Power to external office

[DeanAlani]

Hi

I've now had a quote and just would like some opinion on this. Its not that I don't trust the proposal. Just after some thoughts.

We have an existing 2.5mm swa cable going to our outside office, the proposal is to feed of a 32amp RCBO in the consumer unit in the house (its an RCBO board) with a 2.5mm swa cable and connect this to the existing 2.5mm swa outside so its new circuit from the board, then feed this is into a non RCD rated consumer unit, as the circuit is being fed from is an RCBO so the office CU will be a standard100amp main switch CU with a breaker for sockets and a breaker for lights in the outside office.

Does this sound ok?
Thanks

 

[Spoon]

2.5mm SWA cable is only good for 29A Ref Method D,
How long is the run?

 

[Soi disant]

Office?

Thought it was a garage.

Something doesn't feel right here....

Thanks - I'm really over thinking this. I have my 2391 Exam in the morning

Interesting thoughts, I remember being on a course a while back, my 2391 there were a lot of people on the practical that just didn't have the confidence or experience. Like a rabbit caught in the headlights, it woke them up that its more difficult than they ever imagined. The college were doing refresher days, I was on one of these refresher days to get a feel of it. My thinking is that there are people who start this trade from new, or even guys from different backgrounds who have chosen to do these courses without a lot of experience, generally they don't have the experience but want to get the qualification. They need some confidence & awareness, real world experience is good for this. I want to offer real world refresher courses & courses for beginners/DIY who want to get better at the trade, work safe etc. Level 2 inspection & testing refresher training etc. .... that is my thoughts on this I'm still digging deep into my brain to see if this is viable enough

 

[DeanAlani]

2.5mm SWA cable is only good for 29A Ref Method D,
How long is the run?

Its roughly a 25 meter run

Whats your thoughts, is the proposal correct?
I appreciate you haven't seen the job and can only give your opinion, which will be appreciated

I just want a new supply to our outside office, not much is in there a few computers and LED lighting
Cheers

 

[DeanAlani]

Office?

Thought it was a garage.

Something doesn't feel right here....

its a garage but i want to convert it to an office so technically its a garage that's about to become an office

 

[Spoon]

Its roughly a 25 meter run

Whats your thoughts, is the proposal correct?
I appreciate you haven't seen the job and can only give your opinion, which will be appreciated

I just want a new supply to our outside office, not much is in there a few computers and LED lighting
Cheers

How is the cable run? In the ground?

 

[DeanAlani]

How is the cable run? In the ground?

At the moment the existing cable is in a fused spur from our living room, with the cable going outside underground. As I'm told the new cable will be clipped on the wall. the existing cable will be removed from the spur and the new cable joined to it in a junction box. So new cable clipped, existing underground and staying - bit long winded, sorry :-/

 

[Spoon]

At the moment the existing cable is in a fused spur from our living room, with the cable going outside underground. As I'm told the new cable will be clipped on the wall. the existing cable will be removed from the spur and the new cable joined to it in a junction box. So new cable clipped, existing underground and staying - bit long winded, sorry :-/

No worries mate.
You can't connect 2.5mm SWA cable to a 32A RCBO, as it's only rated for 29A

Edit: The cable is only rated for 29A

 

[DeanAlani]

No worries mate.
You can't connect 2.5mm SWA cable to a 32A RCBO, as it's only rated for 29A

Ok, thanks. good to know
So, if it was suggested a 40amp breaker would be right as 32 is undersized? 40amp RCBO with 2.5mm cable feeding a standard CU?
I don't know why it was proposed like this, I'll get a second quote, I just don't want to get many quotes

 

[MJPD29]

No for 2.5 swa connected to 32a breaker with those parameters

 

[Spoon]

So, if it was suggested a 40amp breaker would be right as 32 is undersized? 40amp RCBO with 2.5mm cable feeding a standard CU?

You have the wrong end of the stick mate.
32A is too big for the cable, you need to go smaller than 32A

I have edited my post #8 as I was a muppet and didn't clearly state stuff.

 

[DeanAlani]

No for 2.5 swa connected to 32a breaker with those parameters

I guess then

You have the wrong end of the stick mate.
32A is too big for the cable, you need to go smaller than 32A

Thanks. That's why I posted here. Something didn't feel right.

 

[Soi disant]

Something didn't feel right.

I know how you feel.

I think I'll leave the others to it.

 

[Pete999]

Hi

I've now had a quote and just would like some opinion on this. Its not that I don't trust the proposal. Just after some thoughts.

We have an existing 2.5mm swa cable going to our outside office, the proposal is to feed of a 32amp RCBO in the consumer unit in the house (its an RCBO board) with a 2.5mm swa cable and connect this to the existing 2.5mm swa outside so its new circuit from the board, then feed this is into a non RCD rated consumer unit, as the circuit is being fed from is an RCBO so the office CU will be a standard100amp main switch CU with a breaker for sockets and a breaker for lights in the outside office.

Does this sound ok?
Thanks

No not really Mate, you need a dedicated feed from your CU suitably fused, not 2.5 something that has been deigned for the anticipated loading of your office to include power, lighting and heating, a length of 2.5mm would hardly cover this, if it's worth doing it's worth doing properly, just saying. Lashing it in with some 2.5mm SWA IS JUST NOT RIGHT.

 

[DeanAlani]

No not really Mate, you need a dedicated feed from your CU suitably fused, not 2.5 something that has been deigned for the anticipated loading of your office to include power, lighting and heating, a length of 2.5mm would hardly cover this, if it's worth doing it's worth doing properly, just saying. Lashing it in with some 2.5mm SWA IS JUST NOT RIGHT.

Thanks. I'm going to get another quote for this.
I was just after some advice, it's currently a garage but with all this working from home we want to use as an office. I just wanted some advice, so when I'm given a proposal to double check rather than keep calling electricians to quote. We have a RCBO consumer unit and was advised against an RCD CU in the outside building. Looking at the thread, SWA isn't suitable from the indoor CU and 32amp is wrong for 2.5mm cable. There be 2 x 2.5kw radiators and a computer with printer, phone charger and a few 2 watt LED lights.

 

[Spoon]

There be 2 x 2.5kw radiators

Any chance of reducing this?

 

[DeanAlani]

Any chance of reducing this?

Its actually 2kw oil filled radiators literally what it says on the radiator

 

[Spoon]

Its actually 2kw oil filled radiators literally what it says on the radiator

Well, that is better.

 

[pc1966]

Tally up what you really want in the office before deciding.

2.5mm should be on a 25A RCBO at most (subject to voltage drop depending on the length) so you have enough for lights (probably 1A max) and around 5.5kW of other stuff. Electronics (TV/PC/etc) has a high in-rush when switched on so if you have 500W expected then treat it like 1-2kW just in case.

So for a basic office with 3kW heater, several LED lights and a few IT things it would be OK. But not on 32A!

However, the extra cost of going for 4mm SWA, or even up to 10mm, is not much compared to the overall installation and it would give you easy/cheap options in the future if you found you needed more stuff out there for any reason.