Power cable to workshop/garage

[jmac80]

Hello all
My garage is badly needing the electrics sorted on it.
The garage is built onto the side of the house and was added about 20 years ago along with a rear extension to the kitchen.
A new hagner consumer unit was fitted in the house about 5 years ago, it has a row of normal mcbs for lights etc and then another row that is protected by rcd for sockets etc.

Currently there is a 2.5mm cable with a 32a mcb on rcd side feeding the sockets to the garage and extension. there is two live wires coming out the top of the mcb suggesting a ring however the sockets in the garage come to a dead end after 5 double sockets, maybe it's 1 radial to the extension sockets and another radial to the garage sockets? would that mean 32a poses a fire risk?

The lights for the garage and extension is a 1.5mm cable coming from 6a mcb, there is just one live cable going into the mcb.

What i would like is a separate small consumer unit in the garage doing the lights and sockets, however i would like to allow for some future 16 sockets, maybe a nice new planer/thicknesser and air compressor

The new cable feeding the garage cu would need to go under the floorboards (joisted floor construction and non insulated) for about 8 metres to get to the garage, probably about 11 / 12m in total.
I'd like to do most of the labouring work myself to save some money so my questions are what size cable would be recommended to feed the garage cu ? And would you advise armoured?

Thanks

 

[EFLImpudence]

Currently there is a 2.5mm cable with a 32a mcb on rcd side feeding the sockets to the garage and extension. there is two live wires coming out the top of the mcb suggesting a ring however the sockets in the garage come to a dead end after 5 double sockets, maybe it's 1 radial to the extension sockets and another radial to the garage sockets? would that mean 32a poses a fire risk?

Yes you are correct.
The MCB could be reduced to 20A (or 25A if available for the CU).

The lights for the garage and extension is a 1.5mm cable coming from 6a mcb, there is just one live cable going into the mcb.

That is fine.
Although, ironically, the cable is larger than necessary (not a problem but a pet hate)

What i would like is a separate small consumer unit in the garage doing the lights and sockets, however i would like to allow for some future 16 sockets, maybe a nice new planer/thicknesser and air compressor

A 'small' CU is not necessary at the moment and...
for the future plans a lot more than a new CU will be required.

The new cable feeding the garage cu would need to go under the floorboards (joisted floor construction and non insulated) for about 8 metres to get to the garage, probably about 11 / 12m in total.
I'd like to do most of the labouring work myself to save some money so my questions are what size cable would be recommended to feed the garage cu ? And would you advise armoured?

You need to employ an electrician who can supervise the work in order that he may certify and notify it therefore your questions should be directed to him.

 

[JohnW2]

Currently there is a 2.5mm cable with a 32a mcb on rcd side feeding the sockets to the garage and extension. there is two live wires coming out the top of the mcb suggesting a ring however the sockets in the garage come to a dead end after 5 double sockets, maybe it's 1 radial to the extension sockets and another radial to the garage sockets? would that mean 32a poses a fire risk?

Yes you are correct. The MCB could be reduced to 20A (or 25A if available for the CU).

It could - and that would make it safe. However, whether 20A (or even 25A) would be adequate for the combined needs of garage and kitchen extension is perhaps not so certain.

Kind Regards, John

 

[jmac80]

Thanks for the quick reply.

for the future plans a lot more than a new CU will be required.

Can you expand on that a little please, what do you mean by "a lot more"

Thanks

 

[jmac80]

Currently there is a 2.5mm cable with a 32a mcb on rcd side feeding the sockets to the garage and extension. there is two live wires coming out the top of the mcb suggesting a ring however the sockets in the garage come to a dead end after 5 double sockets, maybe it's 1 radial to the extension sockets and another radial to the garage sockets? would that mean 32a poses a fire risk?

Yes you are correct. The MCB could be reduced to 20A (or 25A if available for the CU).

It could - and that would make it safe. However, whether 20A (or even 25A) would be adequate for the combined needs of garage and kitchen extension is perhaps not so certain.

Kind Regards, John

There is a washing machine and kettle in the extension which obviously can both be on at the same time, In the garage i could have my 2kw mitre saw on with 1.2kw dust extractor, PC and maybe a couple of things charging.
So quite a few kw potentially.

 

[JohnW2]

There is a washing machine and kettle in the extension which obviously can both be on at the same time, In the garage i could have my 2kw mitre saw on with 1.2kw dust extractor, PC and maybe a couple of things charging. So quite a few kw potentially.

Indeed - much as I expected/feared. If those things were all running simultaneously, it would add up to a lot more than 20A - albeit I doubt that all those things would ever be 'on' simultaneously for very long periods of time.

Whilst reducing that MCB to 20A (or 25A) would make things safe as a temporary measure, I would think that the sooner you get the garage/workshop on its own circuit, separate from the kitchen extension, the better. You may even have to consider whether 20A is enough for the kitchen extension alone and,if not, have that circuit upgraded in some way - maybe by having it turned into a ring.

Kind Regards, John

 

[winston1]

Make the garage and extension sockets into a ring. Problem solved.
Certainly don't need a separate CU in the garage.

 

[JohnW2]

Make the garage and extension sockets into a ring. Problem solved.

As I said, I suspect that, in view of the OP's stated potential requirements, it would probably be better to get the garage onto its own circuit (of adequate capacity) ...

