Powering a shower pump or digital shower/ loft radial 30A circuit

Sounds ok, it's not installing a new circuit, just modifying an existing one. Normally a radial would be protected at 20A using 2.5mm or 30A using 4mm cable, but larger cable is fine, it could be required in certain cases (high current and/or long runs).

You can add more sockets to the radial circuit, it can branch any way required, but you couldn't use cable smaller than 4mm without reducing the MCB to a 20A.

Any time you do branch off into a smaller cable, you can fit a Fused Connection Unit (a switch with a fuse in the faceplate) so the current of the branch is limited to 13A (or a lower fuse rating). For example if the pump was hardwired you might put a FCU with a 5A fuse it in so the current in the pump cable can't be more than 5A without blowing the FCU fuse.

I hope there's an RCD upstream of the MCB?

Theo

I recently installed a Mira Activate Digital shower. Their instructions simply say plug the plug into an available socket no mention of any further fusing requirements other than the circuit is protected by a 30mA RCD and relying on the fuse in the mains plug of the 12V power supply. I simply spurred off a socket on the other side of the airing cupboard wall and fitted a socket high up in the airing cupboard. As for using the existing

Just to add a bit of clarification there, a 32A backbone radial, with multiple spurs in 2.5mm^2 would be acceptable. The rules would be the same as for spurs from a ring, i.e. only one single or double socket on each unfused spur. That splits the over current protection so that overload protection is imposed by the nature of the load, and fault current protection is still provided by the MCB.

The thing you can't do is extend the radial in 2.5mm^2 - i.e. adding a string of multiple sockets etc. To do that you would need to make the extension a fused spur.

Yes, electrically that is fine. Mechanically getting 6mm^2 into socket terminals is a bit harder - but doable.

There would be no need to downgrade the MCB - it is only there to protect the 6mm^2 cable in this instance.

It is a radial - and that is a standard socket circuit design as well.

Yes... there are three approaches that would work:

You could do, but since you already have an available power circuit there, there is no real advantage.

Also worth considering where you will put the socket that the shower will plug into - rules on sockets in bathrooms are quite restrictive. (although they can be used in spaces accessible only with a tool - like in voids, under bath panels etc)

pumps triggered by flow sensors work well IME, so long as you have positive head. The time they can be more difficult is when you have negative head - say wanting to pump to a shower in a loft conversion from a cylinder and cistern on the 1st floor. A pump can also be configured to provide additional flow rate to baths as well if required, where as a "power shower" pump solution is more of a one trick pony.

Also if a pump fails, it is less disruptive to swap it out.

Another option you could consider if you have a decent incoming cold main is an unvented cylinder and a normal thermostatic mixer valve for the shower. No pumps, no noise, no cold water cistern required either, and (close to) mains pressure hot water everywhere.

It looks like this family of products is a contender. How do you find it? Particularly noise wise and shower experience?

I'm lucky that the old 6mm cable is basically a minor diversion and I have plenty to play with.

Indeed the CU has it on the main switch so protecting the whole house

Thanks

And thanks,

Good to know, I'd have been inclined to do the onward radial in 2.5mm were it not for the fact the existing 6mm run goes where I would want sockets. Might have to use junctions going to 2.5mm though for the sockets (i.e. Option 2 in John's other reply). For now the circuit would only be powering the pump, nice to know the additional sockets are possible in future, so I can cut the 6mm where I need to and keep the rest of the old run in situ for future use.

Pump/shower box (therefore socket) will be in loft or airing cupboard, the house layout means too much pipe required for the feed into a pump anywhere in the bathroom since they tend to recommend as short as possible and 5m max.

We have positive head (just) but after reading a couple of pumps' manuals think a universal pump would be safest bet, especially as bar mixers don't seem to be available with under 3m minimum head. We have

At the moment a digital shower with a bath button is a popular idea, but they are £300+ more expensive than a dual outlet shower head version for some reason. Can offset that a bit since you don't need bath taps, it fills from the special overflow but still. Since we're not interested in an overhead drench (overrated and make a mess in our experience) I considered if could somehow make one of those units work, just plumbing so the drench will divert to fill the bath. Or since the bath is basically only required for adequately sheep-dipping a 4 year old maybe we can make do with the existing pipes which aren't that bad!

I thought about it and have been tempted. My wife, being from France is used to that system but I'm not sure I fancy putting our 1950s pipework under mains pressure. And when I looked into costs of the thing itself, plus still need all the plumbing for the shower done decided that's in the future-things-to-think-about pile. That was one plus point for the pump option, in that the shower pipework would be ready with minor tweaks.

It is like night and day compared to the previous electric shower we have the dual shower head but only use the drench head. You can use both heads but they share the flow if you do. As for noise other than the noise of the water through the heads there is a slight noise from the mixer but you are only aware of it in the bathroom and at the moment I have to finish the airing cupboard including fitting doors which will quieten the noise even more. Ours is the high pressure version which is fed of the Combi boiler directly so it does not have a pump compared to the low pressure version which feeds off a cylinder I do not know if that makes a difference or not.

Thanks very much. For us anything will be an improvement over the temporary bath tap shower head we're currently using (The terrible kind that fits over the tap)! I think the main difference with yours will be that the pressure and flow will be controlled by the output characteristics of your boiler, and the hot water is unlimited, whereas in theory the pumped version (having its own dedicated water supplies) are limited more by the temperature of the stored water, the flow restrictions of the pipework and the effect of gravity. The instructions I've seen are nowhere near as stringent as the dedicated shower pumps are but I think there are some good rules of thumb in them.

My 3 flow rate Mira shower is wired in 10mm and has a 40A MCB.

So should have mine, probably. It was 8.7kW which does not go into 30A! You reminded me, been meaning to get that checked out in case that MCB is faulty.

Chances are it is not faulty... if you look at the trip curve for a 32A MCB[1], you will see that it will take 37A indefinitely.

Haha nice try but no cigar :D

It's an MK and the certificate posted on the meter cupboard door is for the 15th Edition and signed September 1984. As it's as old as me, I have a few things not working as well as they used to so would apply that logic to an inanimate object! I'd imagine the shower MCB was added after that though maybe not.

But that's interesting about the curves. Does this new information change anything about your prognosis?

When we were initially looking to replace the electric shower, one electrician said he would straight up fail an EICR based on the age of the CU... I'm led to believe elsewhere there's nothing inherently wrong with it though, unless it actually fails testing. Obviously not going to ask him round again!

Young sparkies, especially those on youtube have a tendency to see "old" and think "£fail£", now there will usually be good reasons to recommend replacement whenever any work is being done, on and old system, maybe they just don't want to explain their thinking ...

Not really - a 30A MCB will allow a smallish overload for a long time.

There were obviously other makers of type 2 MCBs - I tend to find that the Wylex ones seem to be more common that some other brands.

OOI, does the MCB have a type letter or number on it?

An old BS3036 re-wirable fuse CU could still be perfectly serviceable, so just being "old" is not a good reason to fail it.

There may be aspects that are very undesirable (e.g. "whole house" RCD)

- not the way you would do it now, but at best "requires improvement". There could also be things that act as a barrier to doing more work without rectification like missing or inadequate main equipotential bonding.

Either way your cable and MCB are pretty close to a standard radial circuit for feeding sockets - albeit with a thicker cable than the

Use a cooker outlet that also has a 3-pin socket :-)

A bit non standard but the connections will take 6mm^2 cable nice and easily.

To be fair, so will most single / double sockets. It gets more difficult when you want to run a line of several though since not all will take 2 x 6mm^2. There are some that claim to have 16mm^2 capacity terminals though. (A deeper backbox will also help)