consumer units/MCB's etc just trying to avoid digging up my lovely patio or driveway to make this happen.

Thanks in advance for any help.

Cheers.

The "Problem" is voltage drop, which whoever designed your garage installation would have thought of, but they may not have considered you adding an extra room further away at a later date.
Is there any point at which you can physically see the SWA cable? as if you look carefully it will have its cable size "printed" on it and also what will be the distance from the consumers unit to the new extension.



Just saying: An MCB is to protect the cable, nothing else. (It's the same for all protective devices) so for example if you had 1mm cable it can safely carry 11Amps, so if you were to change the MCB to 16A if there was a fault, in theory the cable would overheat and cause a fire before the MCB trips, so that is why you can't just change an MCB to "A bigger one"

Not sure I understand why you think you need so many fuseboards and ultimately the size of the cable to the garage IS THE limiting factor.

I can only recommend you get a couple of local sparks round to quote - these types of installations can only really be assessed by doing a site visit.

Hi Someone-else, thanks for your reply. Yes you can see the SWA for several metres as it is pinned to garage wall and comes up into the garage consumer unit. The cable is 15mm external diameter and has the following stamped on it: PRYSMIAN (A) BASEC ELECTRIC CABLE 600/1000V BS5467. 3 x 2.5. From the garage consumer unit to the proposed garden room is less than 5 metres, most of which is coming down the wall and back up a wall (underground will only be 1m length).

Neelix wrote: ↑Mon Feb 14, 2022 4:49 pm Not sure I understand why you think you need so many fuseboards

Where did I say this? I was just explaining that there are already 2 in play in my existing house and garage and that the garden room I'm interested in is listed as coming with one supplied.

infaddict wrote: ↑Mon Feb 14, 2022 4:50 pmFrom the garage consumer unit to the proposed garden room is less than 5 metres

But how far is the garage from the house? (Sorry, when I asked, I meant from the house consumers unit to the new building)
The reason being the electricity has to get from the house to the garage first, then from the garage to the "new building"

Someone-Else wrote: ↑Mon Feb 14, 2022 5:27 pm

infaddict wrote: ↑Mon Feb 14, 2022 4:50 pmFrom the garage consumer unit to the proposed garden room is less than 5 metres

But how far is the garage from the house? (Sorry, when I asked, I meant from the house consumers unit to the new building)
The reason being the electricity has to get from the house to the garage first, then from the garage to the "new building"

No worries. House consumer unit to garage consumer unit is around 23 metres including the up and down the walls.

What "worries me" is it should be ok IF you use either "room" but not both. Also if you want to go for internet in this garden room you would be better off having it hardwired, and as you will be upgrading the electric supply, you may as well have a CAT6 cable put in too. Then there is the what happens when you want to add more things, or a 2nd heater, better to have a bigger cable than too small.
You don't have to have too much dug up, you can have a "Mole" make a hole for you.
As you are only asking, it wouldn't be a bad idea to get an electrician in now and work with them, as at the end of the day it is they who have to sign it off.

Ok thanks for advice. I agree. I've got someone coming to take a look later in week.

I have quite a large garden with 3 levels. My workshop is at the rear of my garden. Rather than dig a large trench I installed a large scaffolding pole beside the workshop and ran the SWA across a catenary cable to the house then into the CU. In retrospect, I should have just used Tuff cable instead of SWA to save on weight but I'm a bit belt & braces in that respect.

Someone-Else gave me loads of help on this which I'm still very grateful for.

Cable info is here what is important is cooling, but it seems cable is good for around 32 amp, what is the limiting factor is the loop impedance.

There are two loop impedances, one line to earth, and one line to neutral, 1.36Ω is required for the earth loop impedance with a 32 amp B type MCB/RCBO and 0.63Ω more than the incoming to house for the neutral to line loop impedance, which will equate to around 23 meters of 2.5 mm² and that with a design current for circuit Ib of 26 amp, i.e. allowing for not normally being fully loaded.

Using a smaller MCB/RCBO say 20 amp will allow 32 meters, with 0.79Ω more than the incoming to house.
Using a smaller MCB/RCBO say 16 amp will allow 42 meters, with 0.97Ω more than the incoming to house.

In the main we measure the line to neutral in prospective short circuit amps, assuming incomer of 0.35Ω that would be 174 amps for 16 amp MCB/RCBO, 200 for 20 amp, and 235 for 26 amp which I can taken as design current for circuit Ib.

With volt drop it is not so cut and dried as with earth loop impedance, the earth loop impedance has to be low enough to trip a B type MCB/RCBO using the magnetic part of the trip. But the volt drop we normally consider with socket circuits 20 amp at end and 12 amp even spread, clearly not even spread in your case, so at original garage and some at new garden room.

The items really affected by volt drop, are any single phase refrigeration, wire wound ballast fluorescent lights, EV car charging, and solar panels, most stuff today is powered with a switched mode power supply so can have a greater volt drop without any real problem.

Since talking about a garage EV charging may be needed latter. A non hard wired EV charging unit often can have the output user selected. 8A, 10A, 13A, 16A, 20A, 32A is common, clearly a commando type socket required not a 13 amp. But it also needs other protection depending on the make and model.

To use a type A RCD/RCBO it will need 6 mA DC detection.
To use a PME or TN-C-S supply it will need lost of PEN detection.

The latter often uses voltage, and if it goes over 253 volt or under 207 volt it auto disconnects. This would clearly be a problem if you have too much volt drop. If you fit an earth rod and turn the garage to a TT supply then loss of PEN detection not required, however with a different earthing system to house, it must be a safe distance away from the house. With caravans the fire regulations required 2.7 meters, and so the distance house to caravan never really came up.

There is also a consideration if the EV car is charged in the garage then uncertain if a loss of PEN detection is required. Only really needed if car charged outside the garage.

It seems as electricians we can fit a 32 amp socket for a welding set with no special considerations, but if fitted to charge an EV we should follow the EV charging point rules, so all rather a grey area.

However I think you should be aware, as it may mean you select to run a new cable in now ready for any EV. Fixed EV charge points often need extra cables to monitor whole house use, so it does not overload the supply fuse, and normally fixed at 7 kW charge. The plug in type however leave it to user to select charge rate.