Putting larger fuse in CU

[DanAir]

I've posted previously because I have two HW cylinders in my attic which are usually heated via the gas boiler. Occasionally I would use the electric immersion heaters to top up. Each heater is 3kW and at present they both plug into one double socket (I don't switch them on at the same time presently for this reason). When I posted previously I thought they were on the upstairs ring (not sure why) but having traced it back to the consumer unit they appear to be the only items on what appears to be a 2.5mm(2) T+E ring. It's an old Wylex CU with rewireable fuses. The problem is that the fuse at present is only 15A which would be inconvenient if I was using both at the same time. So my questions are:

1. If the fuse was large enough and I wired each heater off a seperate 13A FCU, and the cable is indeed 2.5mm(2) would this size cable be sufficient? I'd rather avoid changing the cable (e.g. for a 4mm(2) radial) as I'd have to put in a new run.

2. If it is sufficient how much spare capacity would there be on the ring?

3. How can I check for sure that the cable is indeed 2.5mm(2)

4. I assume I'd need a 30A fuse but how can I increase the size of the fuse in the CU? In other words, can I simply change the fusewire to 30A and label the fuse as such (I'm guessing not) or do I have to get a replacement, in which case is such a thing available?

Sorry for the longish post, thanks in advance for any help!

 

[Taylortwocities]

I expect that you do not have a ring, just one 2.5mm cable. The max size fuse you could put on that circuit would be 20amps.

Immersion heaters must each have their own circuit from the fuseboard.

Typically they are fused at 15amps (16amp for one of those up to date contact breaker thingys).

Your best bet is to run in another 2.5mm circuit - dont forget to notify if new circuit!!

 

[DanAir]

Thanks for the respsonse. I haven't opened up the CU to see if there are two cables coming off that point but at the plug socket in the attic there are definitely two cables hence my assumption that they are a ring. I'm suprised that you say that each immersion heater heater must have it's own circuit. Unfortunately there are no spares in the CU but I don't see why using a dedicated ring should be less ideal than two seperate circuits, especially if I use FCUs for each one. I don't know the exact capacity of a 2.5mm ring but would assume that since they are usually protected by 32A MCBs they can carry at least 32A safely.

 

[Taylortwocities]

Well, Dan.

If there is one cable going from your consumer unit then it is called a radial. It may go to the first socket in the loft and then on to another socket. Maybe its time to look in that fusebox?

The IEE Guide to Building Regs states that a water heater fitted to a heated vessel exceeding 15 litres must be supplied by its own separate circuit (and not supplied from a ring final circuit). Havent looked to see if this is reflected in 17th regs but I'll bet its there.

It doesnt matter if you havent got space in your fusebox. You need two separate circuits. If you've got space/wiring issues then your better bet may be to install a changeover switch so that you can select one, or the other of the immersions - but not both.

 

[DanAir]

I have no doubt that you're correct although this isn't the advice I was given last time. Regardless of the regs, ans appreciating that it's not a great design, is it dangerous? I put a bigger load than that on the other rings so why not with Immersion Heaters. Not trying to be awkward...

 

[Taylortwocities]

I am trying to advise you what the correct way to do it is.
The regs are there for a reason - usually because someone ended up with a pile of ash where their house used to be.

If you ask for advice and ignore it then its up to you.

Is doing what you are suggesting dangerous -undoubtedly.
The circuit is fused at 15 amps because it is probably a radial with a single 2.5mm cable.
Just because there's two cables on the socket in the loft doesnt make it a ring.
Just because there are two cables in the fuseboard doesnt make it a ring (have you looked, yet????)
You dont know that it is a ring final........I dont know if it is, or isnt, a ring.
An electrician with the correct test equipment can check if what you have is a ring.

 

[333rocky333]

It would be rare to have one socket only on a ring
Therefore when you turn off the mcb ,does any other sockets also go off.
If yes how many.

If you then check how many cables in them ones.

If you find one with only one cable , then it is possible you have at least one radial.
Unless it is a spur

IMO
It would be unwise to fuse a 2.5 T+E at 32amp unless you confirm it as a ring.

 

[DanAir]

There used to be a normal hot water cylinder in the bathroom. A while ago the bathroom was redone and the hot water cylinder was removed and replaced with an unvented system comprising two cylinders in the attic. The builders did most of the job fine but neglected a couple of things, one of which was to get a qualified electrician to wire in the immersion heaters in the cylinders, instead doing it themselves hence the mess. What they seem to have done was to take the cable from the old cylinder (which was connected by 2.5 T+E to a 15A fuse in CU marked "Bathroom") and sensing this wasn't quite enough for a 26A load ran another cable making it a ring. Thus the fuse was insufficient but the installation not unsafe.

