Garage electrics from main house

Neptune · May 19, 2022

We have a garage attached to the property and are having it converted to an office with a small toilet. Electrics will include, internal lights, an external light, a few sockets and a extractor fan.

There is a cable coming into the garage currently from the house and is connected into the main consumer unit. It's a 2.5mm T&E cable wired into a 32amp MCB. It has RCD protection.

I think it would be okay to use the existing cable as a radial circuit for a few sockets and also have it initially split into a FCU (3amp Fuse) and have this running a lighting circuit? I would use 2.5mm cable for the sockets and 1.5mm cable for the lights. Would this be okay?
If yes, what would happen, if the load on the sockets was higher than intended? I assume it would trip the breaker in the house

Alternatively, what would be the advantages of using one of these?

Axiom Metal Garage/Workshop Consumer Unit 3 Way | Toolstation

BS 7671 IET Amendment 3. BS 61439-3. • 1 x 63A 30mA RCD • 3 x Type B MCB (1 x 32A, 1 X16A 1 x 6A) • Dimensions: (HxWxD) 185 x 145 x 110mm

www.toolstation.com

I could have the incoming cable going into this RCD and have a Ring for the sockets and a separate circuit for internal and one further for the external lights.

Can I please have your advice on the above. Thanks in advance.

buzzlightyear · May 19, 2022

Neptune said:
There is a cable coming into the garage currently from the house and is connected into the main consumer unit. It's a 2.5mm T&E cable wired into a 32amp MCB. It has RCD protection.

no the the answer ,you can not put two rcd in series. up grade the supply to the garage ,by the way employ a spark to do it .

UNG · May 19, 2022

Neptune said:
We have a garage attached to the property and are having it converted to an office with a small toilet. Electrics will include, internal lights, an external light, a few sockets and a extractor fan.

There is a cable coming into the garage currently from the house and is connected into the main consumer unit. It's a 2.5mm T&E cable wired into a 32amp MCB. It has RCD protection.

I suspect that what you have is a spur off an existing ring circuit

Neptune said:
I think it would be okay to use the existing cable as a radial circuit for a few sockets and also have it initially split into a FCU (3amp Fuse) and have this running a lighting circuit? I would use 2.5mm cable for the sockets and 1.5mm cable for the lights. Would this be okay?
If yes, what would happen, if the load on the sockets was higher than intended? I assume it would trip the breaker in the house

As it is likely to be a spur given the 32A MCB with what you are proposing may overload / exceed the current carrying capacity of the single 2.5mm² T&E you have which could potentially cause a fire

Neptune said:
Alternatively, what would be the advantages of using one of these?

Axiom Metal Garage/Workshop Consumer Unit 3 Way | Toolstation

BS 7671 IET Amendment 3. BS 61439-3. • 1 x 63A 30mA RCD • 3 x Type B MCB (1 x 32A, 1 X16A 1 x 6A) • Dimensions: (HxWxD) 185 x 145 x 110mm

www.toolstation.com

I could have the incoming cable going into this RCD and have a Ring for the sockets and a separate circuit for internal and one further for the external lights.

It would be a bit pointless what would you use the 32A MCB for as you would have the same problem that I highlighted above

Neptune said:
Can I please have your advice on the above. Thanks in advance.

Wait for it........

It is probably not the answer you want. But get an electrician in as this is a bit more than a simple DIY job

Neptune · May 19, 2022

UNG said:
I suspect that what you have is a spur off an existing ring circuit

The 32amp breaker in the house (on RCD protection) is only being used by this 2.5mm cable. It does not power anything else. I suspect that this needs to be downgraded to 20amp?

The point of using a garage consumer unit would be that I could then have a ring running off a 32amp breaker (2.5mm T&E cable) along with lighting off a 6amp breaker (1.5mm T&E cable). I know the house CU restricts me to 20amp. I thought that this would provide future proofing. If I ever wanted to increase the capacity in the office, I would just need to upgrade the incoming cable and breaker from the house. Everything in the garage would be done to the correct standard.

I don't understand what the issue with RCD's in series is. I have a shed that has RCD protection in the main consumer unit (inside house) and another RCD (as part of a shed consumer unit) in the shed. I have had no issues in 4 years.
I could have a non-RCD board in the office but want to understand if this is entirely necessary.

