Spur circuit cable Q

[rick]

I have a 4mm2 radial cct

One one of the 2 gang socket outlets, I wish to change this to a 1 gang socket and a switched fused spur (as new boiler need this connection)

Easy enough to change back box to a dual, when adding a spur can it be wired in 2.5mm2 (more than capacity for a single fused spur) or is there a requirement to use 4mm2 to match radial cct.

2 reasons for preferring 2.5mm2 ... I have no 4mm2 t&e left :-) and it will be physically easier to add a 2.55 mm2 cable into back box.

Want to do what is correct though.

Not that I think it relevant but fused spur will be fused at 3A, for a new boiler.

[Owain Lastname]

Use 4mm to match the rest of the radial circuit as (a) a radial circuit can be spurred off repeatedly so the whole circuit must be rated to carry the full design current (the rating of the circuit breaker) (b) it is a standard circuit and can be used without calculation

If given the circumstances you can be confident that the fused spur will not be further spurred, and you can show by calculation that 2.5mm is protected by the 32A breaker for short circuit protection, and that overload will not occur because of the nature of the load (3A fuse and a CH boiler with a fixed load), then you can spur in 2.5mm

You can also supply a CH installation from a plug in an *unswitched* socket.

Owain

[Theo]

What's the rating of the MCB?

How long will the cable run be? You can do the voltage drop calc:

For a 3A spur, unless the length is enormous (200 metres kind of enormous), I'd imagine a 2.5mm2 cable should be fine. It's a fixed appliance so no likelihood somebody is going to unplug it and attach an electric cooker.

Theo (IANA electrician)

[Robin]

FWLIW I took Rick to mean he wanted to use 2.5mm2 *from* the SFCU so it would be protected by the (at most 13A) fuse there.

[rick]

Circuit run les than 2m

[rick]

In case I was unclear (apologies)... I have an existing Radial circuit .... there is 2 gang socket on this circuit (not a spur off it) Circuit is 4mm2 t&e

I am changing my boiler and this 2 gang socket is only 0.5m away.

Easiest route I thought of was remove existing back box, change it to a dual back box .... put in a spur and 1gang socket. The 1 gang socket being the original series radial cct, and a cable off this to the Fused spur unit. My question was if the cable to fused spur unit could be 2.5mm2 or had to be in 4mm2

Though based on one comment here I could use a single socket instead of a fused spur unit.

[Dave Plowman (News]

Depending on the length of the run, 1.5mm is often OK after an FCU. 2.5mm more than adequate.

[Robin]

Thanks. It was me who misunderstood.

if the manufacturer accepts that then - depending on what you use other socket for - you could save the bother of changing the back box and fit a double unswitched.

[ARW]

You can use a 2.5mm cable as an unfused spur from a 32A 4mm radial circuit as long as it only powers one double socket or one fused spur (switched or unswitched) or one single socket.

Owain was 99% correct unless I have misinterpreted him in which case he was 100% correct.

There is a difference on a 4mm 32A circuit between an unfused spur and a branch. The unfused spur uses 2.5 a 4.0mm is a branch.

[rick]

Its a 4mm2 radial .... single spur (for boiler) so I'll use 2.5mm2

[rick]

Its easy to change as dry-lined

[Robin]

but then you're back to needing a bit of 4mm T&E?

[John Rumm]

If you extend a 4mm^2 radial with 2.5mm^2, then you have created a spur from the circuit rather than a branch in the radial circuit. So all the normal rules for spurs apply (only one single or double socket, or FCU etc, no additional extensions from that point).

So yes you can legitimately do what you want.

[Robin]

scrub that - I see now Adam has given you a solution

[John Rumm]

One of the reasons for the rules governing spurs is to make sure that should something like that happen, it will either work, or fail safe.

Being limited to a single or double socket (or FCU) makes it unlikely that you can apply enough load to overload the spur cable (2.5mm^2 clipped direct, is good for 27A) - so overload protection is "baked in". And just like on a normal ring circuit, it will have more than adequate fault protection from a 32A MCB.

[rick]

Thnx

[Tricky Dicky]

I have a couple of spurs off a ring circuit which go to FCUs which in turn connect to two double sockets. These are for plugging in a number low voltage, low current PSUs. Technically it is like plugging in a 4 socket bar extension lead into a socket and since the sockets are protected by the FCU I deemed this was acceptable. Am I wrong?

Richard

[John Rumm]

That's fine. You have created a fused spur rather than an unfused one. You can have any number of sockets on a fused spur, since the fuse will limit the total load to what the fuse will take.

Without the fuse, you need to rely on the limited number of sockets to impose the limit.

Fault protection is the same for both, although on the fused example you may trip either of both of the protective devices under fault conditions.