Wiring for a garage

[andrew7043]

Hi all,

I'm new to this and have a few questions about a garage rewire i'm planning on doing. The garage is supplied by a 30amp fuse in my house.

I was wondering if the best consumer unit to get is 40A 30mA RCD. Would a 30amp supply be ok for this consumer unit?

Also the garage consumer unit comes with a 6amp and 32amp mcb. Should I change it to a 16amp mcb as the only things that will be plugged in is a fridge and a separate freezer. Or is a 32amp mcb fine for a radial for this? The cable run is only about 5/6 metres.

Also is it ok for 2x fluorescent lights to run of the rcd?

Thanks

 

[PrenticeBoyofDerry]

I'm new to this and have a few questions about a garage rewire i'm planning on doing. The garage is supplied by a 30amp fuse in my house.

If you are new to designing, installing, inspection, testing and certification. I suggest you do some reading up on this subject as, you will require to follow certain design criteria, regulations and notification procedures.

I was wondering if the best consumer unit to get is 40A 30mA RCD. Would a 30amp supply be ok for this consumer unit?

That would depend on what your load requirements are within the garage and also what type of fuse it is could have bearing on the design

Also the garage consumer unit comes with a 6amp and 32amp mcb. Should I change it to a 16amp mcb as the only things that will be plugged in is a fridge and a separate freezer. Or is a 32amp mcb fine for a radial for this? The cable run is only about 5/6 metres.

Providing the distribution side can deal with 38A demand, it would not matter if you had 32A, 20A or 16A for the socket circuit, what I would consider is if ever I wanted a circuit that could deal with a larger demand than 22A, if ever I wanted extra sockets for power tools or gardening equipment, so provisions might be thought of.

Also is it ok for 2x fluorescent lights to run of the rcd?

It is

 

[JohnW2]

Providing the distribution side can deal with 38A demand, it would not matter if you had 32A, 20A or 16A for the socket circuit ....

I presume that you meant to add that it would "matter" in terms of the cable size required for the OP's radial sockets circuit. If the OP wanted to use ('the usual') 2.5mm² cable, then he would obviously be limited to a maximum MCB size of 20A (or maybe 25A, if he could find one).

We have also been told that the garage supply is protected by a 30A fuse, which theoretically limits the available supply (to less than the 38A you mention), anyway - although that, in itself, is not a reason for not using a 32A MCB in the garage if the sockets radial were wired in 4mm² cable.

Kind Regards, John

 

[PrenticeBoyofDerry]

I presume that you meant to add that it would "matter" in terms of the cable size required for the OP's radial sockets circuit. If the OP wanted to use ('the usual') 2.5mm² cable, then he would obviously be limited to a maximum MCB size of 20A (or maybe 25A, if he could find one).

cable size for socket circuit was not asked for, I was stating that it mattered not if the socket circuit was a 32A, 20A or 16A one with regards to the appliances that were to be plugged in,

We have also been told that the garage supply is protected by a 30A fuse, which theoretically limits the available supply (to less than the 38A you mention), anyway - although that, in itself, is not a reason for not using a 32A MCB in the garage if the sockets radial were wired in 4mm² cable.

Again cable size of the socket circuit has not asked for, but may I point out that I directed the OP to do some research into designing of circuits and the provisions that may be required latter on. This then possibly would mean, that 30A device be increased.
But I agree, if a 30A device were used, it would be sensible to have protective devices adding up to less that at the garage CU.

 

[JohnW2]

cable size for socket circuit was not asked for, I was stating that it mattered not if the socket circuit was a 32A, 20A or 16A one with regards to the appliances that were to be plugged in,...

All true but given that (as per the advice you gave to learn about design) it is apparent that the OP's knowledge of electrical design is very limited, I thought that it might be appropriate/helpful to tell his that the MCB rating does "matter" in terms of what cable size he would be able to use (even though he didn't ask about cable size).

But I agree, if a 30A device were used, it would be sensible to have protective devices adding up to less that at the garage CU.

Possibly but, as I imagine you would agree, it's really down to anticipated load, rather than MCB rating - add up the ratings of the MCBs in the average modern domestic CU and you'll usually get an answer well in excess of the rating of the service fuse (and the Main Switch)!

andrew: in terms of your stated present (very modest) requirements, your simplest course would probably be to change the 32A MCB in the garage consumer unit to a 20A one, in which case you could then use 2.5mm² cable for your radial sockets circuit. You would, of course, then be limited to a total of 20A from your garage sockets, but that's much more than you currently need, and to increase the available supply would probably appreciably complicate your task.