I appreciate your support with this.

i=p/u · May 19, 2022

Course ya can do it, but I smell deviations and not dedication to the cause...

littlespark · May 19, 2022

It seems to be a default answer to “get an electrician” on a DIY question, so forgive us. We don’t know your competency level, although you have said some of the right things…

So basically… the supply from the house is wrong, so either degrade the mcb to 20A for 2.5mm cable, or upgrade the cable to 4mm on a 32A

Having a separate CU is future proofing, and easier to isolate one circuit or the other.

Depending what’s going on the office, a 16A radial circuit may suffice, but there’s nothing wrong with future proofing and installing a 32A ring if you want.

The lighting can easily cope with 1.00mm cable rather than 1.5, (on a 6A breaker)

There is no problem have two RCDs. There will be one in the house to protect the cable from house to garage and possibly another in the garage CU. It just means one may trip quicker than the other. Sometimes a nuisance if they are far apart, such as house and a shed at the bottom of the garden, but not so much on an adjoining garage.

Is it just a toilet? There are special regulations for bathrooms and shower rooms, but not just a WC.
If you’re having an extractor fan in the WC, check the manufacturers instructions as many ask for a 3A fuse to protect the fan, in addition to the 6A mcb on the lighting.

Neptune · May 19, 2022

littlespark said:
It seems to be a default answer to “get an electrician” on a DIY question, so forgive us. We don’t know your competency level, although you have said some of the right things…

No probs. Appreciate your helpful response.
You're right, its often the default response. If you try to challenge it, someone will point out that "you don't understand the basics" because you've note described something completely correctly, etc...

littlespark said:
So basically… the supply from the house is wrong, so either degrade the mcb to 20A for 2.5mm cable, or upgrade the cable to 4mm on a 32A

Having a separate CU is future proofing, and easier to isolate one circuit or the other.

Depending what’s going on the office, a 16A radial circuit may suffice, but there’s nothing wrong with future proofing and installing a 32A ring if you want.

Thanks for validating this

littlespark said:
The lighting can easily cope with 1.00mm cable rather than 1.5, (on a 6A breaker)

I find that my CK Wire stripper doesn't cope too well with 1mm. When stripping the sheath, it goes through to the strands. Its fine with 1.5mm and upwards.

littlespark said:
There is no problem have two RCDs. There will be one in the house to protect the cable from house to garage and possibly another in the garage CU. It just means one may trip quicker than the other. Sometimes a nuisance if they are far apart, such as house and a shed at the bottom of the garden, but not so much on an adjoining garage.

That's very helpful. This feels neater than having FCU's. Wil order the unit from toolstation.

littlespark said:
Is it just a toilet? There are special regulations for bathrooms and shower rooms, but not just a WC.

It's a shower too. Not an electric shower. The light will be one that is suitable for zone 2 and will be situated accordingly. I will have a pull cord switch in the bath.
The fan isolator switch will be outside the bathroom.
Let me know if I should be considering anything else.

littlespark said:
If you’re having an extractor fan in the WC, check the manufacturers instructions as many ask for a 3A fuse to protect the fan, in addition to the 6A mcb on the lighting.

Looking to use this: Xpelair DX100 100mm Extractor Fan Timer | Toolstation - https://www.toolstation.com/xpelair-dx100-100mm-extractor-fan/p14697
Do you know if it will be okay off the 6amp MCB or require an additional fuse?

littlespark · May 19, 2022

Hmm… now it becomes a “room containing a bath or shower” and comes under special locations in the regs.

In England and Wales there is Part P building regs to consider, and as I’m Scottish we don’t have to deal with it, but maybe one of my English / Welsh colleagues can clarify whether a shower room needs notifying to council or not??

Neptune · May 19, 2022

With this CU, Looking at the knockouts, I will need to set it into the wall so that the cable entry holes are below plasterboard level. Is that the case?

Axiom Metal Garage/Workshop Consumer Unit 3 Way | Toolstation

BS 7671 IET Amendment 3. BS 61439-3. • 1 x 63A 30mA RCD • 3 x Type B MCB (1 x 32A, 1 X16A 1 x 6A) • Dimensions: (HxWxD) 185 x 145 x 110mm

www.toolstation.com

Also, which type of glands do I use with this? I have a 2.5mm cable coming in and then going out are : 2x2.5mm and 2x1.5mm

i=p/u · May 19, 2022

Course it does' Notifiable work consists of: the installation of a new circuit. the replacement of a fuse board/consumer unit. any addition or alteration to existing circuits in a 'special location' (i.e. a room containing bath or shower)

UNG · May 19, 2022

littlespark said:
It seems to be a default answer to “get an electrician” on a DIY question, so forgive us. We don’t know your competency level, although you have said some of the right things…

This job is turning into a little bit more than a DIY project and probably warrants getting an electrician in so it is installed and more importantly TESTED and certified I'm not against the odd bit of DIY but this will no doubt end up with another round of posts when the potential problems start
A few days of reading on professional forums would probably give you enough lingo to say most of the right things and make you sound convincing that you know what you are doing

littlespark · May 19, 2022

The forum has had these issues before.

