electric shower

[Spark123]

excuse me for being picky, the load of the shower should be divided by 230..

The iee still use 240v in its calculations

 

[ban-all-sheds]

excuse me for being picky, the load of the shower should be divided by 230..

Ah well - I'll join you.

In being picky, I mean.

The headline rating of showers is invariably given for a 240V supply (which de facto ours still is). So if you want to use 230V for your calculations, you need to rummage in the small print where you'll find that what you thought was an 8.5kW shower has just become a 7.8kW one...

 

[ban-all-sheds]

I prefer Swarfega wrestling myself....

Do you have a link..

Funny you should ask....
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but no.

However, on the principle of "you never know", I googled for "swarfega wrestling", and did get some hits. Tried +fetish +swarfega, and got several hundred hits.

Ah well - I suppose there are worse things to get slimy with..

Found this: http://www.shartwell.freeserve.co.uk/humor-site/swarfega.htm

 

[Albert]

excuse me for being picky, the load of the shower should be divided by 230..

Ah well - I'll join you.

In being picky, I mean.

The headline rating of showers is invariably given for a 240V supply (which de facto ours still is). So if you want to use 230V for your calculations, you need to rummage in the small print where you'll find that what you thought was an 8.5kW shower has just become a 7.8kW one...

Although you might measure or read 240V or even 245V, the theoretical calculations have to be based on 230V. This is not a question of good or bad, this is how it is done. Interestingly you take the load from the sticker as is, of course it might (not necessarily) change the cable size.

I don't see how the load of the shower relats to the supply, you can supply a 10KW load with 230V, 260V or 200V. This does not change the load it will always be 10KW. The current will change with the change of the supply meaning that the you might need a different cable, MCB etc.


BTW thanks for the link...

 

[ban-all-sheds]

Although you might measure or read 240V or even 245V, the theoretical calculations have to be based on 230V. This is not a question of good or bad, this is how it is done.

Yes, but if you do that then to get an accurate answer you have to use the rating of the shower at 230V, not 240V.

I don't see how the load of the shower relats to the supply, you can supply a 10KW load with 230V, 260V or 200V. This does not change the load it will always be 10KW.

Err.... no.

Try Ohm's Law.

If we simplify, and ignore the difference in the impedance of the load at different temperatures, then of course it relates to the supply!

Lower voltage = less current = fewer watts.

A shower which dissipates 8.5kW at 240V will dissipate 7.8kW at 230V.

 

[Albert]

Although you might measure or read 240V or even 245V, the theoretical calculations have to be based on 230V. This is not a question of good or bad, this is how it is done.

Yes, but if you do that then to get an accurate answer you have to use the rating of the shower at 230V, not 240V.

I don't see how the load of the shower relats to the supply, you can supply a 10KW load with 230V, 260V or 200V. This does not change the load it will always be 10KW.

Err.... no.

Try Ohm's Law.

If we simplify, and ignore the difference in the impedance of the load at different temperatures, then of course it relates to the supply!

Lower voltage = less current = fewer watts.

A shower which dissipates 8.5kW at 240V will dissipate 7.8kW at 230V.

You saying that if I have to supply a 100W bulb with 240V, it will consume less watts if we supply with 230V?

Power/voltage = Current. this is a fixed load the maximum demand is (we can say) fixed, the voltage is fixed (whether it is 240 or 230V) so only the current can change when the voltage changes.

Or:

W=I2 * R > R=Constant (not considering changes in temperature as it will effect any supply value) The maximum power needed to oprate this unit is constant, the only parameter that can change is the current demand. that is why I said that it might require a differnt cable size (or not).

 

[ban-all-sheds]

You saying that if I have to supply a 100W bulb with 240V, it will consume less watts if we supply with 230V?

Of course it will

I=V/R. If you reduce V then I must go down.

Power/voltage = Current. this is a fixed load the maximum demand is (we can say) fixed, the voltage is fixed (whether it is 240 or 230V) so only the current can change when the voltage changes.

Yes - so you change the voltage, and Mr Ohm changes the current, so between you you have changed the power, as P=V*I.

W=I2 * R > R=Constant (not considering changes in temperature as it will effect any supply value) The maximum power needed to oprate this unit is constant, the only parameter that can change is the current demand. that is why I said that it might require a differnt cable size (or not).

Yes, but you can't keep the power the same by somehow pushing more current through it to compensate for a lower voltage.

P=V²/R. As we are assuming that R is constant, power is proportional to the square of the voltage.

So a shower rated at 8.5kW on a 240V supply will at 230V dissipate (8.2 x 230²)/240² = 7.8kW

You cannot use the 240V power rating and divide it by 230 to get the current at 230V.

 

[Spark123]

Is it not 7.5kW?

 

[Albert]

I think that I know where we miss each other, you are talking about the power that the load will 'use' if the voltage will be reduced to 230V.
I am talking about what will you do to get a certain load to be used fully in the event that the voltage is lower you will have to supply or it will draw more current.
This is an academic argument because the way to design the circuit is to take the maximum load on the unit and divide by 230V which is the official level of voltage in this country (although in reality it can be 240V or even 245V). (yes in some cases we use diversity... )

 

[ban-all-sheds]

Is it not 7.5kW?

Spot the non-deliberate error

:

So a shower rated at 8.5kW on a 240V supply will at 230V dissipate (8.2 x 230²)/240² = 7.8kW

Whereas it will of course dissipate (8.5 x 230²)/240² = 7.8kW

 

[ban-all-sheds]

I think that I know where we miss each other, you are talking about the power that the load will 'use' if the voltage will be reduced to 230V.

Nothing else makes sense.

I am talking about what will you do to get a certain load to be used fully in the event that the voltage is lower you will have to supply or it will draw more current.

But showers don't work like that. If you reduce the supply voltage you cannot reduce the resistance of the heater to compensate. And until they start making showers with SMPSUs in them they will not draw more current when the voltage drops, they will draw less.

This is an academic argument because the way to design the circuit is to take the maximum load on the unit and divide by 230V

Absolutely not.

Yes - use the 230V figure, but don't use the rating for a different voltage!

Let me illustrate it this way. Suppose the specifications for the shower only quoted the 230V performance. So there you are, designing your circuit, and you look at the spec and it says 7.8kW.

What current calculation do you do?

a) 7800 ÷ 230

b) 8500 ÷ 230

If you answer (b), please explain why.

If you answer (a), then please explain why if you read the spec and it said

7.8kW at 230V
8.5kW at 240V

you wouldn't still do (a).

which is the official level of voltage in this country (although in reality it can be 240V or even 245V).

It is officially allowed to rise to 253V.

IMO there is a case to be made, for high current appliances, for at least looking to see what the current would be at 253V.

So in the case of this shower, what would you calculate that to be?

a) 37.3A
b) 41A
c) 30.8A

?

 

[blueblob]

This is really interesting to an amatuar like me.....

but the real question is....












































IS CUSTARD BETTER THAN SWARFEGA!!!!!

 

[Albert]

It is officially allowed to rise to 253V.

I know, but you don't use it for design purpose, you would use 230V.

 

[Spark123]

How do the iee get the figures for table 41b1? I think it is from 240v divided by the figures in the charts in appx 3 isn't it?

 

[ban-all-sheds]

IS CUSTARD BETTER THAN SWARFEGA!!!!!

I don't know - I've never eaten Swarfega.