Too many Sub mains?

[DavidWick]

Hi guys

Hope you can offer some experience on to me, as im quite new and with that tend to stick with basic domestic stuff.

Basically a builder/friend of mine called and asked if I could take a look at an outbuilding they were building as there was an existing 2core 16mm SWA cable running from house CU (not terminated just coiled on floor) to site and wanted to know if he could use it.

When I popped over, it was a big country house and they are building a four car garage 20mtrs or so away, with living space above. There will be 7kw electric boiler, two immersions and a typical power and light supply.

The existing house supply is TNS, 100a Main fuse and a Henley block feeding Three different fuse boards! This is already more then im used to but my concern is surely if he then adds this out building it will exceed the 100a main fuse? so what do you do in that situation? do you have to contact mains supplier to change to three phase? bigger fuse? The more experienced out there must have had this many times so what did you do? did you find its not rearly a problem and its just me over complicating it as 100a is a lot taking account diversity?
Also what is the view on cable size? I think its ok and would TT outbuilding?

I have been Honest and ducked out of getting involved as I know my level at this time but would still like to know for future knowledge.

Many thanks in advance

 

[telectrix]

should be OK, allowing for diversity. and why TT the outbuilding, when you have a perfectly good TN-S you can extend?

 

[davesparks]

As far as the old cable goes I would test it, including a sheath test before making any comment on its suitability.

I would carry out a load study to find out the current average demand of the installation, but I own the gear to do this with so it's easy for me. If not then you can calculate the maximum demand and make an assessment from there.

 

[Marvo]

When I popped over, it was a big country house and they are building a four car garage 20mtrs or so away, with living space above. There will be 7kw electric boiler, two immersions and a typical power and light supply.

Is this just in the garage/flat or is this including the main house?

 

[DavidWick]

should be OK, allowing for diversity. and why TT the outbuilding, when you have a perfectly good TN-S you can extend?

I thought TT would be safest way as two core cable would mean earth reliant on the termination of a gland.

 

[DavidWick]

Is this just in the garage/flat or is this including the main house?

This is just for the garage/flat

 

[davesparks]

I thought TT would be safest way as two core cable would mean earth reliant on the termination of a gland.

The armour will have to be the cpc of the submain regardless of whether you establish a new earthing system for the outbuilding or not. The SWA gland is designed to provide that earth connection so there is no reason why you shouldn't use it if the armour meets the requirements for the protective conductor.

 

[Fin170]

earth reliant on the termination of a gland.

Irrelevant, whatever way you do it the armouring would still need to be earthed properly (luckily that's not a hard job or people would be wasting miles of copper on unnecessary 3rd cores.....)