replacing old consumer unit in garage with new

[kshah86]

hello, i need some help. im a trainee electrician. you probably going to say i should know this but i just want to make sure.
basically im changing an old consumer unit in a friends garage for a new one. garage is in the back of garden. the power to the unit is coming from the main consumer unit which is in the house ( roughlt 10 to 15 meters distance between main and garage boards). right now the garage is feeded from a 45a mcb on the main consumer unit.

currently on garage consumer unit is 1 main ring and 1 lighting circuit

im going to buy a garage consumer unit from screwfix.

MK 6-Way 8-Module RCD Garage Consumer Unit with 6 MCBs


this come with 1 x 63A 30mA RCD. this will be the main isolation switch so my question is this:

will this be ok as it is coming from a feed which is running of a 45a mcb?

 

[Jamzie]

What is installed at the moment and why are you replacing it? Is the existing board fed from an RCD?

 

[dan86]

I'm wondering the same thing tbh. 6 way seems a tad overkill for 2 circuits!
and why replace..?

 

[crow11]

what type and size of cable is the old garage supply on

 

[crow11]

whats wrong with the BG garage c/u at screwfix £19.99, 40A rcd and 6A and 32A mcb's included, if you need to reduce down new mcb £3.49, less than half the price of the one your thinking of

 

[Octopus]

And what on earth does your "friend" want to do/run in their garage??

6 way CU for a garage - madness.

 

[dan86]

seen houses with less than 6 circuits!

 

[Engineer54]

What is installed at the moment and why are you replacing it? Is the existing board fed from an RCD?

If the main house consumer unit is just as old, and doesn't have RCD protection, i'd be telling his friend to spend the money on replacing the house CU!! lol!!

6 way's on a garage CU, that has 2 circuits, come on your got to be having a laugh with us!!

 

[sajeel]

I would go with crow advice and use 2 way consumer unit. Feed should be via armour cable. 45a is more than enough for some sockets and lighting. Unless he is trying to convert into a room and rent ?

 

[kshah86]

Thanks for all the replies.

The consumer inside is fairly new (last 5 years) and seems to up to 17th edition regs.

The feed to the garage is via swa cable.

Current consumer unit in garage is very old. No rcd protection in the unit. I have a picture but I can’t upload as i’m new to this forum.

The feed coming from inside is on a 45a mcb which is on an rcd protected circuit. Does this mean that it is ok? And should I leave it as it is.

Reason for the 6 way consumer unit as that right now there is a old two way unit installed. One is a ring circuit which is fine. However the second is lighting and currently has 4 live wires going into 1 mcb. So that needs to be changed to 2 lighting circuits if I am right?. I can’t find a 3way unit so the next step up from a 2 way is the 6 way. I will only use 3 circuits and put blank plates on the rest. Also the garage is fairly big so if they want to convert it later or add more things then is it not better to have a 6 way?

Also do I need to do periodic on this before I start or do I just do all the testing once finished new install?

Apologies if my questions sound a bit dumb. This is my first install on my own. Us sparkies all got to start somewhere. This will be a real confidence builder for me and hopefully make me more competent!

Another reason for changing the unit apart from it being old is that recently there was a fault on an appliance which was on the ring. Basically there was a water heater connected to the ring. My friend said he went into the garage and switched on the lights and he heard a pop and smelt burning. Then the lights went out. I went around to investigate the problem. The 45a mcb inside had tripped. In the garage I opened up the plug which the water heater was on to see that the fuse had burned out and plug was all melted. Also had a look at the water heater and identified that the switch on the water heater had also melted from the inside. I fixed the water heater (replaced plug and fuse, and switch and all internal wiring in heater as it had been burnt) and now its working fine after doing continuity and insulation resistance tests on appliance and could not find any other faults.
Before I switched the power back on I done a few dead tests on both the lighting and ring (continuity and insulation resistance, polarity) to check weather there was any faults like loose wiring or bad insulation). I could not find any.
Am I right in thinking that the cause of this fault was a bad fuse in the plug or something in the water heater like a faulty thermostat?, or could it have been a surge which caused an overcurrent to flow to this appliance which blew the fuse?

Also got me thinking that if there was an rdc on the garage ring, would this have tripped and preventing the burning of plug?

 

[telectrix]

if the SWA is fed from an RCD protected way, you don't want another RCD in the garage. also, what CSA is the SWA. if it's 6mm, a 45A MCB is about on the limit. any extraneous metalwork in the garage needs to be bonded as well. if it's TNC-S earthing, then it needs 10mm back to the house MET.

