3kW load. What do you regard as the design current?

[echoes]

If 230V is applied to a 240V element, how could the current 'be a bit more'?

Take a 240V incandescent light bulb. (Don't worry, I wont tell the EU!)
Measure the current at the rated voltage and again at a lower voltage. What happens?

The resistance might not be constant, but I would be most surprised to find a higher current at a lower voltage - this would imply a negative resistance or a step-change somewhere on the I-V curve.

 

[SgtTrojan]

Well how can a valid assessment of the maximum demand ever be carried out? I sense Armageddon!! Rioting!! HOuse's without power!! The chaos!! :O :O :O

 

[ban-all-sheds]

You need to get out more.

 

[SgtTrojan]

hahaha

 

[Taylortwocities]

Untitled

• Taylortwocities
• 1 Nov 2010
• 3

 

[SgtTrojan]

http://www.vphase.co.uk/

If the voltage was limited would you use 220v (in the case of using a unit in the above link) or would you base it on 230v? In the domestic environment there is likely to be very little difference but on an industrial scale with private supplies this could be quite substantial.

 

[stillp]

If the voltage was limited would you use 220v

I'd use the manufacturer's specified voltage, or the highest foreseeable voltage (as required by BS7671) and I wouldn't touch one of those voltage optimisation devices with a bargepole.

 

[holmslaw]

..

 

[stillp]

Voltage and kilowatts are directly proportional

Only when the resistance is constant. Lamp filaments and some heating elements have a positive temperature coefficient, so when cold they will draw a much higher current that when hot.
Until I have a chance to make some measurements I have no way of knowing if an immersion heater has a resistance that is constant with temperature or not.

 

[ricicle]

Voltage and kilowatts are directly proportional

Only when the resistance is constant. Lamp filaments and some heating elements have a positive temperature coefficient, so when cold they will draw a much higher current that when hot.
Until I have a chance to make some measurements I have no way of knowing if an immersion heater has a resistance that is constant with temperature or not.

The resistance of an immersion heater element is more or less constant over it's normal operating temperatures (ie when you measure it's cold resistance it is virtually the same as when it has been operating for a couple of hours.)
Filament lamps have a large PTC as the cold resistance of the filament is low, but does not draw the current that Ohm's law says it would because the resistance rises almost instantaneously when the lamp is switched on, although this rise is much slower in halogen lamps.

 

[holmslaw]

..

 

[stillp]

The resistance of an immersion heater element is more or less constant over it's normal operating temperatures

Thanks, that's what I've been meaning to measure.

the cold resistance of the filament is low, but does not draw the current that Ohm's law says it would

Oh yes it does, just not for very long.

 

[bernardgreen]

Voltage and kilowatts are directly proportional, so for your 240V 3Kw immersion heater the power will vary as shown below.

If directly proportional why have you squared the voltage

240
240 ×
19.2 ÷
--------------------------
3,000 Kw

Current is directly proportional to voltage, wattage is proportional to the square of the voltage.

 

[holmslaw]

..

 

[ajrobb]

directly proportional = if one increase so does the other and vice versa.

Directly proportional would require that they increase in proportion but they don't.

For a constant resistance, Mr Ohm stated that current and potential difference are directly proportional.