Shower power drop

Scoby_Beasley · 17 May 2004

Ok. I know this has been answered before for me, but now I can't find the answer.

.
A [240v] 10.5kw shower at 230v is a 9.6kw shower. Can someone please remind me of the formulae ? TIA

plugwash · 17 May 2004

assuming a shower is largely resistive

V=IR
P=IV

from this we can derive

I=V/R
P=(V^2)/R
RP=(V^2)
R=(V^2)/P

so for your 240V 10.5KW shower
R=(240^2)/10500=5.4857142857142857142857142857143 ohms

when running it at 230V
P=(230^2)/5.4857142857142857142857142857143 = 9643.2291666666666666666666666672W

dingbat · 17 May 2004

Or, to put the same thing another way, if Power = V^2/R then power is proportional to the square of the voltage, resistance being constant for the unit.

So the difference in power between 230V and 240V = 230^2/240^2 = 0.92 (to two sig figs)

A 10.5kW shower would then deliver 10.5 x 0.92 = 9.66
An 8.5kW shower would become 8.5 x 0.92 = 7.8
Etc...

AdamW · 18 May 2004

But what about the +/- 10% tolerances?

All you can tell from the mains spec is that your 10.5kW shower is between 7.8kW and 11.7kW

From what I have read, UK mains voltage is usually nearer 240V than 230V anyway, so your 10.5kW is a fairly true figure.

dingbat · 18 May 2004

Tolerances? Are you mad, man? This is Britain... we don't tolerate anything here!

The tolerances are merely a clerical sop to the EU overlords (not that I'd want to give away my political leanings, you realise) - the lowest domestic supply voltage I've ever meaured in the UK was 238V. But it's a good point about the spec - it's 10.5kW at 240V.

securespark · 18 May 2004

[Adopt schoolboy style chanting]

"Dingbat's a Tory, Dingbat's a Tory, Dingbat's a Tory......."

No offence, mate!

ZenStalinist · 18 May 2004

Hmm... the voltage here measures 238 volts one average (usually between 235 and 242 volts, 238 seems most common reading)

Seen it go as low as 226... Eh, the wonders of semi-rurality..

Stod · 18 May 2004

Do you think, even for one minute that your local Distribution Network Operator will spend money reinforcing your local network if your voltage is still above 207 V (the 2008 lower tolerance)?

If you have a satisfactory voltage now it's because you are not at the end of a feeder, your local network was designed for 240V and/or it's not had extra load added.

Scoby_Beasley · 18 May 2004

Thanks for the formulae m8's
Now how do I get the lid back on this tin of worms ??

Scoby_Beasley · 18 May 2004

dingbat said:
So the difference in power between 230V and 240V = 230^2/240^2 = 0.92 (to two sig figs)

Suits me Sir !! I prefer the 'rounder' figure

kendor · 18 May 2004

Just to throw a spanner in the works, would the resistance actually be constant with the change in the temperature due to the different vd's across the resistance and therefore the different power consumptions?
Take a lamp on a dimmer for instance the filament will be hotter at 240v and slightly cooler at 230v so you will see the lamp dim I know that the tungsten filament has a negative coefficient of resistance and it's resistance will change accordingly to allow the current to stay constant in a fixed voltage arrangement but when you change the voltage and the temperature what happens then ? the element(in the shower unit) runs cooler.

dingbat · 18 May 2004

Blimey O'Riley Kendor, calm down... it's a commercial...

So anyway, I was saying to Maggie just the other decade...

ZenStalinist · 18 May 2004

It'd better have included profanities.

*Produces Labour party membership card*

No offence.

dingbat · 18 May 2004

ZenStalinist said:
It'd better have included profanities.

*Produces Labour party membership card*

No offence.

Profanities? Yes indeed. I distinctly remember her saying something along the lines of, "F**k those Labour f**kers."

Oh, happy days... and oh look, here they come again!

U-KIP, U-KIP. U-KIP...

securespark · 19 May 2004

It's worse than I thought.

We need the euro to compete fairly in the union.

Ooops!! How the heck did that slip out?

Let there be a referendum on europe.

Crikey - there goes another one....I'd better sign out.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Shower power drop

Stod

Similar threads