Will my garage handle my planned usage?

LHensh · Jan 20, 2022

Hi,

Just looking to get some advice before I get too carried away with plans for using my garage as a home office/ManCave.

The garage has an old fusebox (photo attached) with 15amp and 5amp fuses which is fed by a 2.5mm cable coming from the house.
The feed to the garage comes from the main consumer unit in the house.

I'm trying to work out if my expected maximum power consumption could be safely handled by both fuses.

Sockets:

Oil filled radiator (photo attached) 1000w @ 240v = 4.17A
Router = <0.5A
TV = <0.5A
Beer Fridge = 0.75A
Coffee Machine = 5.5A
Laptop = <0.5A
Monitor = <0.5A
Projector = 1.9A
LED strip lights =<0.5A

Total Amps = 14.8

Lights:
10x LED downlights 5W X 10 X 240V = 0.21A
2x LED outside lights (4 bulbs) 5.5W X 240V = 0.09A

Total Amps = 0.3

For the sockets, assuming that all of the lower usage appliances all run at 0.5A and are all running at the same time (which wouldn't be the case) I should be within the 15A limit.

Lights are no concern as would be well under the limit.

Am I on the right lines here?

littlespark · Jan 21, 2022

The 2.5mm cable from house could carry 20A depending on installation method… One issue is what is feeding this 2.5mm cable and distance between house and garage.

It should be rcd protected, and if not… the board in the garage needs to incorporate rcd protection.

timhoward · Jan 21, 2022

Exactly as @littlespark said. Main questions being:
What is the circuit breaker rating for the garage circuit in the house? A photo of this consumer unit would help.
Is this an RCD protected circuit?
What is rough length of circuit?

My gut reaction is that this will probably be ok from a loading point of view as the heater, coffee machine, and fridge will cycle on thermostats. While you wouldn't do it like this if starting again, subject to answers above it should meet the needs.

My advice is to pay a sparks for 2 or 3 hours to IR test the current wiring garage supply, confirm the total earth loop impendence (making sure it will trip the house breaker in fault conditions), and stick a modern garage consumer unit in. It's not a big job.
(You'd also get an installation certificate and building control notification certificate which helps if you ever sell the house.)

LHensh · Jan 21, 2022

littlespark said:
The 2.5mm cable from house could carry 20A depending on installation method… One issue is what is feeding this 2.5mm cable and distance between house and garage.

It should be rcd protected, and if not… the board in the garage needs to incorporate rcd protection.

Yeah the board in the house is RCD protected. May well have someone replace the fuse board with something a bit more up to date. Not sure if they'd put a second RCD on the garage end or of that would be counter intuitive?

Hard to say the length of cable run from house to garage as it not clear where/how it runs, but I expect it will be 10-15 metres

littlespark · Jan 21, 2022

There is no benefit of having 2 x RCDs,one at source and one at garage… If there’s one at source already, that’s fine.
The cable feeding the garage may demand it anyway.

The sub main fuse board in the garage needs replaced though.

LHensh · Jan 21, 2022

timhoward said:
My advice is to pay a sparks for 2 or 3 hours to IR test the current wiring garage supply, confirm the total earth loop impendence (making sure it will trip the house breaker in fault conditions), and stick a modern garage consumer unit in. It's not a big job.
(You'd also get an installation certificate and building control notification certificate which helps if you ever sell the house.)

Thanks, this is exactly what I was thinking/hoping would be the case.

I have attached pictures of the board. The garage supply runs off the far right breaker which is also used for the kitchen sockets.

From there it runs to a module faceplate (not sure on correct term) which has a switch dedicated to the garage supply.

DPG · Jan 21, 2022

That puts a slightly different slant on it if all your kitchen stuff is fed from the same circuit as your garage feed.

littlespark · Jan 21, 2022

Yes… that’s more of an issue. It’s now a spur from an ring final circuit and not a radial from source.

Quite a lot of high power items from that one switch bank already.

DPG · Jan 21, 2022

Is there an easy route from your consumer unit to the garage? I'm thinking probably not and that's why the feed was taken from the ktichen grid switch.

LHensh · Jan 21, 2022

Unfortunately no easy route to the garage from the consumer unit.

How much of an issue would it be having my planned usage run through the setup as it stands?

littlespark · Jan 21, 2022

As it’s a spur, and then having other things off it, it needs fused down to 13A.

It doesn’t actually conform to regulations as it stands.

But that can be solved by adding a fuse in the grid switch where the blank is.

Highly recommend getting an electrician to do that as they are notoriously difficult to work on.

timhoward · Jan 21, 2022

I'm not quite so sure the garage arrangements break the regs on their own, after all an unfused spur to one double socket in the garage would be allowed, with total potential current draw of 2 x 13A (26A), and there's currently a 20A limit (5A + 15A will blow eventually!).

I'm assuming that the ring comes both in and out of that grid switch, with each 20A DP isolator on the ring, feeding an FCU near each appliance.

Good old 433.1.204 says
"if under the intended conditions of use the load current in any part of the circuit is unlikely to exceed for long periods the current carrying capacity (Iz) of the cable"

My main concern is that having a few appliances on could get close to the load current exceeding the CCC of the cable, depending too on how the cables are run. Especially in the short length's behind that switch.

Regarding the garage - I had exactly the same idea of sticking a 13A module in that grid switch if there's no practical way to improve the arrangements.

littlespark · Jan 21, 2022

But it’s not one double socket on the end of the cable… it’s an old fuse board. No idea what size of fuse wire is in that 15A and 5A carrier….

I stand by what I said.

Change the garage fuse board to a double socket…. That’s ok.

Then have everything come off extension leads.

Will my garage handle my planned usage?

LHensh

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