Domestic Shed Installation

LGRSol · Mar 3, 2018

Hi All,

Having recently qualified as an electrician, I am beginning to understand the adhocs and curveballs that customers can throw at you.

Last week I went to quote for a shed installation. The shed is approximately 25m from the property and would be a relatively straight forward notifiable job i.e. SWA buried to a safe depth (500mm), 2 way DB in shed for 1 way lighting and ring final sockets.

Now in my limited experience, as far as I am concerned the install would be directly to the main DB and either protected by a 30mA RCD or alternativly using an RCBO. However the customer has said that they really want the source to be from a fused spur out of the ring final circuit in the garage.

Obviously I am fully aware of the regs and in particular 5.7 of Part P. As far as I am concerned, this is an absolute no go, however I am struggling to find if this is stated anywhere in the wiring regs. Before I say this to the customer please could I have some advice / thoughts.

Much appreciated,
Lee

Andy5678 · Mar 4, 2018

Hi, have you had a look at the best practice guide covering wiring of sheds? Should get it with a quick google search.

Madocks · Mar 4, 2018

Do your cable calcs, dont forget max vdrop on lighting etc.

25m run off the ring may prove high ZS values.

Much better job to run a new cable from C.U

Midwest · Mar 4, 2018

Just thinking if his Zs is at or near its max at the RFC source, the R1+R2 for the 25mm cable would be an issue.

123 · Mar 4, 2018

Given it will need fused down to a max of 13A, Zs in the shed shouldn't be an issue.

If the likes of a washing machine, tumble dryer and freezer are put in the shed then there will be issues.

telectrix · Mar 4, 2018

Leesparkykent said:
How come it might prove tricky?

think it's confusion over whether it's a 25m run or 25mm cable.

Midwest · Mar 4, 2018

123 said:
Given it will need fused down to a max of 13A, Zs in the shed shouldn't be an issue.

If the likes of a washing machine, tumble dryer and freezer are put in the shed then there will be issues.

I always thought it was the rating of the ocpd at the origin?

123 · Mar 4, 2018

Midwest said:
I always thought it was the rating of the ocpd at the origin?

As long as the fused spur is before the 25m run of cable then the 13A fuse will be the protective device for that cable

Leesparkykent · Mar 4, 2018

As above. If the RFC is within permitted Zs values then I can't see 2.5mm SWA spurred of the RFC via a Fused spur being a problem at a length of 25 meters, regarding volt drop and Zs.

westward10 · Mar 4, 2018

This is getting very complicated.

Midwest · Mar 4, 2018

123 said:
As long as the fused spur is before the 25m run of cable then the 13A fuse will be the protective device for that cable

Learnt something else, today. I have always used the ocpd at the origin, to decide my maximum Zs value (in such a scenario), but I guess the above makes sense. Although I suppose this would only be the case with such a design using a 13A fuse for the max limiting value, as a smaller fuse could always be replaced by a larger one.

Midwest · Mar 4, 2018

westward10 said:
This is getting very complicated.

I'm finding it very therapeutic

123 · Mar 4, 2018

Midwest said:
Learnt something else, today. I have always used the ocpd at the origin, to decide my maximum Zs value (in such a scenario), but I guess the above makes sense. Although I suppose this would only be the case with such a design using a 13A fuse for the max limiting value, as a smaller fuse could always be replaced by a larger one.

I don't see how or why it couldn't be done for any fuse size. DNO fuse can be changed for larger, as can fuses in the likes of KMF Switches. MCB's are hard to swap out either - you can't make everything idiot proof.

Midwest · Mar 4, 2018

123 said:
I don't see how or why it couldn't be done for any fuse size. DNO fuse can be changed for larger, as can fuses in the likes of KMF Switches. MCB's are hard to swap out either - you can't make everything idiot proof.

I was just thinking, if you designed your altered circuit, using the limiting factor with a 3A BS1362 fuse in your FCU, it could be replaced with 13A by a homeowner for example. It would have to be a bit of a long run mind, but still feasible.

davesparks · Mar 4, 2018

123 said:
Given it will need fused down to a max of 13A, Zs in the shed shouldn't be an issue.

If the likes of a washing machine, tumble dryer and freezer are put in the shed then there will be issues.

I can’t see anyone having their washing machine and tumble dryer 25m away from the house.

davesparks · Mar 4, 2018

gazdkw82 said:
Secondly, why fit a RFC? Just for a 4mm radial

Don't know why anyone would choose to install a ring over a radial.

A 4mm radial seems overkill for a shed.

Ring circuits have advantages in some situations, particularly in supplying a large number of sockets in multiple rooms of domestic premises where they are more economical.

To rule out one installation method over another based on opinion is narrow minded.