Replacing electric shower- cable?

[Freddie]

Hi all,

I wonder if anyone can give me some advice.

At the moment, I have a broken Mira 7.5kw electric shower and it's due to be replaced with a Triton T100XR 9.5 kw electric shower.

The concern I have with a straight swap is the power rating of the currently installed cable.

It's 6mm2 T&E fed straight from it's own 40A MK MCB in the consumer unit. The length of cable is at the very most 3 metres and majority off it is in a non insulated partition wall. The shower is switched via 45A switch on the wall outside of the bathroom.

The question is, can I do a straight swap or am I looking at changing the cable to a 10mm2 T&E due to the increased wattage of the shower?

TIA

Freddie

[Andy Burns]

According to

a 9.5kW load at 230V with 3m cable run clipped direct, 6mm^2 is fine, you will be taking marginally over 40A though so the MCB could give nuisance trips.

[Freddie]

Does it matter that the cable isn't clipped? It's...erm..free standing in the wall. Not exactly a brilliant job of the previous owners

[Richard]

Up the cable to 10mm increase the breaker to 45A you do not want to be marginal on your breaker, you can guarantee it will trip when you have a head full of shampoo.

[Freddie]

Yup Richard...I'm thinking that to be honest.

But, having said that, the manual for the shower recommends a 40A mcb

*shrug*

Freddie

[Stuart]

And you might just want to upgrade the shower in the future .

[John Rumm]

I would say that this is not required in this circumstance due to the short length and the installation method.

A 40A MCB will carry 50A almost indefinitely. Hence your load is not going to trip it. You may well find that you only get a true 9.5kW out of it at 240V (39.5A ish). Which suggests at 230V the load will fall further to just over 38A. Either way, it will be fine on a 40A MCB.

[Richard]

Not my experience 10-15% is the norm

[Richard]

Hmmm I think you will find when the voltage lowers the current goes up.

230v would take you to 41.3A at 9.5kW

[Bob Mannix]

Hmmm I think you will find it doesn't.

A shower *rated* at 9.5kW at 230V will draw more current than one *rated* at

9.5kW at 240V but, for the latter case, if the voltage is lowered to 230V (as suggested by the previous poser) the current and the power will be lower. Mr Ohm sorted that out a long time ago :o)

[Christian McArdle]

I think you will find the opposite.

Christian.

[Tim..]

This is *only* the case with an inductive load- eg a motor.

Tim..

[John Rumm]

So by your experiance, you are suggesting that what you wrote before is wrong then ;-)

40A + 10% is well in excess of the maximum load the shower will present.

(BS7671 fig 3.4 (characteristic curves for type B MCBs), show a load of

50A on a 40A device as off the top end of the scale (i.e. trip time in excess of 10,000 seconds). Even a 60A load is shown as susstainable for over 15 mins. Exact times will vary with manufacturer and circumstance obviously).

[John Rumm]

Shower heaters are mainly resistive loads and hence the loading should follow ohms law quite well. Hence if you lower the voltage the power will fall in proportion.

[Harry Bloomfield]

Richard used his keyboard to write :

Oh no it won't.

Resistance will be more or less constant therefore as the voltage decreases so will the current.

[Matt]

It was all so easy when we had proper 240v and not this foreign 230v crap :)