Nevertheless I may be wrong, so, is there any way I can determine for certain whether it's a ring or do I need an electrician? I'm not looking for short-cuts but I prefer to do things myself when it is reasonable and safe to do so. I also have some respect for the regulations.

 

[333rocky333]

It is possible they may done that but would be no point if they left it fused at 15 amp.
It is not unheard of for people to WRONGLY put 30a wire in 15a holders, which they may have done.

If it is definitely a ring and 2.5mm they should have replaced the fuse unit with a 30 amp one.

The only proper way to confirm a ring is as TTC said earlier, for someone competent , with test gear.
to locate the 2L 2N and 2E at the fuse, disconnect all 6 of them,

insulation resistance test between them, and then

check continuity from one leg at the CU , out to the socket and back to the CU on the other leg.

Repeat for all 3 conductors

Once confirmed it is a ring and 2.5 with continuity for all 3 cores, you could then form your ring with two fcu's fused at 30amp.
What you then connect to them ,is down to you and the relevant regs!
It may become notifiable work though.

 

[DanAir]

Once confirmed it is a ring and 2.5 with continuity for all 3 cores, you could then form your ring with two fcu's fused at 30amp.

Why 30amp? Surely 13 each is sufficient?

 

[333rocky333]

4. I assume I'd need a 30A fuse but how can I increase the size of the fuse

I have two HW cylinders .
Each heater is 3kW )

especially if I use FCUs for each one.

Once confirmed it is a ring and 2.5 with continuity for all 3 cores, you could then form your ring with two fcu's, fused at 30amp.
What you then connect to them ,is down to you and the relevant regs!
It may become notifiable work though.

Once confirmed it is a ring and 2.5 with continuity for all 3 cores, you could then form your ring with two fcu's fused at 30amp.

Why 30amp? Surely 13 each is sufficient?

was that not what you asked at the beginning.

I missed out a " , "
The biggest fuse that fits in a fcu is a 13 amp
You can not get a 13a or 26a mcb and you can not get 13a or 16a fusewire to fit the cu
so the 30 a fuse is obviously in the CU, and the 13a fuse in each fcu
Surely you realised that.

 

[DanAir]

was that not what you asked at the beginning.

You're quite right - my mistake, I meant 13A - they do sound the same though so at least my poetic reputation is intact

Is testing things like insulation resistance and continuity beyond the scope of a DIYer?

Regardless, given the state of the of the electric work the builders did it's probably worth getting a PIR done. I actually had an electrician who was recommended by a friend who came round and spent around 1.5 hours doing the testing for a PIR, but never sent paperwork through. I haven't paid anything so not too bothered, but a bit strange - and given their level of professionalism I think I would probably be sceptical about their testing anyway!

 

[daytona]

was that not what you asked at the beginning.

You're quite right - my mistake, I meant 13A - they do sound the same though so at least my poetic reputation is intact

Is testing things like insulation resistance and continuity beyond the scope of a DIYer?

Regardless, given the state of the of the electric work the builders did it's probably worth getting a PIR done. I actually had an electrician who was recommended by a friend who came round and spent around 1.5 hours doing the testing for a PIR, but never sent paperwork through. I haven't paid anything so not too bothered, but a bit strange - and given their level of professionalism I think I would probably be sceptical about their testing anyway!

testing is beyond the scope of diy mate sorry.you need very expensive test equipment and the knowledge to interpret the results.

i too would be very sceptical,to the point of not even wanting to look at a PIR that was completed in 90 mins

 

[DanAir]

Let me preface this question by saying that I will be getting a PIR done but I'd like to do as much simple DIY work beforehand rather than afterwards.

So let me ask a question on continuity: Let's say there's only one socket on the ring, with Cable 1 and Cable 2 feeding in. If I remove Cable 2 and check the socket works, and then remove Cable 1 and replace Cable 2 and check the socket works, isn't this a pretty reasobable test for continuity (I assume the only risk is that when reattaching the cables is that a good connection is not achieved?)

 

[ban-all-sheds]

I expect that you do not have a ring, just one 2.5mm cable. The max size fuse you could put on that circuit would be 20amps.

Not if it's a rewirable..