shed consumer unit

[gsc]

Hi,

I am looking to have a she/garage consumer unit running from my house 10 way 100A consumer unit.

An electrician told me he will get an rcbo into my house consumer unit and then run armoured cable from that to the shed unit. I am just wondering if that sounds right and how you think he's going to connect it to my house consumer unit..

I am having the shed unit which is 25m away from house unit because I want outside lighting and power to run power tools.

Any advice is greatly received.

GSC

 

[Richard Burns]

From what you describe the electrician probably knows what he is doing, it is not how I would do it but it is a viable option.
If he can put an RCBO into a spare way on the consumer unit then he is good to go.

 

[NDG Elecs]

Yep agree with RB. It depends partly on matters such as earthing arrangement and the current consumer unit, if a high integrity type then all good to go. If it's not I would used a sub board to eliminate possible nuisance tripping in the future. Especially if the armoured cable is being buried underground.

 

[Spoon]

Hi gsc. Welcome to the forum. What power tools are you intending to use in the shed? What size RCBO is the electrician thinking of using?

 

[gsc]

Rcbo 40A according to the electrician... things like a table saw, drill, circular saw and bits like that. Also a pressure washer but all these will not be used at the same time...

He said he would put the rcbo next to the mains and connect it with a busbar...

I don't know much but didn't know if that was acceptable...???

Armoured cable was going to be 6mm and running 40m.... He looked at all my things I would use including lighting and thinks it won't go over 6kw ever...

Does that sound acceptable?

Thank you for your help and advice because I would rather have other electricians agree with his method haha...

GSC

 

[Guest77]

Hi,

I am looking to have a she/garage consumer unit running from my house 10 way 100A consumer unit.

An electrician told me he will get an rcbo into my house consumer unit and then run armoured cable from that to the shed unit. I am just wondering if that sounds right and how you think he's going to connect it to my house consumer unit..

I am having the shed unit which is 25m away from house unit because I want outside lighting and power to run power tools.

Any advice is greatly received.

GSC

What do you do for a living?

 

[Darkwood]

As you have variable loading it must be assumed the rating of the rcbo is the load factor here for calculating volts drop even giving some diversity here the volts drop will not comply if referencing from the BS7671 and your the info supplied is correct, so no this is not designed correctly IMHO.

 

[Spoon]

There are different ways you can do this installation. People do things differently.
An alternative way would be to use a breaker in the house CU, as armoured cable doesn't need RCD protection. Then have a CU with RCD in the shed. This is also better in that if the RCD trips for any reason you don't have to go back to the house to reset it.
Also 6mm cable over 40 meters: depending on the manufacturers specs, its close to the 3% voltage drop allowed. I'm sure your electrician has got the manufacturers specs and worked all this out, so all should be fine.
Can I ask why you don't trust your electrician?

 

[telectrix]

There are different ways you can do this installation. People do things differently.
An alternative way would be to use a breaker in the house CU, as armoured cable doesn't need RCD protection. Then have a CU with RCD in the shed. This is also better in that if the RCD trips for any reason you don't have to go back to the house to reset it.
Also 6mm cable over 40 meters: depending on the manufacturers specs, its close to the 3% voltage drop allowed. I'm sure your electrician has got the manufacturers specs and worked all this out, so all should be fine.
Can I ask why you don't trust your electrician?

unless, of course, the house installation is TT.

 

[Darkwood]

There are different ways you can do this installation. People do things differently.
An alternative way would be to use a breaker in the house CU, as armoured cable doesn't need RCD protection. Then have a CU with RCD in the shed. This is also better in that if the RCD trips for any reason you don't have to go back to the house to reset it.
Also 6mm cable over 40 meters: depending on the manufacturers specs, its close to the 3% voltage drop allowed. I'm sure your electrician has got the manufacturers specs and worked all this out, so all should be fine.
Can I ask why you don't trust your electrician?

Manufacturers specs may give a different max load for cable but the voltage drop for a 70c rated cable 6mm would be the same I would have thought as coppers has a fixed resistive value at any given size whether you reference from the manufacturers or the BS7671 and will only change if the temperature rating of the cable was different.

 

[Spoon]

unless, of course, the house installation is TT.

Sorry tel, forgot about that.

 

[Guest77]

Yeah Tel has to chuck in a spanner haha

 

[Spoon]

Manufacturers specs may give a different max load for cable but the voltage drop for a 70c rated cable 6mm would be the same I would have thought as coppers has a fixed resistive value at any given size and will only change if the temperature rating of the cable was different.

I think it changes darkwood. Regs say 7.3. Eland say 7.9

 

[Darkwood]

I think it changes darkwood. Regs say 7.3. Eland say 7.9

Hmmm ! ... you sure the cable your reference isn't 90c rated ... as 7.9 is the mA/m figure for that?... manufacturers may give a different load value to the BS7671 but they cannot change the natural resistive value of copper as its a fixed constant at a given temp' and size regardless where you source the information.

