Another Shower Question...

[mjames]

I am installing a Mira Sport 10.8 KW on 10mm cable. At 230V this pulls almost 47 amps, by my reckoning.

Mira have told me that I should use an RCD and an MCB in the CU. They have told me that this is better than an RCBO as if one fails it can be replaced.

Can anyone tell me what type (ampage and rating) RCD and MCB I should use? I guess that a 50A 30ma MCB and 50A RCD are the correct bits to get?

Also, I need to run the cable from the CU to the ceiling, then through the old lath and plaster ceiling and through the floor space to the bathroom. I don't need to chase the wall as the cu is boxed in and the box comes to the ceiling but do I need to put the 10mm cable in any sort of conduit where it passes through the roof?

I am getting an electrician to do the actual connection to the CU but I want to save money by getting all the bits in place first.

Thanks!

 

[brown-nought]

Please don't take this the wrong way but ask the sparky doing the connection what he would recommend. If he/she is commissioning the system they will need to sign the test certificate.

However I'd go for 50 A MCB 'B' and 63A 30ma RCD.

 

[shavvy]

Does your CU already have an RCD? You only need one as long as its at 30 mA trip rating. And yes, you`ll need a 50A mcb.

 

[ban-all-sheds]

I am installing a Mira Sport 10.8 KW on 10mm cable. At 230V this pulls almost 47 amps, by my reckoning.

It would, if it was rated at 10.8kW at 230V.

But it isn't - it's only 9.9kW, which is 43A, so you could use a 45A breaker. 50A would be better, but if you can only get a 45A for your CU then you are not stumped.

 

[Damocles]

The shower is a resistive device. So it says 10.8KW, but what voltage is marked on it 230 or 240? That is the one you need for the calculation.

If it was a 12V shower, then current would be about 900A. But in reality, if you connected it to 12V it would only draw about 1/20 of its normal running current. So what? the actual heating power and current depends on what your supply voltage really is.

 

[ban-all-sheds]

The shower is a resistive device. So it says 10.8KW, but what voltage is marked on it 230 or 240? That is the one you need for the calculation.

See p3 of http://www.mirashowers.com/onlinecatalog/pdf/p3279_2a.pdf

 

[Damocles]

Yes exactly. It's a 45A device at 240V and 42A at 230V.

guess you must have broadband. I never get allowed to keep the phone long enough. Mira seem to have a slightly skewed idea about supplementary bonding 'to earth'

james, did you read the instructions? 45A mcb.

Their reasoning about protective devices seemed a bit doubtful to me. Are RCBO known for failing? If you have the option of using the existing CU then a RCBO might be the simplest way to set things up.

RCD just needs to be rated at more than 45A if it is running only the shower. If you opt to use the existing one in the CU (if there is one), then what is its rating? If only 60A or less, then you might be pushing it to run a shower as well as whatever else is there already.

which roof are you talking about?it is normal for cables to run unprotected through the space under the floor or in the attick. Just route it out of harms way, and so it is not covered by fibreglass insulation or the like.

 

[mjames]

Hi - thank you all for answering!

Yes, but my supply is probably 240V isn't it? I used the tlc direct voltage drop calculator to try to work out the amps, I used 230V because that is the recommended voltage for doing that calculation, I think.

I asked Mira for their technical guide and they are sending it out. I have looked at that PDF which contains what I need, however, surely a 45amp mcb is a bit low so I should use a 50amp? However, at 240V it would pull exactly 45A wouldn't it? I am not sure how to "err on the side of caution" here. The line to the shower will be dedicated to the shower.

The CU is an old wylex and I am having it replaced - I am trying to save money 'cos we ain't got any, but I am also trying to remain within my limitations as a DIY'er and let the sparks take over when there is actual live bits involved...

We are going for a split load one. The CU will be near the ceiling at the front of the house and the bathroom is directly above it. We can put in whatever we need, really, space permitting. The 10mm cable will pass straight through the lath and plaster ceiling into the bathroom floor space.

 

[brown-nought]

Hi - thank you all for answering!

Yes, but my supply is probably 240V isn't it? I

Europe has harmonized on 230v but with a tollerance of + 10% to -6%. This means your supply voltage could be up to 253v. The old UK standard was 240 +/- 6% hence in reality nothing changed very much.

 

[dingbat]

Mira seem to have a slightly skewed idea about supplementary bonding 'to earth'

A lot of manufacturers' instructions are misleading. I glanced at the sheet that came with a lamp I was fitting today and read that the circuit '...should be protected by a 6A residual current device...'. No wonder JP gets confused!

 

[shavvy]

As to the rating of mcb it may depend on what make of CU you use for the replacement. If you go for a new Wylex CU they don`t do 45A mcb`s so it would have to be a 50A one. I`d have thought the higher rating would be better anyway as running an mcb at full load will cause it to get hot and you could find tripping problems.

 

[Damocles]

James, the online mira booklet says how much power it will take if the voltage is actually 240, and if it's actually 230V. It's 45A if you really have the higher Voltage.

A 45A breaker will run happily at a few amps over for a short time. Showers are not normally run continuously. In fact it might never trip at 10% overload. This might be affected by how cramped it gets in the CU. My inclination on a large power device is to get the current control as tight as possible, but I guess the difference between a 45 and 50 is not great. Current rating of a 10mm cable is a bit marginal for 50A if it is enclosed.

And on the subject of enclosure, you will make sure there is no chance of a leaky shower dripping down into the CU? Lots of nice holes and wires for the water torun down.

 

[mjames]

Having looked at the existing cabling, we would need to rip out the airing cupboard to be able to replace the wiring - the bathroom is tiled so breaking into the walls is not an option. We are going to go for an 8.7 KW shower instead...! Then we can use the existing 6mm wiring, I think.

Thanks for all your help!