Spur direct from CU with another circuit

[j s i]

Domestic Scenario
There is an existing 32A ring circuit protected by BS60898 MCB. The wiring is flush <50mm deep.
Upgrading a storage heater I need to add a FCU for the boost funtion on the storage heater. The wiring is surface for the FCU.

A colleague says it is ok to come directly from the top of the 32A fuse for a single spur (but not from anywhere else, ie: JB on one leg of the ring circuit as it leaves the CU on the surface).

My problem is how can this comply with regs.
Whilst I can see the cable will not be overloaded due to the fuse size in the FCU, Is it classed as 1 circuit or 2 circuits.

If it's 1, then the circuit has been altered and needs to be brought up to current standards, RCD protection to the circuit.
or
If it's 2, then the circuits have been bunched which is not allowed.

Cheers

 

[kingeri]

It would still be one final circuit. A spur may come off any point on a ring, including the CU, so long as total no. of spurs doesn't exceed no. of socket outlets. If it's a fused spur then the FCU should be within 3m of the spur point. Your colleague isn't exactly right, a spur could come off an accessible junction box or a concealed maintenance free one....best avoided if possible to leave a clearer picture of the circuit for the next spark who looks at it in the future.

And why can't you bunch 2 circuits together, as long as your CCC calcs come out ok?

 

[Deleted member 26818]

The dfinition of a circuit is: "An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)."
As such the protective device9s) is/are the origin of a circuit.
Your colleague is correct that it is ok to come directly from the top of the 32A MCB, but incorrect to say 'not from anywhere else'.
An unfused spur can be taken from anywear on a RFC, from the CU, from a socket-outlet or even from a JB connected to one leg of the RFC.
As a rule of thumb, as long as the spur is no more than 3m in length there is no problem.
Over 3m, then you have to start looking more closely at circuit impedances, to determine whether the overload protection will suffice.

 

[j s i]

Thanks for the replies, but just to clarify

If I was to spur from somewhere else on the RFC, then I would need to upgrade the circuit with RCD protection to bring the entire circuit up to current standards. But if I come direct from the CU, are you saying that the entire circuit does not need RCD protection.

Thanks

 

[Deleted member 26818]

No, only the work you conduct has to comply with the current edition of the Regulations.
It may be that it would be safer to upgrade the whole circuit, it may even be easier and more cost effective, but it's not a requirement for you to do so.
For instance, if you were to spur a new socket back to back with an existing socket on the other side of a wall, only the new socket would require RCD protection.
You could achieve this by installing an RCD socket, or by installing an RCBO in the Consumer Unit.
Probably not a lot in it as for cost, unless of course, the Consumer Unit is an old Wylex 3036 type. Then you'd be looking at replacing the Consumer Unit etc.
If you were to extend an existing radial, you could perhaps use an RCD FCU and thereby protect both the new wiring and sockets.

 

[ses]

Something that i have always wondered is technically is it allowed to feed one socket or spur outlet from an empty 32 amp mcb in a 2.5mm radial

 

[Deleted member 26818]

No, because Ib < In < Iz will not be satisfied and 433.1.1 won't be complied with.

 

[ses]

So this is what puzzles me if you take a spur direct from mcb on a ring
I can see how further upstream it works but direct from the mcb?

 

[Geoffsd]

Why not, what's the difference?

The cable will be protected by the fuse(s) in the plug(s)

However, as above, the spur must not exceed three meters without fusing down and must satisfy fault protection requirements - regs. 433 & 434

 

[Deleted member 26818]

So this is what puzzles me if you take a spur direct from mcb on a ring
I can see how further upstream it works but direct from the mcb?

You're not thinking perhaps that sometimes the Regs might contradict themselves are you?

 

[ses]

Blasphemy the regs are sacred :/

 

[kingeri]

Obviously there is actually no difference between a spur from a ring at the CU and a radial coming of the CU supplying one socket with no ring present, but likewise there is no reasonable situation where you'd really consider doing the latter.

 

[lauriehurman]

In the beginning there was the word ...

(and then there was the amendment to the word)

 

[ses]

Ok scenario for you

Job 200miles from home turf end of the day you need to add 1 single socket under a DB which is rare as muck so there is already a nice empty 32amp mcb in the board for you
Rcd socket face plate will be installed

 

[Deleted member 26818]

Fine if wired in 4mm, or two 2.5mm conductors.

 

[Geoffsd]

As ses knows, it would be fine as he suggests.

(subject to fault conditions being met which they will be for such a short run).

Max. load 13A.