Shower cable thickness dilemma?

[dave163]

My ensuite bathroom has a very elderly 8.5kW shower that I want to replace. Flow rate is generally poor despite excellent water pressure to the house, temperature adjustment is usually either "too cold" or "too hot" and if water is drawn anywhere else in the house (e.g. flushing a toilet) when using the shower it scalds you. Not good.

I recently fitted a Mira Sport Boost shower to the main bathroom and really like it - the air boost feature makes it feel much more powerful and it's pretty good at maintaining a constant temperature. I have the more powerful 10.8kW version but I see there is also a 9KW version. On the maximum power setting I never turn the temperature setting more than halfway as it's plenty hot enough.

Going back to the ensuite bathroom, the existing shower is supplied by a 6mm cable. As this is only visible at the consumer unit I have to assume it's buried in the wall and don't know if it's in a conduit or just covered in plaster. The consumer unit is at ceiling level in the room below, almost directly underneath the shower, and the cable goes up to the pull cord isolation switch in the ceiling above the shower before coming back down again into the shower. I doubt the total cable length from consumer unit to shower is longer than five or six metres.

I got in an electrician to look at the type of cable and he said I should be looking for showers up to 8.7kW. At the time I hadn't seen the Mira came in a 9kW version so I would like to know how much difference there is between 8.7 and 9kW over a fairly short 6mm cable? Initial Googling suggests lots of people asking about the maximum rated shower that can run on a 6mm cable; some responses say even more than 9kW is OK if the cable is short (define short?), others that even 8.5kW is too heavy a load for a 6mm cable. How do you determine the maximum power rating that a given cable can safely deliver?

Unfortunately the "obvious" solution of swapping over the cable for one that's 10mm is simply not an option as it would involve having to totally strip the bathroom and get laminate flooring up in two rooms.

If a 9kW shower isn't an option, can anyone suggest a lower rated shower that would be acceptable (constant temperature, and ideally have a similar feature to the Mira Airboost)? I don't want to simply replace my existing elderly cr@ppy 8.5kW shower with an equally cr@ppy new 8.5kW shower.....

 

[flameport]

6mm² cable can be rated up to 47A, depending on how it is installed, which is 10.8kW at 230V.
Length of 6m is not a problem.

Two high power electric showers in one house may be a problem in itself. What cooking and heating arrangements are used?

 

[davelx]

6mm² cable can be rated up to 47A, depending on how it is installed, which is 10.8kW at 230V.
Length of 6m is not a problem.

Two high power electric showers in one house may be a problem in itself. What cooking and heating arrangements are used?

and the MIRA 9kW shower ratings are for 240V (full specs are on their web site, they give power ratings at both 230 and 240V), so you are looking at 37.5A, so minimum of a 40A MCB which should be OK for the relatively low duty cycle.

Note that any other installation method other than Method C - clipped direct/buried in plaster - would require a heavier cable due to the reduction of the capacity of the cable. So you really should confirm the details of the cable run.

 

[dave163]

Thanks - to answer questions above:

1. Kitchen has electric oven, washing machine and tumble dryer and other bathroom has the 10.8kW Mira. But unlikely to be cooking and showering, or using both showers at the same time. The one being replaced is for guests so won't often be used (as long as we are living here at least!)

2. Cable goes up wall behind rules and plaster. Not sure if it's plaster or plasterboard (any way of knowing other than smashing into wall?) but it's probably just plaster. Passes through floor between ground and first floor then again inbetween bathroom ceiling and attic floorboards so no contact with insulation wool etc.

At a push I could surface route 10mm cable from consumer unit to ceiling pull switch if I could get away with keeping the 1m length of cable going down into the shower as that goes behind rules and I don't want to have to redecorate (guests coming to stay in a few weeks and want a decent shower for them!)

Where does a 6mm cable and a 9kWh 40 amp shower leave me given that set up?

 

[flameport]

Where does a 6mm cable and a 9kWh 40 amp shower leave me given that set up?

Given the answer 2, not a problem.

 

[dave163]

Thanks - can you expand on why that's OK for 6mm then? I'm confused about Method A, Method C etc for cables in wall. Pretty sure wall is plaster over brick (rather than plasterboard) as it is totally dull when tapped unlike rooms in my house that I know are plasterboarded over brick (three storey Victorian house with some original plaster and some new plaster depending on the room; bathroom in question is on middle floor).

I don't know if the cable is just covered in plaster or if it runs in a conduit which I guess could have either a plastic or metal sheath between cable and plaster. The wall is neatly tiled over the plaster which is why I don't want to go cutting new channels into the wall to run new cables.

 

[flameport]

Cables have a maximum current rating, which in the case of 6mm² is 47 amps.
Cables get warm/hot when current passes through them, and the maximum ratings are only applicable if the heat can be removed. Cable fixed to a surface or under a floor allows the heat to escape, as does being directly buried in plaster as the heat can be dissipated into the wall.

If the cable is installed in other ways such as through insulation or in plastic conduit, the heat cannot escape as quickly, which will cause the cable to be hotter for any given current. The current rating is therefore reduced to ensure the cable temperature doesn't exceed 70C.

Conduit matters because it traps a layer of air between the cable and the conduit, and air is a very poor conductor of heat.