 

[Spoon]

Hmmm ! ... you sure the cable your reference isn't 90c rated ... as 7.9 is the mA/m figure for that?... manufacturers may give a different load value to the BS7671 but they cannot change the natural resistive value of copper as its a fixed constant at a given temp' and size regardless where you source the information.

Sorry darkwood, you are correct, the spec sheet I had was for 90c.
Looks like the 6mm is not good enough for the 40 meter run then.

I think I may stop posting today.... just can't get thing right......

 

[Guest77]

6mm2 isn't enough on this occassion if a 40 amp design current, but a 20 amp supply may actually be adequate lol, but what I liked about your initial post was your question regarding why the op doesn't trust his spark, I like to know the answers to those questions, and asked what he did for a living as well, and I wasn't being sarcastic, I genuinely would like to know.

 

[Darkwood]

Sorry darkwood, you are correct, the spec sheet I had was for 90c.
Looks like the 6mm is not good enough for the 40 meter run then.

I think I may stop posting today.... just can't get thing right......

We all do dont fret... just wondered how Elend cables had managed to change the laws of Physics - a first since Einstein did it

 

[Spoon]

Just to say to the OP. The voltage drop for 6mm over 40 meters is not recommended. For 37 meters it's ok.

 

[gsc]

I hope I explained okay...

The House CU is a 10 way split load with 2 RCDs.. running different circuits... He said he would plug the RCBO (40A) into the mains switch of the House CU as there is a spare way and connect it via the busbar. Then run the armoured cable above ground to a CU with RCD on the shed...

It's the bit about the RCBO I am worried about... is that the correct way or is it possible that it could be an issue?

Thanks again..

Also MDJ I am a computer technician but I hope you aren't asking that question as a way to make me feel bad for asking if its safe...

GSC

 

[Spoon]

We all do dont fret... just wondered how Elend cables had managed to change the laws of Physics - a first since Einstein did it

haha... At least it's nearly Friday.

 

[markc123]

What do you do for a living?

Funny isn't it ... there was a time when this thread would actually be quite offending in the fact that the OP doesn't believe what his 'professional' spark is telling him. Undermining him bit ect...

However, with the state of some of the 'electricians' these days + the media stories of 'cowboy tradesmen' - you can hardly blame people for coming online and getting a second opinion now.

...although, some of the 'electricans' online are still dodgy - reference a thread you've literally just been commenting on in the last 10mins mike lol.

 

[Spoon]

OP:
Are you sure he is fitting a RCBO to the house CU as it already has RCD protection so doesn't need it.
What is it about the electrician that makes you want to question him?

 

[Guest77]

I hope I explained okay...

The House CU is a 10 way split load with 2 RCDs.. running different circuits... He said he would plug the RCBO (40A) into the mains switch of the House CU as there is a spare way and connect it via the busbar. Then run the armoured cable above ground to a CU with RCD on the shed...

It's the bit about the RCBO I am worried about... is that the correct way or is it possible that it could be an issue?

Thanks again..

Also MDJ I am a computer technician but I hope you aren't asking that question as a way to make me feel bad for asking if its safe...

GSC

I am just interested in why you have a gut instinct about the design, I think you are right to feel a bit wary, perhaps you just have instincts for these things lol, the reason though why he is using an RCBO in the house is because the armoured is above ground, some may say he should have fitted a small db with double pole rcd protection but others may feel the single pole RCBO enough.

 

[Guest77]

OP:
Are you sure he is fitting a RCBO to the house CU as it already has RCD protection so doesn't need it.
What is it about the electrician that makes you want to question him?

The armoured is above ground and therfore the spark isn't taking any chances lol, if it was 2 foot under he probably wouldn't fit RCD protection at the house end.

 

[Spoon]

But as the OP says "The House CU is a 10 way split load with 2 RCDs.." It already has RCD protection.

 

[Guest77]

But as the OP says "The House CU is a 10 way split load with 2 RCDs.." It already has RCD protection.

yes but I would not add a sub main to an existing Trip myself, especially as power tools will be used, the house lights will be going off otherwise I suspect, with a different RCD nothing else in the house can be effected if a fault occurs mate.

 

[Darkwood]

I hope I explained okay...

The House CU is a 10 way split load with 2 RCDs.. running different circuits... He said he would plug the RCBO (40A) into the mains switch of the House CU as there is a spare way and connect it via the busbar. Then run the armoured cable above ground to a CU with RCD on the shed...

It's the bit about the RCBO I am worried about... is that the correct way or is it possible that it could be an issue?

Thanks again..

Also MDJ I am a computer technician but I hope you aren't asking that question as a way to make me feel bad for asking if its safe...

