Lights dimming


I need to be lead gently (but not too gently through this one!).

In my home in California (110V) why is it that (for example) a light will noticeably dim when a heavy duty appliance is turned on on the same circuit, but not when it is turned on on another circuit?

Why didn't I see this effect when I lived in the UK (240V)?



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ckt1 A============Bappliance====lite

ckt2 A======================Blite There will be a voltage drop on the conductors on ckt1 from point A to point B when the appliance turns on. ckt2 conductors will not experience a voltage drop when the appliance turns on, since they are not connected to the appliance. Therefore, when you plug the lite into ckt2 it does not dim like it does when plugged into ckt1

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OK thanks for this ... but why wasn't the same effect visible on 240V ... is it an effect that is proportional to the voltage or square of the voltage?

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Sort of, square of the current actually, all other things being equal. It's probably actually worse than this as all other things aren't equal (in particular conductor crossectional area for equivalent voltage drop losses would need to be 4 times those in the UK for same power circuit). See a recent thread where exactly this point was discussed.

Filamant lamps themselves accentuate voltage fluctuations in their light output -- you might find retrofit compact fluorescents and regular fluorescents with electronic control gear are better at masking the voltage fluctuations.

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Andrew Gabriel

| OK thanks for this ... but why wasn't the same effect visible on 240V ... is | it an effect that is proportional to the voltage or square of the voltage?

UK circuits are typically higher amperage, such as 30 amps, and have a correspondingly higher amperage wire, less you plug in too many things on several outlets and overload it. US circuits are typically lower amperage, such as 15 or 20 amps, and have a smaller wire, thus more resistance.

For a given amount of power, a heavy duty appliance at 120 volts will draw twice the current as the same appliance at 240 volts.

So lets say some appliance needs 1200 watts, but draws 12000 watts just briefly to start up. In the UK that would be 5 amps and 50 amps on a

30 amp circuit. In the US that would be 10 amps and 100 amps on a 15 amp (typically) circuit.

Also the "other circuit" where it is not seen could be on the other side of the split phase wiring used in the US (and not in the UK). You get

240 volts between opposite ends of the transformer winding, but only 120 volts between either and the neutral (grounded) wire from the center. And that 120 volts is what is typically used, but the full 240 volts is available on circuits wired for it.

Heavier appliances tend to be wired up to their own circuit and also tend to be wired up to 240 volts instead of 120 volts (so they use only half the current, same as in UK).

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