 

[kshah86]

if the SWA is fed from an RCD protected way, you don't want another RCD in the garage. also, what CSA is the SWA. if it's 6mm, a 45A MCB is about on the limit. any extraneous metalwork in the garage needs to be bonded as well. if it's TNC-S earthing, then it needs 10mm back to the house MET.

so if i install a new consumer unit, should i just have and isolating switch instead of an rcd?

also water pipes in garage are fed from inside house, and they are bonded already inside so do i still need to bond in garage?

if i do need to still bond in garage, can i not just bond the pipes to the new consumer unit as this will create an earth path going back inside?

 

[telectrix]

as long as the copper is continuous into the garage from the house, then they're fine as they are. but no harm in adding an extra bonding conductor to the garage CU, as long as the garage earthing comes from the house. the only problem with having the RCD in the house CU is that a fault in the garage will trip in the house. if the feed to the garage does not have any non-SWA buried in walls then it's better to feed from a non-RCD way in the house and fit RCD in garage, or from a dedicated RCBO, so that it will not trip any other circuits in the house.

 

[kshah86]

as long as the copper is continuous into the garage from the house, then they're fine as they are. but no harm in adding an extra bonding conductor to the garage CU, as long as the garage earthing comes from the house. the only problem with having the RCD in the house CU is that a fault in the garage will trip in the house. if the feed to the garage does not have any non-SWA buried in walls then it's better to feed from a non-RCD way in the house and fit RCD in garage, or from a dedicated RCBO, so that it will not trip any other circuits in the house.

thanks for the reply, the swa is buried in the wall, and also when that recent fault happened in the garage, only the 45a mcb was tripped in the house and not the rcd

 

[sajeel]

But there is potential of rcd tripping and losing electrics in house. I would move to non rcd mcb add 2 way if no heaters are used. If you add heater then the best way is to wire a spur circuit as plug melt on a ring is common and will occur again over time you need to work out load of all heaters and wire accordingly maybe 6 way with 3 or 4 circuit s before that total load calculation is necessary.

 

[trev]

"the consumer unit is fairly new and seems up to the 17th edition regs"
I'd expect a trainee to be able to tell the difference between a 17th compliant CU and a one that wasn't compliant unless of course that trainee had only got a few weeks under his belt, in which case the trainee should be nowhere near a CU.

The 4 lives going into one MCB could simply mean that this garage was originally wired by an idiot! How big is this place? Surely the most it would need is two flourescents and there's absolutely no reason why you couldn't put them on one circuit.

I note you say it's fairly big and that they want to convert it at some point in the future but convert it into what exactly? My guess is that it's being converted right now into some sort of accommodation. As a trainee I think you should steer clear of this because it could and probably will come back to bite you on the arse.

I agree that we all have to start somewhere but it comes across that you're very unsure about what is happening here, as most trainees would be.

 

[Octopus]

thanks for the reply, the swa is buried in the wall, and also when that recent fault happened in the garage, only the 45a mcb was tripped in the house and not the rcd

So what does that tell you then??

 

[ratkins]

What about equipotenal zone?

 

[Engineer54]

so if i install a new consumer unit, should i just have and isolating switch instead of an rcd?

also water pipes in garage are fed from inside house, and they are bonded already inside so do i still need to bond in garage?

if i do need to still bond in garage, can i not just bond the pipes to the new consumer unit as this will create an earth path going back inside?

Yes you do. Once a metallic pipe leaves one building and enters another, it requires main bonding. As this is a PME/TNC-S supply to the house, you can only bond from any extraneous pipework etc, to the garage CU's EMT, if the supplying SWA is a 3 core 10mm or larger. If the supply cable is less than 10mm or only 2 core, then you will need to bring in a separate 10mm main bonding cable from the house MET to the garage EMT, to extend the house equipotential zone in to the garage....

 

[electric will]

The original board has a 6 amp mcb for lighting, with 4 lives coming out of it? I'd want to know where these were going.
and the water heater fault took out a 45 amp mcb? But only when the lights where turned on?
ummmmmmm

 

[Engineer54]

The original board has a 6 amp mcb for lighting, with 4 lives coming out of it? I'd want to know where these were going.
and the water heater fault took out a 45 amp mcb? But only when the lights where turned on?
ummmmmmm

Haha, we don't even know, because the OP has neglected to tell us, what size this SWA cable is, or if it's a 3 core or a 2 core?? All we know is that it's protected by a 45A MCB!!