9.5Kw Electric Shower

[markadams]

Hi,

I am replacing an electric shower from a 9kw and can only find a new 9.5Kw replacement.

It is wired in a 6mm T&E plastered in the wall then running across the loft (which has very little insulation) into the CU which is about 6 metres away so only about a 10m run. So I make this reference method C

According to BS7671 Current Carrying Capacity of 6mm T&E Reference method C would allow a load of 47 amps.

The current shower is on a 40amp MCB and I am sure that this would be fine for the new shower as it is rated between 8.7 - 9.5Kw (37.8 - 41.3 amp)

The user manual advices 10mm cable and a 40/45 amp MCB. What is the opinion of other sparks.

 

[telectrix]

stick with the 6.0mm on the 40A MCB. you could in fact use a 50A MCB with the 6.0mm as overload protection is not required ( fixed load ). might be an idea to fix a notice in the loft..... " DO NOT COVER CABLES WITH INSULATION". (use pictures so the insulation monkeys might understand).

 

[Andy78]

I would be looking to fit an 8.5kW shower in this instance as my first option.

 

[markadams]

I would be looking to fit an 8.5kW shower in this instance as my first option.

Normally I would go on the side of caution but on this occasion the 8.5kw is £65 more than the 9.5kw and it is out of stock. The customer will not pay the extra.

All the calculations add up to the current install being fine but I would be happier if it was on a 10mm cable as I would have installed from scratch.

 

[Andy78]

Normally I would go on the side of caution but on this occasion the 8.5kw is £65 more than the 9.5kw and it is out of stock. The customer will not pay the extra.

All the calculations add up to the current install being fine but I would be happier if it was on a 10mm cable as I would have installed from scratch.

Just for argument's sake, can you not get another make of shower ? Mira multifit showers for example have multiple water pipe and cable entry points to enable retrofit with other brands.

 

[markadams]

Just for argument's sake, can you not get another make of shower ? Mira multifit showers for example have multiple water pipe and cable entry points to enable retrofit with other brands.

I would but the customer wants the Triton T80 Thermostatic, I just want to make sure that others agree that my calculations are correct and the 9.5kw should not be a problem.

Mark.

 

[Dave OCD]

When you consider the duration of a shower I'd say around 10 minutes - so bearing in mind that the published cable current ratings are for continuous load I'd say 6mm is absolutely fine.

 

[baldsparkies]

Its not just about calcs its also about efficiency.
Yes its fine as the calcs have confirmed, and the customer will see no difference in performance from old to new.
If it was me, and cost not an issue, I would run the shower your installing from a 10mm and here's why !!
Improved flow rate during winter months simple as that.
From supply to load, keeping volt drop and losses to a minimum can, and does, make the whole system much more efficient.
In the summer when ambient temps are higher its not a problem.
In the winter when the temp of the incoming water is much colder, the shower element, in relation to flow rate, needs as much go juice as it can get.
So with a 10mm you will find you can run the shower with a higher flow and still maintain a nice working temp.
The 6mm will suffice but the flow rate will need to be lower to maintain the same working temp.
I have known of and experienced this situation on quite a few occasions.
But in your case what they haven't had there not going to miss.
All about real world scenario's, hope that helps.

 

[Dave OCD]

At the length of run in the example on this thread volt drop shouldn't really be an issue.

 

[baldsparkies]

Technically your right, but with a 9.5 kw load across the supply, the overall effect is higher flow rate for the 10mm than the 6mm at the same running temp.
And 10 meters is enough to make a difference with those kind of loads present.
As said real world not whats on paper.
Although the manufacture does say 10mm and that will supersede BS7671 I believe.

 

[Dave OCD]

Don't worry I'm very much real world too.

 

[markadams]

Don't worry I'm very much real world too.

I think we all agree that in an ideal world I would install a new 10mm cable, with 50 amp MCB and install a 10.5 KW shower.

 

[Dave OCD]

No I wouldn't install one of those things, the lights dim in the whole street when they fire up lol.

 

[davesparks]

I think we all agree that in an ideal world I would install a new 10mm cable, with 50 amp MCB and install a 10.5 KW shower.

Or fit the 8.5kW

 

[Lucien Nunes]

From supply to load, keeping volt drop and losses to a minimum can, and does, make the whole system much more efficient.

Not sure I agree with your usage of 'much'.

If the 6mm circuit is just compliant with the whole 5% vd in the final, then to a first order approximation the best you are going to do by changing the cable to 10mm is reduce that 5% to 3%, i.e. an improvement of 2%. The efficiency will increase from 95% to 97%, while the shower heat output increases about 4%, which might be worthwhile although I'm not sure I'd notice an improvement of 4% in flow rate at a given temp.

Obviously if the circuit is not at 5% vd already, the improvements will be less marked. Plugging in top of head figures to the 9.5kW / 10m run scenario gives me an improvement of about 0.5% efficiency and 1% flow rate for a given temp.

 

[Spoon]

When you consider the duration of a shower I'd say around 10 minutes -

You need to have a work with some of the guys at my works. Daughters spending 45 mins in the shower and sons who turn the shower on and then go into their bedrooms for 10 mins and listed to music while 'the shower warms up'

 

[telectrix]

You need to have a work with some of the guys at my works. Daughters spending 45 mins in the shower and sons who turn the shower on and then go into their bedrooms for 10 mins and listed to music while 'the shower warms up'

so fit a coin meter to the shower feed. set it at £1.50 / 10mins.

 

[Spoon]

so fit a coin meter to the shower feed. set it at £1.50 / 10mins.

or a contact that switches the voltage from the shower to the water supply after 15 mins..... too harsh....?? I would put a count down on the wall....

 

[telectrix]

or turn on the cold water taps when daughter is in shower. unless you have really good water pressure, it will run cold.

 

[Spoon]

yep, same as flushing the toilet... happened to me quite a few times

 

[Dave OCD]

I had a bloke get me to fit a coin meter to his snooker table lights so that his son and mates would pay to play !! His boy was in there a few weeks later practising his shots whilst wearing a head torch !

 

[Guest77]

After reading the thread I agree with Andy in post 3, that would be my choice in this instance.

 

[markadams]

MDJ,

I agree with you, but the 8.5Kw is £60 more expensive and the customer will not pay the extra. I have the choice of either fitting the 9.5kw (only if it is safe to do so) or not do the job. According to BS7671 it is safe but it is against my normal rule of thumb so I just wanted confirmation from others before I went ahead.

It has had a 9kw installed for several years without a problem, personally I really don't understand why a 10mm cable was installed from the start but too late for that now.


Mark.