.... i would like to allow for some future 16 sockets, maybe a nice new planer/thicknesser and air compressor

Kind Regards, John

 

[ericmark]

A new hagner consumer unit was fitted in the house about 5 years ago, it has a row of normal mcbs for lights etc and then another row that is protected by rcd for sockets etc.

That seems a little odd for 5 years ago would have expected all circuits to be RCD protected. 2008 was the date of change so maybe before that date?

Have you any spare ways? It is possible that the person fitting the consumer unit wanted to reduce the number of ways and as a result doubled up garage and kitchen extension. It is possible with LED and CFL lighting that other circuits can be combined leaving a spare way for the garage. It will depend if RCBO's can be fitted to your board as to what can be swapped.

The other idea is to fit a second board feed from maybe a 40A MCB so allowing more circuits.

In other words there are many ways around the problem but not sure it's really a DIY job.

 

[jmac80]

A new hagner consumer unit was fitted in the house about 5 years ago, it has a row of normal mcbs for lights etc and then another row that is protected by rcd for sockets etc.

That seems a little odd for 5 years ago would have expected all circuits to be RCD protected. 2008 was the date of change so maybe before that date?

Have you any spare ways? It is possible that the person fitting the consumer unit wanted to reduce the number of ways and as a result doubled up garage and kitchen extension. It is possible with LED and CFL lighting that other circuits can be combined leaving a spare way for the garage. It will depend if RCBO's can be fitted to your board as to what can be swapped.

The other idea is to fit a second board feed from maybe a 40A MCB so allowing more circuits.

In other words there are many ways around the problem but not sure it's really a DIY job.

Hi
You are correct it was longer than 5 years ago, mrs informs me it was 2008, the following year after we bought this house from my mother.
Yes there is 1 free way and potentially 2.

All I really want to do myself is the labouring parts to cut down on costs, I'd rather put a cable from cu to garage and spend the entire day doing it than pay someone £40 an hour or whatever to do it for me.

 

[ericmark]

I would say step one is two 20 or 25 amp MCB's and simply split the two supplies if there are no problems no need to go further. If there are problems with tripping then you need to work out what is required.

In the main it is as simple as fitting new thicker cable but not always and you need to do some reading about the supply types and decide what is required.

We tend to jump to conclusions and expect every house has a TN-C-S supply (Also called PME) but this is not always the case. With a TN-C-S supply we assume Ze is 0.35Ω and we select cable on that basis but if for example the supply was TN-S then Ze could be 0.8Ω.

When the new consumer unit was fitted we would hope you got an installation certificate (Plus either a compliance or completion certificate) the installation certificate should tell you what system you have and all the readings taken. You can use that as a starting point to work out what is required.

There are plug in testers with a loop test but most seem to be designed for the old 250 volt system and pass at 1.5Ω the pass however is now under that figure but better than nothing.

 

[deadshort]

Hi,i would advise you have an electrician confirm that you do not have a ring circuit before you split the circuit onto 2x 20A breakers. You could end up with a 40a ring main !

If you run a dedicated supply to you garage i would advise you to fit a DP switch at the house end, this will enable you isolate the supply in the event of a fault.

Regards,

DS

 

[JohnW2]

Hi,i would advise you have an electrician confirm that you do not have a ring circuit before you split the circuit onto 2x 20A breakers. You could end up with a 40a ring main !

Agreed - although, in electrical (not necessarily regulatory) terms I might not have too much of a problem with a "40A ring final", per se (see **), to have the two ends of a ring connected to different MCBs could obviously present potentially serious hazards.

** lest people wonder why I said that .... although obviously not compliant with the explicit dispensation in the regs regarding ring finals, if (as is likely) the 2.5mm cable were 'clipped direct' (hence CCC=27A), a 40A ring final would, in electrical terms, if anything be fractionally better/safer than a (compliant) 32A one with the (regs-permitted) minimum CCC of 20A - since the ratio of CCC to OPD rating would be 0.675, rather than the 0.625 in the case of the regs-dispensated 32A ring final.

If you run a dedicated supply to you garage i would advise you to fit a DP switch at the house end, this will enable you isolate the supply in the event of a fault.

That certainly wouldn't do any harm. However, given that we're talking about an attached garage, rather than any sort of 'outdoor installation', the garage circuit is probably no more likely to develop a fault than any other circuit in the house.

Kind Regards, John

 

[EFLImpudence]

But we don't know what the stipulated minimum CCC would be for a 40A ring.

Does the 0.625 have any relevance?

If the regulations did consider the same figure for a 40A ring then the minimum CCC would be 25 which, as you will obviously see, is the same 2.5mm² conductors as are normally used.

This then begs two questions, doesn't it? ...

 

[JohnW2]

But we don't know what the stipulated minimum CCC would be for a 40A ring. Does the 0.625 have any relevance?

My point was that the regs allow a ring final for which the CCC of the cable is only 62.5% of the In of the 32A OPD - so a hypothetical 40A ring in which the CCC of the cable was 67.5% of the In of the OPD would be 'at least as safe, wouldn't it?

If the regulations did consider the same figure for a 40A ring then the minimum CCC would be 25 which, as you will obviously see, is the same 2.5mm² conductors as are normally used.

Are you not saying the same as me - namely that a hypothetical 40A ring with cable CCC of 25A (I was suggesting 27A, for Method C 2.5mm²) would be at least as safe (in fact, fractional 'safer' when CCC=27A) as a 32A ring with cable CCC of 20A (which is compliant)? ... and, in general, there is no objection to doing something which is 'safer' than what it allowed by BS7671!

Kind Regards, John