Downrating cable?

Joined
25 Jan 2004
Messages
6,317
Reaction score
2
Country
United Kingdom
This one will probably wind up a few sparks here!

I am moving the switch for my immersion heater from one side of my airing cupboard to the other. On inspection of the existing cable it turns out the immersion heater is supplied by a 4.0mm2 radial. This radial is protected by a 15A rewireable fuse at the CU.

Is there any point to this? I mean, surely 2.5mm2 would have been far more appropriate in this situation! Bigger cable can't hurt but it just seems pointless.

The real question is, though, I have had some trouble sourcing 4mm cable locally so have some coming from TLC next week. However, I would like to get the job done today if possible so was wondering if it is acceptable to have a 4mm radial feeding into a 30A JB, then a 2.5mm cable from this to the immersion switch? There is no point in me doing this job today if I have to get a fused unit to feed the 2.5mm cable, I would just wait for my order.

I suspect the answer is a resounding "NO!", but I am wondering why.
 
Sponsored Links
If you do it today with 2.5mm, what will you do with the 4mm when it comes? Re-do it?
 
No, I would only do it if it were suitable for a permanent fit.

Whereas it is obvious that you don't spur off a 2.5mm circuit with a 4mm cable, I was really just wondering if the opposite is OK. I think I will just wait for the 4mm to come, it is a bit late in the day to start shutting off the power and I won't have time in the week, but it is just a question in my head.
 
You can use 4.0mm on a 2.5mm circuit.....but not the other way round....

Or maybe thats what your saying.....

I would use the 2.5mm as a temporary job only....
 
Sponsored Links
provided the breaker rating is appropriate for the cable you are adding it is perfectly safe to add it
 
Ah, you see it has a 15A fuse, which is what made me think "why is this in 4mm?!".

Think I will just wait for my 4mm2 to come in. And mutter about why Wickes don't stock it.

Perhaps the guy who built the extraordinary electric shower that FWL saw used to live here :D

Possibly it is wired in 4.0mm because this place was built in 1984, and a 4mm radial could be used (just) with the 7kW showers of the day. It wouldn't surprise me if some of the other flats in the street have electric showers instead of immersion heaters.
 
or it may be that part of the run goes suffican'tly far in insulation to demand it
 
Actually, it is interesting that you mention insulation: all of the walls in my flat are stud partitions. Every wall has insulation in it apart from the one wall that the immersion radial runs in.

I had wondered "why did they insulate every wall but this one?" perhaps this is why.

From the CU to the immersion the cable run is about 15m in all. Does this sound about right for 4mm for a 3kW load?
 
4mm is overkill unless there are special factors like insulation to be considered
 
ban-all-sheds said:
BECAUSE WICKES IS A SHED!!

This is the daft thing: 4mm is the ONLY size they don't stock! 1mm, 1.5mm, 2.5mm, 6mm, 10mm, 16mm, 25mm singles, 10mm singles, it is all there. But no 4mm! :evil:

I reckon it is just their conspiracy to annoy me. They also sell copper pushfit plumbing fittings, but the tool they sell to remove them doesn't work! :rolleyes:
 
And they sell a full range of conduit and fittings, but no single cables.

And they reckon that choc-blocks are so delicate that they need to be individually wrapped in polythene bags.

And they don't sell any PAR timber long enough to reach from floor to ceiling in even modern houses.

And
And
And
And
And
And
And
And
And
And
And
And
 
plugwash said:
provided the breaker rating is appropriate for the cable you are adding it is perfectly safe to add it

Plug, sorry but this is misleading information. If you reduce the size of the cable, it should be done in a Fused Connection unit or similar device which allows the two sections of the circuit to be isolated, regardless of the rating of the MCB in the distribution board
 
fwl is there a reg requireing that or is it just good practice

i can't see the problem with 16A mcb--4mm cable--junction box--2.5mm cable--fused connection unit--appliance
 
plugwash said:
fwl is there a reg requireing that or is it just good practice

i can't see the problem with 16A mcb--4mm cable--junction box--2.5mm cable--fused connection unit--appliance
My concern would be someone in the future going "Ah - 4mm - that'll be 30A then" and putting in a bigger MCB. Unlikely, I know, but...

In your example, why not simply put the FCU in place of the JB?
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top