[Question] Need some advice

[Jacques#1]

A main installation is fed with 16mm cable to a junction on the side of a house. The house has a flat, tin roof.

A granny flat is built on. Can I tie in to the junction box, will be using 16mm armoured cable, a main breaker + E/L at the new flat with its own DB? I went through the book a few times, and I cannot find a reference that says its not allowed, do not want to lift the roof to chase into the old DB??

[Dave A]

What you probably need to consider is you need a readily accessible mains switch for the entire installation...

You could convert the junction box into the main DB. Just a York box deal with a mains switch and a circuit breaker for each of the two sub db's. What was the main db in the house then becomes a sub db.

[Jacques#1]

"Theoretically" I'll for this turn around see the installations as the seperate buildings, which will be controlled by their own main switches, and earth leakages. The meter box has its own main switch that can switch the power off to the entire installation and also protects the cable splitting to the different sections.

Thing is, its a facebrick house, flat tin roof, I can't break open the wall to fit a bigger box then the one currently installed.

[Jacques#1]

After sleeping on it, I might just go your route Dave.....I can surface mount a box over the junction box, extend the cables, fit one Double pole CB, earth + neutral bar, and call it the main isolation point.....The house DB would still be the "first" db on the property if not the main db, so I'll mark it "main" as I understand it.

[Dave A]

We had a similar situation once before....

I know that apart from the main switch, we also fitted either a circuit breaker or isolator (not sure which) for each feed to the two sub db's. Whether that's overkill in terms of the regs I couldn't tell you, but it's neat and covers all potential problems including making it easy for the next electrician to figure out what's going on.

(Please bear in mind I'm a manager here, not the IE).

[Jacques#1]

Well then you're supposed to know more than your subordinates, aren't you!

[Dave A]

Different skills, mate
I do get sucked into interpretation issues, facilitating technical debates and converting "electrician's code" into consumer-speak at times, so my exposure is far from nil.

I can hunt down the exact details, why's and wherefor's if you like. It's just not the sort of stuff I carry around in my head. That's my IEs' job. When they start talking about section this and that, the first thing I do is reach for the book.

[murdock]

the meter box should have a fuse then from the fuse mount a DB over the junction box...fit a main isolator into this DB...either fit an earth leakage into this DB (which will become the main DB) to protect the entire site which i would not advise...or make sure there is an earth leakage in the house db which will now become a sub main DB1 fed from a circuit breaker to protect the size wire you use to feed this sub main DB 1...

for the outbuilding make sure you fit an e/l unit into this DB which will become sub main DB 2...fed from a cicuit breaker in the main DB sized to protect the cable you decide to use...if you do it this way you can reduce the size of the cables feeding sub main 1 and 2 to reduce costs if required.

it is all about the planning...and more importantly the budget.

isolator to isolate the supply...circuit breaker to protect the cable from thermal overload and short circuit.

short circuit protection is the one people forget about especially when you start working with DBs over 10 KA...cables are designed to carry a fault for one second at a specific fault level...if you reduce the protection time you can reduce the cable size...but this is a risky bussiness because if the electrician doesnt know about the reduced time he might not fit the correct breker of fuse required for this application if the original one requires replacement...

there are different ways to do this one being a cascade system...another semiconductor fuses..