Domestic Shed Installation

[LGRSol]

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

 

[telectrix]

the customer can have the shed supply from the ring if he wants.however, he will be limited to the 13A FCU as to load. far better to take feed from DB, fused appropriately. SWA will not require RCD protection ( unless TT).

 

[LGRSol]

Really appreciate your advice...
Thank you!

 

[Risteard]

Far better than taking a feed from the DB would be glanding into a switch fuse. Ultimately this all depends on what the client is willing to pay for.

 

[Spoon]

As Tel has already pointed out, if the customer wants it from the FCU then he is limited to what he can use in his shed.
Ask them what they intend to plug in... They might want a heater and other stuff..
Does the house CU have RCD protection?

 

[buzzlightyear]

The shed is approximately 25m from the property

taking a 25mm from a ring circuit or radial is a bit much in IMO, it should be on its own circuit to the shed .

 

[LGRSol]

Hi Spoon.

The DB is RCD protected. I'll speak to the customer on monday and find out their intended usage and advise accordingly.

If they do go with the 13A FCU option, do you have any advice on the best way to do the testing the installation. It will be the first time that i've fitted a second DB. I'm assuming testing should be at the original DB (specific breaker)?

Many Thanks

 

[buzzlightyear]

you test in the shed at the farthest point at the D/B to the MCB or r/fuse.to get the results .

 

[Leesparkykent]

Why bother with a CU in the shed if your coming off the RFC using a SFCU?

 

[Octopus]

If, and I say if they decide to go with the fused spur option, just make sure your quote states maximum design load 13 amps

 

[LGRSol]

Really appreciate EVERYONE'S input & advice on this one!

 

[Wilko]

Hi - might be good to do a paper design before any site works. Sizes will vary with loads and MCB etc. Any bonding required etc.

 

[gazdkw82]

Personally I'd take it from the C/U into a 2 way board, however I would put an rcd local to that board not the board in the house. Problem being that if the end trips in the house it's a nuisance to reset. The SWA will not need rcd protection.

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

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

 

[Midwest]

And make sure the source RFC for the FCU is tickety boo, before you hand them a quote.

And as Buzz said, that 25m run might prove tricky off a RFC.

 

[Leesparkykent]

And make sure the source RFC for the FCU is tickety boo, before you hand them a quote.

And as Buzz said, that 25m run might prove tricky off a RFC.

How come it might prove tricky?

 

[Andy5678]

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

 

[Madocks]

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]

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]

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]

How come it might prove tricky?

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

 

[Midwest]

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]

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]

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]

This is getting very complicated.

 

[Midwest]

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]

This is getting very complicated.

I'm finding it very therapeutic

 

[123]

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]

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]

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]

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.

 

[gazdkw82]

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.

Could be overkill but it gives more scope in the future for added equipment. All depends on what's being used.

Could do A3 radial. 20mcb, 2.5mm.

Narrow minded? Most decisions are based on facts and opinions. IMO rings have more disadvantages than they have advantages. It's fact radials are easier to install and less complicated to test/understand.

 

[Leyland30]

The iet released a wiring matters document (google) detailing the use of a spur from ring circuit to serve an outbuilding - accepted arrangment subject to the considerations already mentioned by other posts.

You may also want to consider the distance of the shed from the origin of the supply. The greater this is, the stronger the case for a dedicated earth at the shed end I.e. TT arrangement. Presumably there are no metallic services entering the shed e.g. gas/water pipe work.

 

[Leyland30]

The iet released a wiring matters document (google) detailing the use of a spur from ring circuit to serve an outbuilding - accepted arrangment subject to the considerations already mentioned by other posts.

You may also want to consider the distance of the shed from the origin of the supply. The greater this is, the stronger the case for a dedicated earth at the shed end I.e. TT arrangement. Presumably there are no metallic services entering the shed e.g. gas/water pipe work.