4mm live cable- any good use?

[SJRSJR]

Hello

While knocking tiles off a wall, I’ve discovered a 4mm cable with a bit of black tape on the end - turns out it’s live and is a spur off the downstairs ring which is otherwise 2.5mm. (It’s in a room that was the original kitchen and is now the utility room).

If it had its own FCU could it be used for an oven or a hob? Thanks

 

[ericmark]

Ring finals can be overloaded if a heavy load is taken from near the origin, so it depends where on the ring final the cable comes from, so would want to investigate further before using it for anything more than a single socket. We were taught to test for a fig of 8 supply, but on reading the regulations can't see anything to say not permitted, but there is clearly an overload danger.

 

[EFLImpudence]

If it had its own FCU could it be used for an oven or a hob?

The two things aren't really connected.

An oven or hob that could run on a 13A fuse will likely be supplied with a fitted plug.

As it is 4mm² cable it doesn't need a fuse and could be used for some sockets.

 

[Jurassicspark]

Just to add, the current will be limited to the rating of the MCB protecting the ring and not the rating of the 4mm cable.

 

[JohnW2]

Just to add, the current will be limited to the rating of the MCB protecting the ring and not the rating of the 4mm cable.

"Should be", rather than "Will be" A 32A MCB will allow something like 46A to flow for about an hour

But, yes, and the (I assume 32A) MCB would theoretically be adequate to support an "oven or hob", even though such things are better supplied by dedicated 'cooker circuits', rather than being supplied from a ring final.

 

[SJRSJR]

Thanks. It’s half way round the ring. And yes 32A RCBO.

So essentially it’s a spur so no need for a FCU (unless there were going to be more than one). May as well put a socket on the end of it rather than leaving it taped up.

Incidentally, is taping it up an appropriate way to terminate a live cable? I’ve seen terminal block taped up before but not just tape. Neither seem very safe.

 

[plugwash]

So essentially it’s a spur so no need for a FCU (unless there were going to be more than one). May as well put a socket on the end of it rather than leaving it taped up.

Pretty much, yeah best to just forget the cable is slightly bigger than normal and treat it as you would any other spur.

You could maybe make a first-principles argument that running a "fat" spur off a ring to run multiple sockets or maybe a single larger load is ok as long as it's supplied from a relatively central point on the ring, but it's certainly not the "done thing".

Incidentally, is taping it up an appropriate way to terminate a live cable?

Certainly not on a permanent basis. Even on a temporary basis it's questionable.

If an unused cable can't be disconnected at source, it should be terminated with appropriate terminals in an appropriate enclosure.

 

[JohnW2]

You could maybe make a first-principles argument that running a "fat" spur off a ring to run multiple sockets or maybe a single larger load is ok as long as it's supplied from a relatively central point on the ring, but it's certainly not the "done thing".

I would be inclined to suggest that it's "not the done thing" because most people don't do it, and they don't do it because those they learned from didn't do it (and they didn't do it because ....., etc. etc.).

However, as you imply, in terms of electrical principles and regulations, there is no specific reason why it should not be done, provided that it is done with regard to those principles and regulations. It is, after all, not really different from taking two or more spurs (of same size cable as the ring), each supplying one double socket, from the same or very close points on a ring, and most people would be happy to do that (particularly the latter).

As for, "as long as it's supplied from a relatively central point on the ring", I would say that's a bit of an exaggeration - since (with a 32A Method C 2.5mm ring) even an entire 32A load applied at one point on a ring is 'OK' provided that one point is no closer than about 16% of the total ring length from one end of the ring (about 32% of distance from one end to the centre of the ring) - which I wouldn't personally necessarily call "relatively central".

 

[sparkwright]

Hello

While knocking tiles off a wall, I’ve discovered a 4mm cable with a bit of black tape on the end - turns out it’s live and is a spur off the downstairs ring which is otherwise 2.5mm. (It’s in a room that was the original kitchen and is now the utility room).

If it had its own FCU could it be used for an oven or a hob? Thanks

Sorry to sound patronising, but what makes you think it's a 4mm cable?
(I'm only asking because old imperial stranded cable can look a bit like newer 4mm cable, but in reality is smaller.)