Best way of supplying summer house

[Uplight99]

Hi guys

I am taking on a job of supplying/wiring a summer house which is in the process of being built.

Consumer unit has no spare ways so my only option really is splitting the tails in the meter cupboard into a fused isolator to supply the SWA. However as far as I’m aware I am not allowed to install an isolator in the meter cupboard and the client is getting this signed off by the council so I need to do everything properly. Are there any ways around this? Even if I wanted to add a new CU I would still have to split the tails so it seems to me that there is no way around this.

Also the summer house is only going to have 10x sockets and 6x spot lights so I don’t see any reason to use anything bigger than 6mm SWA. Main cutout is 100A so I will need a fused isolator to step the amps down to 32A (SWA is running in duct under the floor) do they even make an isolator like this? 63A is the lowest I’ve seen so I will have to run in 16mm SWA to support the 63A from the isolator? Seems overkill to me

 

[buzzlightyear]

Also the summer house is only going to have 10x sockets and 6x spot lights so I don’t see any reason to use anything bigger than 6mm SWA. Main cutout is 100A so I will need a fused isolator to step the amps down to 32A (SWA is running in duct under the floor) do they even make an isolator like this? 63A is the lowest I’ve seen so I will have to run in 16mm SWA to support the 63A from the isolator? Seems overkill to me

could you draw out the sub mains to consumer unit in the out house ,loading factor ,design .

 

[Dustydazzler]

in the olden days when there were no spare ways in often already cramped fuseboards we would add a 20amp single way shower consumer unit to the downstairs rfc to feed a small shed our garage. exactly the same way you would add a 13a swi spur to a ring but the 20a consumer unit gives you a bit more load capacity

 

[Uplight99]

could you draw out the sub mains to consumer unit in the out house ,loading factor ,design .

Meter cupboard is on exterior wall. Consumer unit is literally on the other side of the meter cupboard under the stairs (inside obviously) PME system

Then about a 25m run from the meter cupboard to where the consumer unit in the summer house will be going. Trenches have already been dug I just need to lay in the SWA and duct-in

I’d say even 6mm is overkill for a couple sockets and lights but I’m thinking of future proofing it.

I’ve done work like this countless times for my boss - splitting tails for sub-mains etc but it usually specs for 16mm t&e coming out of the fused isolator so obviously I can use a 63A fuse. But in this situation I cannot fuse 6mm SWA at 63A as I believe highest it can handle is around 47A clipped direct but I will be running under the ground.

Basically what I’m asking is how do I fuse the amps down to 32A or 40A for the SWA from a 100A cutout. Am I being stupid and just overthinking this? Can I not just buy a 40/32A HRC fuse and stick that in the fused isolator?

 

[Uplight99]

in the olden days when there were no spare ways in often already cramped fuseboards we would add a 20amp single way shower consumer unit to the downstairs rfc to feed a small shed our garage. exactly the same way you would add a 13a swi spur to a ring but the 20a consumer unit gives you a bit more load capacity

She wants it to have its own separate supply and also said in the future she may add heaters and some other things

 

[buzzlightyear]

Even if I wanted to add a new CU I would still have to split the tails so it seems to me that there is no way around this.

Basically what I’m asking is how do I fuse the amps down to 32A or 40A for the SWA from a 100A cutout. Am I being stupid and just overthinking this? Can I not just buy a 40/32A HRC fuse and stick that in the fused isolato

10mm swa , fused switch 40amp .job done you need to work out the vd on your loading .

 

[Uplight99]

10mm swa , switch fuse 40amp .job done

Yes this is an option but the meter cupboard is quite tight just feel like 10mm is overkill and then I’ll struggle terminating it

Also still in the blue in regards to putting equipment in DNO meter cupboards?

 

[ipf]

What's wrong with using a 2 way consumer unit (garage unit), 63amp rated.
You've 2 ways available to take whatever supply you want....under 63amp.

 

[buzzlightyear]

Also still in the blue in regards to putting equipment in DNO meter cupboards

the op needs to put options A or B .why cant the peaple show the plans what they are doing . words dont come easy.

 

[Uplight99]

What's wrong with using a 2 way consumer unit (garage unit), 63amp rated.
You've 2 ways available to take whatever supply you want....under 63amp.

How will this protect the SWA? Main switch on the garage CU is after the SWA not before

 

[Uplight99]

the op needs to put options A or B .why cant the peaple show the plans what they are doing . words dont come easy.

I really don’t see what drawing it out would achieve. There really isn’t much to the install - literally just straight from the meter box 25m run

 

[buzzlightyear]

How will this protect the SWA

switch fuse .or other .

 

[ipf]

How will this protect the SWA? Main switch on the garage CU is after the SWA not before

Split via Henleys at CU, to second CU (2 way), out from 32 or 20 amp CB/RCBO.

 

[Uplight99]

Split via Henleys at CU, to second CU (2 way), out from 32 or 20 amp CB/RCBO.

Ah you’re saying split the tails by the consumer unit instead of splitting them in the meter box?

Didn’t think about that

Forgot to mention the neutral tails have already been split in the meter box for the off peak board

 

[westward10]

Replace the consumer unit.

 

[Uplight99]

Replace the consumer unit.

Not an option

 

[buzzlightyear]

She wants it to have its own separate supply and also said in the future she may add heaters and some other things

me, give her a option on what you going to do .

 

[westward10]

Not an option

Why is that.

 

[pc1966]

Meter cupboard is on exterior wall. Consumer unit is literally on the other side of the meter cupboard under the stairs (inside obviously) PME system

Then about a 25m run from the meter cupboard to where the consumer unit in the summer house will be going. Trenches have already been dug I just need to lay in the SWA and duct-in

I’d say even 6mm is overkill for a couple sockets and lights but I’m thinking of future proofing it.

If there are extraneous conductive parts to be bonded (e.g. service pipes) then on PME it is 10mm copper equivalent (might have changed with AM2 - not got mine ordered yet).

As a general rule, the cost-saving on the cable will be dwarfed by the cost of future replacement if it proves to be too small for whatever reason (CCC, VD, or bond size).

I’ve done work like this countless times for my boss - splitting tails for sub-mains etc but it usually specs for 16mm t&e coming out of the fused isolator so obviously I can use a 63A fuse. But in this situation I cannot fuse 6mm SWA at 63A as I believe highest it can handle is around 47A clipped direct but I will be running under the ground.

Basically what I’m asking is how do I fuse the amps down to 32A or 40A for the SWA from a 100A cutout. Am I being stupid and just overthinking this? Can I not just buy a 40/32A HRC fuse and stick that in the fused isolator?

You can fit a 40A fuse in many, just check what fuse type it takes and there is your part to search for.

However, if you are looking at 40A or less then a fuse won't have much useful selectivity with a 20A or 32A MCB/RCBO down-stream so you might as well use a 40A C-curve or D-curve MCB (end of cable Zs permitting) instead. Yes, you still have poor hard-fault selectivity, but at least it is a simple thing to reset post-fault.

Which is why I would agree with @westward10 that a new CU with more space makes a lot of sense.

EDIT: Just seen your post saying not an option, in that case you are looking to split tails and either a fused-switch, or a small CU with a MCB in it for sub-main protection. If SWA then no RCD needed on it, but obviously the out-building will need RCD/RCBO for the sockets.

 

[Uplight99]

Why is that.

Customer doesn’t fancy paying an extra £500+ on the job when it isn’t really necessary. She will just go and find another electrician who will do it