GSC

With the given info the install would not comply due to it not meeting Volts Drop requirements of 3% ?

A rcbo is an acceptable choice of front end device, I assume you have a spare way in your consumer unit and he is fitting the appropriate rcbo for that board in a part of the board which isn't covered by the 2 existing rcd units?

We sometimes question DIYers or other curious people when they reference an Electrician as it can be a ruse to get free advice to do the work themselves where we would normally advice to use a competent person for their own safety or because the work is notifiable, in MDJ's case he just wondered on this occasion why you had a 6th sense to query the design - so please don't take offense over any questioning.

 

[Spoon]

A rcbo is an acceptable choice of front end device, I assume you have a spare way in your consumer unit and he is fitting the appropriate rcbo for that board in a part of the board which isn't covered by the 2 existing rcd units
.

Now it makes sense.....

 

[Richard Burns]

You have said that the design current is never above 26 A (6 kW) so this is the design current he is using to calculate volt drop which then does comply using a 6mm² cable over 25m (and adding some for the internal circuitry).
If you have a split load CU then there is no spare way that is not already RCD protected so any additional RCDs such as the two suggested (RCBO in board and RCD in shed) are redundant but could be used in case of failure of the primary RCD.
If you have a high integrity CU then there may be a spare non RCD way and this could be protected with an RCBO, but in the shed would be more convenient.
Overall a few minor niggles but each to their own.

 

[gsc]

OP:
Are you sure he is fitting a RCBO to the house CU as it already has RCD protection so doesn't need it.
What is it about the electrician that makes you want to question him?

Hi Spoon,

So I have spoken with him again today regarding this.
He has said that what he wants to do is:
In my existing CU I have a spare slot next to my MainsSwitch, where he wants to connect the RCBO via the a Busbar. Then run 6mm2 3Core Armoured Cable to a length of roughly 38m give or take a couple or metres.From there the cable will fit into a Garage/Shed CU via an RCD which will have 2MCBs connected. One for lights and one for power sockets.

Looking at my equipment and lighting needs for the shed andgarden, he doesn’t feel I will go over 6KW of power usage at any one time andfeels that what he has stated will be ample. The Armoured Cable will go inPlastic trunking down a wall and along a concrete floor.
I am not an Electrician but understand it a bit so wanted todouble check that other people felt this was good enough?


I have been screwed over before by an electrician with ahouse rewire and so am very cautious.



Thanks
GSC

 

[Spoon]

Do you know why the armoured cable is going in plastic trunking?

 

[gsc]

Do you know why the armoured cable is going in plastic trunking?

He just said he will run it through plastic trunking to makeit look nice….

 

[Spoon]

He just said he will run it through plastic trunking to makeit look nice….

ok.
I understand you having doubts if last time you got shafted.

 

[gsc]

ok.
I understand you having doubts if last time you got shafted.

So do you feel that what he has offered is good enough? and seem legit?

Regards,

GSC

 

[Spoon]

All depends mate. Its hard to say exactly over a forum, without looking at what he is doing. Regs say not more than 37 meters, so I cant advise on "roughly 38m give or take a couple or metres". It's the same as I can't say to you it's ok to drive 31mph in a 30 zone.
The way the electrician has recommended seems ok as long as he follows the regs. (CYA.... I'm good at that)

 

[gsc]

All depends mate. Its hard to say exactly over a forum, without looking at what he is doing. Regs say not more than 37 meters, so I cant advise on "roughly 38m give or take a couple or metres". It's the same as I can't say to you it's ok to drive 31mph in a 30 zone.
The way the electrician has recommended seems ok as long as he follows the regs. (CYA.... I'm good at that)

Well it's a better response than no..... at least I know he isn't trying to rip me off or anything

Thanks Spoon.

GSC

 

[Spoon]

He still might be trying to rip you off with the price????? Not my place to ask...

 

[Guest77]

So do you feel that what he has offered is good enough? and seem legit?

Regards,

GSC

The job is probably okay, to get over the problem reduce the breaker size, job done.

 

[NDG Elecs]

The job is probably okay, to get over the problem reduce the breaker size, job done.

My thoughts exactly. I would drop down from 40 to 32A at house end. Hell, you could probably get away with a 20A unless you are an (extremely clever!) octopus and capable of using four power tools at once! Even in winter with a heater on probably no more than 20A will be flowing at any one time.

I am guessing that the spark in question is setting the RCBO up unprotected from the dual RCDs...as a high integrity CU. At least that's what I have inferred.

Should we ask if there is any extraneous metal work in the shed....is it PME.....best not go down that path eh!!

Yeah OK, octopuses are very clever.... I mean even clevererer than that tho!

 

[Guest77]

It will take a lot of power to trip a 20 amp supply, I bet my house only draws 40 at christmas with the cooker on and every socket in the house with something plugged in lol