Electric shower heater wiring (a bit long)

[newshound]

A mate of mine has just bought an early 70's house. This has an 8.5 kW shower heater which has just failed completely, he was wondering whether he might be able to upgrade to 9.5 with the existing wiring.

I have three questions.

He still has the original CU. Later (but maybe not much later) a 6 mm^2 circuit has been added for a shower heater (probably a predecessor of the present one). The layout of the present board is as follows.

Incomer -> Supply Company Fuse -> New Smart Meter -> Old Wylex current operated ELCB (100 ma trip) -> Henley Block.

From this the supply splits to the original CU which has plug-in MCBs, and what I can only describe as a one-way CU for the shower, this has a switch (havn't checked if it is one or two pole) with an integral 30A plug-in MCB. This suggests to me that the original shower had a lower rating than 8.5kw, I'd have expected to see a 40A trip to protect an

8.5kW heater circuit but maybe you can get away with 30A for short showers.

From there is a simple short run of 6 mm^2 t&e. It's surface mounted on the "board", disappears into the ceiling, and runs up about two metres of wall to the shower unit. It runs behind ceramic tiles on an outside wall which (I guess from the age) might be normal rather than lightweight concrete blocks for the inner skin. The outer skin is a single layer of brickwork. Exterior walls are plastered inside, not plasterboard.

So, 8.5kW is 37A @ 230V while 9.5kw is 41A. (39A and 43A at 240V).

Clipped direct is OK to 47A. What does the team think about the wire run behind the tiles? I'm inclined to think that while it *might* run slightly hotter than clipped direct, the insulation is not particularly good and it can't set fire to anything sandwiched between blockwork and tiles (presumably chased into the original plaster). Do you think he could go up to 9.5kW?

Second question, the overall RCD protection with 100mA trip is a bit out of date, should he get this changed to 30mA? I know, in an ideal world he'd swap the lot for a metal split CU compliant with 17th edition, but he's also renovating his original flat prior to sale, so he's not rolling in cash. The rest of the electrics doesn't look too bad and doesn't appear to have been hacked about significantly.

Third question, the overcurrent protection for the shower circuit is this unit with a combined MCB and switch. I guess he needs to go up to

40A especially if he changes to 9.5kW. But does he need a switch here? After all, he can isolate the shower circuit by tripping the RCD device (at the price of turning off the rest of the house). He has, of course, the normal double pole ceiling mounted pull switch outside the shower cubicle. If he does need a switch, can anyone suggest a reasonably compact switch and overcurrent protection unit? I guess it doesn't have to have an MCB, could he use an industrial type 40A cartridge fuse?

[tabbypurr]

If money is really tight, I'd replace the small shower CU with a new one th at includes a modern 30mA RCD. There are lots on ebay, but it must be all m etal cased amendment 3 compliant. Putting the whole house on a single 30mA RCD is not a great plan & not compliant.

Fuses are still permitted, but the cable must then be derated. I can see no upside though, MCBs are only a few pounds.

NT

[John Rumm]

Possibly...

Probably worth a read of:

Yup, if you look at the time vs current trip curve for a 30/32A MCB, the likely load is going to be below the trip threshold near enough indefinitely.

Yup, from a design point of view that is correct - although with showers that 8.5kW may well be quoted at 240V...

"In masonry" is generally treated as if clipped direct.

He would be pushing it from a cable life point of view - its likely to run hot. That in turn can cause more problems with overheating at the terminations.

10mm^2 would be a better choice.

The 100mA trip will offer no protection from electrocution.

However if the existing installation meets proper design criteria, and the equipotential bonding is adequate, then it could still be considered "safe".

(without more details of the installation it would be hard to say for sure). The fact that the shower has probably already been uprated does case some doubt however as that will invalidate any previous calculations and testing.

Any new installation though really aught to have a RCD (even if that just means swapping the MCB for RCBO)

There is plenty of choice of small one and two way CUs to choose from. You will probably even find some described as a "shower CU".

[ARW]

Are you posting this sort of stuff to piss me off?

[tabbypurr]

your logic escapes me

NT

[dennis]

Isn't everything he posts to piss someone off? Like woders and TNP. .

[Andy Burns]

I know the 18th ed is available now, does it have a grace period before it comes into effect?

