Shower

[RMS]

Hi, i'm training to be an electrician and was looking at a fault with my shower at home. There was no power to the unit and i found a melted phase wire in the pull switch. the shower is rated at 9.8kw@240v and 9.0kw@230v. The cable run is approx 10m and wired in 6mm twin and earth and protected by a 40amp type b 60898 mcb. Done some calcs and it seems that this would be okay for volt drop and current carrying capacity of the cable. Question is, am i right???? and would i work out the power using 230 nominal or approx 240 actual voltage???

 

[Steve]

whats the actual voltage? use your multimeter to find out, I found ours to be 235.4V, slap bang in the middle .

We had a similar fault with our shower, melted neutral wire. it was probably caused by a loose connection, but was so badly melted and corroded it was stuck together. Think it had been like that for some time, glad we got at it when we did really. Replaced it all with 10mm².

 

[RMS]

Have not measured voltage yet. Will the power rating of the shower be dependant upon the actual measured voltage and corrected to find the acurate power consumption.

 

[il78]

"quick" cable calculations are based on 230V and information supplied from tables in BS:7671, although cable calculations can be done using actual and manufacturers data.

What reference method are you using....is the cable run through a loft?

 

[RF Lighting]

If you work the power out using the nominal voltage (230V) this will cover worst case scenarioas your voltage may drop as low as this. This rates your shower at 39.13A So providing there are no derating factors to apply to the cable, it is clipped direct and the run is not too long (for volts drop), it is capable of carrying 46A so no problem there. What was the pull switch rated at? if it was 40A or 45A rated this should have been ok. I think it is more likely that it was a badly fitted pull switch with a loose connection. Do you have access to a clamp meter or do you know someone who you could borrow one off? This will give you a reading of what power is actually being used by the shower (providing you can get it working)

 

[RMS]

The cable runs out the cu in trunkin so it is grouped with 6 other circuirts, it is burried directly into plaster up to the loft where it will be covered in 500mm plus insulation. The pull switch is rated at 45amp.

 

[RF Lighting]

Bad news.
Time to rewire the shower.

From the derating factors you provided its going to have to be 16.0mm²

 

[RMS]

Any chance you can show me how to work that out?

thanks rms

 

[Adam_151]

16mm² , re-consider the route and 10mm² should be possible

(take it out from under that insulation is the big thing)

 

[RF Lighting]

See here http://www.tlc-direct.co.uk/Book/4.3.9.htm

I made a mistake earlier you need to either wire it in 35mm² or re-route the cable out of the grouping and thermal insulation.

I hope i'm not just doing you college work for you

 

[RMS]

Thanks for that Rf, i was dividing the overcurrent device by all the correction factors instead of dividing them simultaneously. I work the current carrying capacity at 140.35 amps. The cable runs with the other circuirts above the fuse board to under the first floor in trunkin, surely this is a problem with alot of installations.

 

[RF Lighting]

As far as I am aware but I could be wrong, providing the trunking is big enough and the cables have enough room to 'breath' then grouping factors do not need to be applied, just the factor for enclosed in trunking.

My slightly out of date OSG states that in domestic premises conventional circuit design permits upto 5 touching, single layer circuits when clipped to a non-metallic surface (I am paraphrasing cause I can't be bothered typing it all)