Kind Regards, John

 

[EFLImpudence]

If the garage supply is protected by a 30A cartridge fuse then the cable should already be at least 4mm² and if a rewirable fuse then 6mm².
(more involved design excluded)

If this is the case and the circuit is sound a Garage Unit is only necessary for the RCD protection and a fuse for the lights.

I think there are too many variables at the moment to advise.

 

[JohnW2]

If the garage supply is protected by a 30A cartridge fuse then the cable should already be at least 4mm² and if a rewirable fuse then 6mm². .... If this is the case and the circuit is sound a Garage Unit is only necessary for the RCD protection and a fuse for the lights.

That would be the case if one didn't mind how large a cable one would have to use for the sockets for the OPs fridge and freezer. [even if you wanted to invoke 'downstream protection by plug fuses', and even if there were only two single sockets, that would still be potentially 26A worth of overload protection - I suppose just about OK with 2.5mm² - but if either of the sockets were doubles (or if there were more than two) that argument presumably wouldn't work].

I think there are too many variables at the moment to advise.

True, if one wanted to 'maximise' the supply to the garage. However, as I recently wrote, if the OP's requirements are not appreciably greater than he indicated, then not many variables are involved and (assuming that the supply to the garage is 'OK') the simplest solution would probably be for him to wire his ('fridge/freezer') sockets in 2.5mm² cable and protect them with a 20A MCB in the garage CU.

Kind Regards, John

 

[EFLImpudence]

True but then the house fuse may just as well be reduced.

I find it odd that there is already a supply to the garage on a 30A fuse.

Is it perhaps just a spur from a ring?
Does it require RCD protection?

 

[JohnW2]

True but then the house fuse may just as well be reduced.

I suppose so, but one would, in practice, have to go down to 20A, if not 15A, and that would be total for both sockets and lighting circuits in garage - albeit that would probably still be adequate for the OP's stated requirements.

I find it odd that there is already a supply to the garage on a 30A fuse. Is it perhaps just a spur from a ring? Does it require RCD protection?

Yes, I agree - that's why I qualified what I wrote with "assuming that the supply to the garage is 'OK' ".

Kind Regards, John

 

[andrew7043]

Thanks everyone for your quick response.

As there was a lot of different views what i can make of it is:

The 30amp fuse (not mcb) seperate circuit not part of a ring, supply coming from the fuse board in my house which is not a rcd can supply a 40amp rcd consumer unit in the garage.

I will change the 32amp mcb in the new garage consumer unit to a 20amp which will supply a fridge and seperate freezer. The sockets will also supply an electric lawn mower the odd time it will be used. Is a 20amp still ok? The current wired fuse in the old consumer unit in the garage is a 15amp wired fuse.

The 20amp mcb will be wired in a radial circuit using 2.5mm t&e.

Thanks again

 

[PrenticeBoyofDerry]

Domestic fridge and freezers only require a small current, so having a lawn mower on at the same time as running fridge/freezer on a 20A MCB, should not be an issue.
20A radial circuit providing there are no de-rating of cable within the garage, such as cable routed through thermal insulation and installed in conduit, then it should be fine. If there are derating that reduce the capacity of the cable then the cable size would need increasing to 4.00mm.
It is unlikely that it will be an issue but you/we never known.
Cable clipped direct of routed in containment such as conduit and trunking would be okay, but when thermal insulation is an added factor the derating must be considered.

 

[ericmark]

I have little faith of existing wiring of any garage or shed even when wired as house was built. An estate near me had 2.5mm twin and earth feeding a box with 4mm SWA going to garage with B16 MCB feeding from house and garage having a B32 and B6 for sockets and lights.

It is not uncommon to find a junction box with the transition between twin and earth and SWA cable and one does need to check what is installed.

As to MCB and RCD size and where it would depend on what the garage is used for. As a workshop you want the garage MCB or RCD to trip before house but as a store for fridge and freezer then the reverse is true you want to be able to check in house if supply is likely sound specially with RCD which may trip with storms.

Also when deciding where to place the RCD one has to consider the type of cable used in the house and the earth loop impedance.

So to me before attempting to answer the question more information is required. The estate I am thinking about was designed by an electrician but then semi-skilled staff was used to wire it and then electrician to test I am sure most of the problems were due to poor instructions to the semi-skilled staff but that's life.

So first what protection is in the house, what size cable or cables are used and what is the loop impedance both earth and line to neutral once one knows that then one can start advising what can or can't be done.

 

[AndyPRK]

Is the garage attached to the house ?

 

[andrew7043]

No it's not attached.