Garage electrics from main house

[Neptune]

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]

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]

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

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

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

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]

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]

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

 

[littlespark]

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]

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...

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

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.

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.

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.

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]

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]

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]

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]

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]

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]

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]

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]

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.