We have a DIYers asking questions, so we are expected to assist in some way, without giving step by step instructions of course. If we don't guide in the right direction, then some of these DIYers may carry on regardless, causing danger to themselves and others.

We can advise.... We can state the safety implications.... We can take a guess at the type of person asking the question, and of course, if we think the job is beyond a particular persons ability, then we can say so.
But as mentioned... they may carry on regardless. That is their prerogative (and their fault if anything goes wrong) after being given advice.

I don't agree that i should get a 'dislike' for giving advice.

nicebutdim · May 19, 2022

littlespark said:
There is no problem have two RCDs. There will be one in the house to protect the cable from house to garage and possibly another in the garage CU. It just means one may trip quicker than the other. Sometimes a nuisance if they are far apart, such as house and a shed at the bottom of the garden, but not so much on an adjoining garage.

I'd be interested to learn how an RCD provides adequate protection for that cable.

littlespark · May 19, 2022

nicebutdim said:
I'd be interested to learn how an RCD provides adequate protection for that cable.

I was just explaining why 2 RCDs (one at source, one at garage) isnt really a problem if you don't mind which one trips.
The OP stated it was a 2.5 T&E, on a 32A (which has already been advised to derate) and it was already protected by an RCD

nicebutdim · May 19, 2022

littlespark said:
I was just explaining why 2 RCDs (one at source, one at garage) isnt really a problem if you don't mind which one trips.
The OP stated it was a 2.5 T&E, on a 32A (which has already been advised to derate) and it was already protected by an RCD

Was wondering more about the circuit, but now realise that OP states garage is attached to the property - a point I missed and one which prompted me to ask this question.

loz2754 · May 20, 2022

littlespark said:
I don't agree that i should get a 'dislike' for giving advice.

I wouldn't worry too much about getting a dislike. I just got one on another thread for giving my opinion. From the same 'disliker', in fact.

I think the dislike button should be removed, as it only serves to provoke ill feeling.

People may by all means disagree, preferably backed up by a reason for such disagreement.

UNG · May 20, 2022

littlespark said:
The forum has had these issues before.

And will continue to have the these problems after all electrics is only a few wires and there is nothing speciai in connecting them all up

littlespark said:
We have a DIYers asking questions, so we are expected to assist in some way, without giving step by step instructions of course. If we don't guide in the right direction, then some of these DIYers may carry on regardless, causing danger to themselves and others.

Guidance = get an electrician in to advise and install in the safest way possible

littlespark said:
It seems to be a default answer to “get an electrician” on a DIY question, so forgive us. We don’t know your competency level, although you have said some of the right things…

IMO this isn't guidance it's carry on regardless

littlespark said:
We can advise.... We can state the safety implications.... We can take a guess at the type of person asking the question, and of course, if we think the job is beyond a particular persons ability, then we can say so.
But as mentioned... they may carry on regardless. That is their prerogative (and their fault if anything goes wrong) after being given advice.

Sometimes it is best to not reply with any advice no matter how limited it is, then you cannot be implicated if an incident occurs

littlespark said:
I don't agree that i should get a 'dislike' for giving advice.

I gave you my reason in the post prior to yours but you appear to be playing the devils advocate

loz2754 said:
I think the dislike button should be removed, as it only serves to provoke ill feeling.

How woke. Maybe all the buttons should be removed so as not to cause offence anyone by someones choice

loz2754 said:
People may by all means disagree, preferably backed up by a reason for such disagreement.

We have a disagree button I chose not to use it and if I do I give a reason

Neptune · May 20, 2022

I've cancelled the plans for a shower. Thanks for informing me of the additional regs on that.
I can also see we are getting into the spiral of "get a electrician", "but I can do this", "no you can't, get an electrician"...

As a DIY'er who has been involved in many electrical projects and takes safety very seriously, I do find this frustrating. I know I can do this with some guidance from you. It's your experience that draws us to these forums rather than finding this information in piecemeal across the web.

I do have a couple of questions I would like your advice on please.

1. The garage consumer unit seems to be designed for surface mounted cables. This is not the case in my office as the cables will be under the plasterboard. The unit only has top and bottom entry. Is it a case of sinking the unit into the wall deep enough so the cable entry is below the plasterboard?

