Planning garage sub-main project

Liam151807 · Jun 19, 2023

Hi all, 2nd year apprentice.

I'm looking to take a supply out to my garage, which is detached, from the DB at the front of the house (2 bed terraced, built 1978~)

The length of run is about 30m (having to go through either loft or under floorboards).

Intend to have a 20A radial and a 6A lighting circuit so am I right in thinking I'd allow 66% diversity for lighting and 50% for the power circuit as it's over 10A? So 13.96A total demand?

Thanks

James · Jun 19, 2023

in practical terms, a garage supply will always (well nearly always) want a 32A supply as a minimum.
In the winter, there will inevitably be a 3Kw fan heater and the starting current of the angle grinder, chop saw, welder, hot tub, lawn mower, or any other of the common things plugged into a garage.

Liam151807 · Jun 19, 2023

James said:
in practical terms, a garage supply will always (well nearly always) want a 32A supply as a minimum.
In the winter, there will inevitably be a 3Kw fan heater and the starting current of the angle grinder, chop saw, welder, hot tub, lawn mower, or any other of the common things plugged into a garage.

For my needs it will just be for a fridge/freezer, tumble dryer and use of garden/home equipment to hoover my car and trimming hedges.

But yeah if and when I move on one day someone else may want the extra capacity for more. Let's say I took 40A out there, what would I do for a mains switch in the dB?

James · Jun 19, 2023

ha, ha.

fridge/freezer 2A continuous 10A start
tumble drier 9A continuous 13A start
Vac. cleaner 8A continuous 13A start.

I rest my case.

MJPD29 · Jun 19, 2023

Liam151807 said:
For my needs it will just be for a fridge/freezer, tumble dryer and use of garden/home equipment to hoover my car and trimming hedges.

But yeah if and when I move on one day someone else may want the extra capacity for more. Let's say I took 40A out there, what would I do for a mains switch in the dB?

What do you mean mains switch in db? You could just add a 40a mcb to your existing db if using swa and fit a small board in the garage, with rcd protection of course

Liam151807 · Jun 20, 2023

MJPD29 said:
What do you mean mains switch in db? You could just add a 40a mcb to your existing db if using swa and fit a small board in the garage, with rcd protection of course

Well I was going to use RCBOs due to the built in RCD protection at the main DB in house. Then at the garage sub main dB have a main switch to isolate the two breakers on that board.

Hope that makes sense haha

diditrain · Jun 20, 2023

If you fall out with the miss’s and are crashing in the shed one night and your RCBO trips right in the middle of a good movie, you will have to trail back to the house in the dark to reset the RCBO. Better design required, ideally with your RCBO at the shed end.

littlespark · Jun 20, 2023

Detached garage, so buried cable between? Need SWA.
SWA does not need rcd protection.
The circuits from the board in the garage will.

What is in the house board at the moment? Mcb? Rcd? Photo please.

Are you working with a company? What do your colleagues suggest?

pc1966 · Jun 20, 2023

You will always end up needing more than you planned for!

There are a few ways to approach the design and you should go with the largest of:

What loads do I expect to use?
What is my basic cable choice capable of?
What cable size can I afford if looking to future usage?

You are unlikely to select less than 2.5mm SWA so buried it should be fine with a 32A MCB, but what is your route and hence the "methods" involved? They might lower the CCC if it runs through insulated regions.

Will you meed voltage drop with this choice?

Do you have extraneous conductive parts in the garage? If so do you need to size for PME bonding requirements?

What is your supply Ze and so can you meet sub-main disconnection time with the cable length and your chosen protection?

MJPD29 · Jun 20, 2023

Liam151807 said:
Well I was going to use RCBOs due to the built in RCD protection at the main DB in house. Then at the garage sub main dB have a main switch to isolate the two breakers on that board.

Hope that makes sense haha

Your swa cable does not need rcd protection. Personally I would have it on an mcb and either small board with rcbcos or rcd in garage.

westward10 · Jun 20, 2023

SWA supply to garage may need RCD protection, we don't have enough information.

James · Jun 20, 2023

westward10 said:
SWA supply to garage may need RCD protection, we don't have enough information.

Although it might not need to be 30mA

mainline · Jun 20, 2023

40 amp mcb, 10mm 3 core swa, garage Cu, would cover most garage supplies and allow for future upgrades.

If there really is the likelihood of say an Ev point or hot tub, then I would use an SP+N fused connection unit.

But like already said, more information is needed.

Liam151807 · Jun 20, 2023

Cheers for the replies, much appreciated.

So the db is in the porch on the ground floor (highlighted red) and the first floor pic with red lines is my proposed route going under the floor boards or up into the loft.

Liam151807 · Jun 20, 2023

Here is the circuits on the board

Liam151807 · Jun 20, 2023

Here is the swa being ran to the garage

Liam151807 · Jun 20, 2023

I will run the T&E down the wall.either from the loft or from under the floor on the first floor (half way down) into a joint box with the swa which will go to the garage

Dave OCD · Jun 20, 2023

The right hand side of your board with the lighting MCBs isn't RCD protected and the blank will be a spare way- so subject to a decent Ze you could simply fit an additional matching MCB for the new SWA, but it will have to be glanded properly, possibly into a small metal enclosure to ensure the armour is earthed. EDIT= Ah I've just seen your post stating T&E for the first part of the run, so the SWA can be glanded into a weatherproof enclosure with an earthing nut and lead to the CPCs.

MJPD29 · Jun 20, 2023

Skip the twin and earth in conduit and run swa straight to board. What is the purpose of the t+e? Looks like part of that board isn’t rcd protected, get an mcb to fit it

Liam151807 · Jun 20, 2023

MJPD29 said:
Skip the twin and earth in conduit and run swa straight to board. What is the purpose of the t+e? Looks like part of that board isn’t rcd protected, get an mcb to fit it

Well I was thinking twin and earth (e.g. 6mm) would be cheaper and easier to run under floor boards etc.

Tbf I could use that blank space for a 40A MCB to supply the garage couldn't I

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