The BSI website slipped-up for a few days last week while they updated the editions and allowed library subscriptions online access to the 17th ed

[newshound]

Thanks, hadn't thought of eBay (had a quick look at TLC and SF sites without spotting anything obvious). Most seem to be 32A or 50A, neither ideal, but there's at least one 40A.

Space is a bit tight, but some of these look as though they will be OK.

He'll be getting a Sparks in, but I wanted to get the requirements clear before people start confusing him.

Didn't realise it would need to be metal cased.

[ARW]

Why would you derate a cable for fixed known load just because you are using a fuse?

[ARW]

Have you had a sip out of the shit goblet again?

[newshound]

Hadn't had time to hunt up the curves, sorry!

Didn't realise that

That's really useful to know. Makes sense to me (but from finding quotes from some of the Sparks' forums it looks as though some people take 34A for 6mm^2 (except clipped) which seems to be Method 4).

Good point. Although there is probably a foot of cable "in air" at the bottom, and several inches at the top.

Changing the cable is not a straightforward option, that would mean replacing tiles, although I did wonder whether a sparks might suggest a new run of 10mm^2 in trunking outside the cabinet, and then over the top of the door into the unit. This would not look *too* untidy if done nicely.

I recognise that

I havn't tried to check this, only had a chance for a *very* quick look and I was concentrating on confirming the supply cable size.

Trouble is the present "shower MCB" is plug in, not DIN rail, so (presumably) direct replacements not available.

Found some now thanks to NT.

We'll see if we can find an electrician who is prepared to do a relatively small job.

[newshound]

Correction factor 0.725 for semi enclosed fuses to BS 3036 (according to my old On Site Guide).

[ARW]

Correct but irrelevant in this case, you have a known fixed load.

[John Rumm]

sorry =, I intended to post the link:

Method 4 would be instillation type B (38A). 34A sounds like method 100.

Yup, no reason to follow the existing path in every case. Any chance the old cable could pull through a new one?

That will need a new enclosure then... but for a one or two way one, they are cheap.

[John Rumm]

Shame I missed that... an electronic searchable version would have been handy.

[tabbypurr]

Fuses allow more overload than MCBs, they're not type B. Hence the regs permit less cable current when protected by fuses.

NT

[tabbypurr]

Wylex plug-in MCBs, like anything else, can be found on Ebay. Those old CUs can get hot on >30A loads but they claim to handle them. But you need a ne w mini-CU with RCD anyway.

FWIW 100mA RCDs do give partial shock protection, but only partial.

NT

[tabbypurr]

It means the cable requires a lower fuse - and the shower is then an overcu rrent on a 30A fuse, though it will run on it, but it'd run hot (the fuse).

NT

[NY]

Can you buy MCBs which are interchangeable with the sockets for wire fuses, so you can remove the socket and fuse and replace it with an MCB, without needing to replace the whole fuse box?

The only MCBs I've seen are in houses which had a "fuse box" which was designed for MCBs, maybe with an overall master RCD as well. I've never seen box which has had its fuses replaced with MCBs.

The only time I've seen a 30 A fuse blow was when my immersion heater element developed a short to earth. At 1 AM, when my Economy 7 timer kicked into night mode, switching on the full-tank heater (as opposed to the half-tank one which could be run at any time of day/night) I was woken by a very loud bang from downstairs as the fuse blew, and a loud metallic ping from the hot water tank. The fuse left a huge stain of sputtered metal on the outside of the ceramic fuse holder and its surroundings, which I never managed to scrape off. The metallic ping became clear when I unscrewed the immersion heater: the element had blown itself to pieces and shot part of the coils into the copper cylinder. (And, yes, I *did* remember to drain the cylinder, in addition to just draining the header tank, before unscrewing the element - unlike a colleague who forgot this and when the element was on the last thread, which could no longer hold the element in place, the water pressure shot it out, gouging a hole in the wall and depositing a couple of hundred litres of still-hot water onto the landing, down the stairs and all over the ground floor carpets. He described the horror of seeing a "plug" of water several inches of diameter emerging from the cylinder, with no way to stop it.)

[Andy Burns]

You can, though there are disadvantages such as a lower fault current breaking capacity, and maybe others that I haven't remembered

e.g for old Wylex fuseboxes ...