2. In my situation, can I use the 20mm knockouts with the socket grommets used for metal boxes? I can then use the fire intumescent sealant to close the gap after the cables have been fed through.

3. I need to join the incoming cable (from house) to a longer length as it wont reach the office consumer unit in its current state. I have sunk a 1 gang box in the wall and will make the join here and use a blanking plate once finished. I am planning to use a 10mm cable from this box/join to the consumer unit. Again, provide future proofing. In the future I can change the incoming cable to this join and everything downstream should be decent enough.
How do I join the 2.5mm cable and 10mm cable within this 1 gang box? I normally use wagos...

Thanks for your support

westward10 · May 20, 2022

From your past threads you have had plenty of guidance for your 'electrical projects' but the forum will draw a line at giving step by step advice even though this has been the case with some of these. You take safety very seriously but having a rudimentary knowledge of the works will hinder your take on this because without truly understanding the principles electrical safety it is doubtful you can employ them. All the questions you ask in #18 and previous posts suggest you should not be undertaking these works and there is also the question of notification to Building Control under Part P of the Building Regulations.

Neptune · May 20, 2022

westward10 said:
All the questions you ask in #18 and previous posts suggest you should not be undertaking these works

Which question in post #18 is concerning you?

westward10 · May 20, 2022

Neptune said:
Which question in post #18 is concerning you?

All of them.

Neptune · May 20, 2022

westward10 said:
All of them.

OK...
Let's go through the first one on the "garage consumer unit" not having back entry for cables and asking whether people sink some of it in the plasterboard to hide the incoming cables on top and bottom.

Why is that a shocking question?

westward10 · May 20, 2022

Neptune said:
OK...
Let's go through the first one on the "garage consumer unit" not having back entry for cables and asking whether people sink some of it in the plasterboard to hide the incoming cables on top and bottom.

Why is that a shocking question?

I am not going to entertain you with an answer.

buzzlightyear · May 20, 2022

Case closed what the man's says above , I mentioned this in post #2.

DPG · May 20, 2022

Neptune said:
OK...
Let's go through the first one on the "garage consumer unit" not having back entry for cables and asking whether people sink some of it in the plasterboard to hide the incoming cables on top and bottom.

Why is that a shocking question?

I'm someone who is all for people doing their own things within their comfort zone and ability. Electrics, car repairs, plumbing, whatever. However, I think you do need to get an electrician in for this one. Some of the things you are asking (eg. how to join 2 cables of differing sizes, and how to get cables into the CU) show that you lack some key knowledge to complete this job. And that's before we get on to doing the necessary testing.

There's no shame in knowing your limits. You seem more capable than most DIYers and you should be proud of that.

It's like car repairs - I can do the basics such as servicing, brake replacement etc. but anything beyond that goes to the mechanic so I know it gets done safely.

littlespark · May 20, 2022

DPG said:
I'm someone who is all for people doing their own things within their comfort zone and ability. Electrics, car repairs, plumbing, whatever. However, I think you do need to get an electrician in for this one. Some of the things you are asking (eg. how to join 2 cables of differing sizes, and how to get cables into the CU) show that you lack some key knowledge to complete this job. And that's before we get on to doing the necessary testing.

There's no shame in knowing your limits. You seem more capable than most DIYers and you should be proud of that.

It's like car repairs - I can do the basics such as servicing, brake replacement etc. but anything beyond that goes to the mechanic so I know it gets done safely.

Agree with this, but need to question the bottom paragraph.

I would say brake replacement should be done by a mechanic. How would your insurance fare if you crashed because your car didn’t stop when it should have…. Because you replaced the brakes yourself.

Personal experience. One of my first cars, my dad changed the pads… They worked fine, but didn’t release. Had to change them again 3 months later because it was braking all the time.

Could also argue that anyone shouldn’t be responsible for changing a wheel, as it might fall off at speed if not done properly.

Im guilty of it myself, instantly telling someone to get a professional in… sometimes it is the best advice when the OP asks something completely out of order….
This thread started off ok, but the mention of joining onto 10.00mm is unnecessary.

A couple of years ago, @Dan the forum boss posted guidance on how to help DIY, which I alluded to by saying DIYers would do the work anyway, with or without our help.

DPG · May 20, 2022

littlespark said:
Agree with this, but need to question the bottom paragraph.

I would say brake replacement should be done by a mechanic. How would your insurance fare if you crashed because your car didn’t stop when it should have…. Because you replaced the brakes yourself.

