Cable sizing to Annexe

[anorakdiyer71]

HI
what size & type of cable is required if putting sub mains to an annexe & a garage .
The main house is on external meter box
Garage & annexe to have their own supply and consumer unit fed back from just before the house internal consumer unit
(so all metered from main meter)

Not sure what size of cable and does it need too be armoured ? there will be ducts used.
I could run separate earths back to house as well (someone suggested)

From a 1st fix / groundwork perspective , accepting this is subject to a proper calculation what would be needed as a rule of thumb ??

16mm 25mm SWA ??

 

[chivers67]

Have to work out the likely load to both the Annexe and Garage before committing to a conductor size but 25 or 16 sound highly likely. Can you run the garage from the annexe rather than two cables from the Incomer or do you feel they both require a separate feed?? What will be protecting them just the DNO fuse from a Henley block or similar?

If the cable is in a Duct High Tuff or similar would suffice

 

[oldbutnotdead]

Depends on distance and anticipated load but you'll probably not be a mile away with 16, 25 might be overkill.
You really need to get an electrician on board for the design phase- it is a new circuit and new cu so notifiable work- they can advise on cable size, trench depth, type of fill etc.
While you're digging the trench don't forget a duct for Comms (network, burglar alarm etc) and think about water supply and drainage.

 

[ericmark]

If the DNO fuse is 60 amp then cable needs to carry 60 amp, if 100 amp then needs to carry 100 amp, it is permitted where the cable is protected from damage to have up to 3 meters before the overload, but I would want either cable able to take 100 amp or a fuse where the cable splits.

Current rating of cable depends on installation method, you can't simply buy 100 amp cable. This is not really a DIY job, in Wales I know it needs notifying, not sure in England, new circuits need notifying, but in Wales the garden is a special location so no argument.

 

[oldbutnotdead]

PS Your plan (split off at Henley blocks before the house CU) is absolutely the best way to power your garage/outbuilding but you will need some sort of overcurrent protective device before the run to the garage (a single MCB appropriately rated in a box is fine). Again you MUST get your electrician involved at design stage because they will be the one certifying the installation and most won't (understandably) be interested in signing off on a job where they have not been involved with the design. They will usually be happy with someone else doing the donkey work (digging trench, drilling holes through walls) to their specification.

 

[Murdochcat]

your questions suggest you really need a local spark to do this …..

 

[pete01]

(a single MCB appropriately rated in a box is fine).

Would you say this was needed if the Dno fuse can protect the cable?

 

[JohnW2]

Would you this was needed if the Dno fuse can protect the cable?

As you imply, it would not be needed if the cable to the garage were meaty enough to be adequately protected by the DNO fuse - just as the 'tails' from the DNO fuse to the CU don't need any additional protection.

However, that would require that the cable to the garage were of a similar CSA to the cables going to the CU (i.e. 16mm² or 25mm²). Although those sizes have been mentioned, they could be bigger than actually needed for a garage supply (obviously depends upon what is going on in the garage!!).

If smaller cable than that is used, then, yes, as oldbutnot dead said, some sort of additional overcurrent protection (appropriate to the size of the cable to garage) would be needed.

Kind Regards, John

 

[pete01]

As you imply, it would not be needed if the cable to the garage were meaty enough to be adequately protected by the DNO fuse - just as the 'tails' from the DNO fuse to the CU don't need any additional protection.

However, that would require that the cable to the garage were of a similar CSA to the cables going to the CU (i.e. 16mm² or 25mm²). Although those sizes have been mentioned, they could be bigger than actually needed for a garage supply (obviously depends upon what is going on in the garage!!).

If smaller cable than that is used, then, yes, as oldbutnot dead said, some sort of additional overcurrent protection (appropriate to the size of the cable to garage) would be needed.

Kind Regards, John

I agree

 

[anorakdiyer71]

Thanks to all that have responded. Useful information.
An electrician will be consulted and on board , was just trying to evaluate things with groundwork etc really, ahead of the game.
I'll get a design size/calc done as advised