Personal experience. One of my first cars, my dad changed the pads… They worked fine, but didn’t release. Had to change them again 3 months later because it was braking all the time.

Could also argue that anyone shouldn’t be responsible for changing a wheel, as it might fall off at speed if not done properly.

Im guilty of it myself, instantly telling someone to get a professional in… sometimes it is the best advice when the OP asks something completely out of order….
This thread started off ok, but the mention of joining onto 10.00mm is unnecessary.

A couple of years ago, @Dan the forum boss posted guidance on how to help DIY, which I alluded to by saying DIYers would do the work anyway, with or without our help.

Brake replacement is well within the scope of a decent home mechanic - there is nothing particularly complex about changing brake pads or even discs. But again, it relies on being in your comfort zone and understanding what needs to be done. I know a couple of people who replace springs and shock absorbers, and I know they are capable of doing this safely. Personally I'm not comfortable doing this so I would get it done by someone else.

Neptune · May 20, 2022

In case someone else wants to come along and provide some advice but also to clear this up. The question on joining 10mm cable had some context. Its there to provide future proofing as any future armoured cable from the house can connect to this.
The proposed join is an accessible join which I assume does not need to be maintenance free and I also mentioned that I’ve used wagos in the past. I’m not aware that wago do a suitable product for this situation.

I was looking for some validation and clarity around this.

littlespark · May 20, 2022

10mm is big enough for an electric shower or cooker… overkill for a handful of sockets and lights…

Neptune · May 20, 2022

littlespark said:
10mm is big enough for an electric shower or cooker… overkill for a handful of sockets and lights…

If in the future, if we wanted additional draw, it will provide scope for that. I appreciate its well over spec’d for todays requirements.

It’s difficult to explain but if we wanted to upgrade the existing 2.5mm supply cable in the future, it could come and connect in this box and everything downstream is setup for higher loads e.g. 10mm to CU, RFC on 32 amp using 2.5mm cables, etc. There will be no need to rip up the plasterboard. That was my rationale.

DPG · May 20, 2022

Neptune said:
If in the future, if we wanted additional draw, it will provide scope for that. I appreciate its well over spec’d for todays requirements.

It’s difficult to explain but if we wanted to upgrade the existing 2.5mm supply cable in the future, it could come and connect in this box and everything downstream is setup for higher loads e.g. 10mm to CU, RFC on 32 amp using 2.5mm cables, etc. There will be no need to rip up the plasterboard. That was my rationale.

The idea is sound, and it makes sense to future proof things.

Neptune · May 20, 2022

And…I plan to use 60amp terminal strip to join the 10mm and 2.5mm cable inside the box I have situated in the wall. Will be covered with blanking plate.

UNG · May 20, 2022

Neptune said:
In case someone else wants to come along and provide some advice but also to clear this up. The question on joining 10mm cable had some context. Its there to provide future proofing as any future armoured cable from the house can connect to this.
The proposed join is an accessible join which I assume does not need to be maintenance free and I also mentioned that I’ve used wagos in the past. I’m not aware that wago do a suitable product for this situation.

I was looking for some validation and clarity around this.

This just seems to be a penny pinching solution
Why not just replace the cable back to the CU from your garage unit in one continuous length would probably give you a bit more kudos.

Neptune · May 20, 2022

UNG said:
This just seems to be a penny pinching solution
Why not just replace the cable back to the CU from your garage unit in one continuous length would probably give you a bit more kudos.

May be at some point…
Will cause havoc by way of redecor that will be required. Also, this is ample capacity for what I need at the moment. Should it need to be increased, I’ve tried to future proof and avoid re-work wherever possible.

Thanks for your help

brianmoooore · May 21, 2022

DPG said:
I know a couple of people who replace springs and shock absorbers, and I know they are capable of doing this safely. Personally I'm not comfortable doing this so I would get it done by someone else.

No one should be comfortable changing coil springs on a strut. The effects of the energy released when a clamp slips off of a compressed coil spring can be frightening. I always tie some strong string between the spring and a fixed object to at least stop it bouncing off every wall in the workshop.

nicebutdim · May 22, 2022

brianmoooore said:
No one should be comfortable changing coil springs on a strut. The effects of the energy released when a clamp slips off of a compressed coil spring can be frightening. I always tie some strong string between the spring and a fixed object to at least stop it bouncing off every wall in the workshop.

It's not difficult to do safely with the right equipment, but those cheap clamps that people use are frightening.