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What happens when 240 volt equipment is plugged into a 120 volt source or if 110-120 volt equipment is plugged into a 60 volt source assuming all alternating current?

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Assuming all alternating current and all at the same or similar frequency (say, 50-60Hz), nothing much will happen on a device using a linear transformer. A linear transformer (one that is made of core and coils) will produce half its nominal output voltage, which is unlikely to meet the requirements of the circuitry following that transformer.

With a switched-mode power supply, results will vary subject to the power supply design. The attached circuitry is unlikely to work within its specifications, but you may find that it works under light load.

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The short answer is that normally the power taken - and the resultant power output - would be only one quarter of the power at the equipment's rated voltage.

However some small electrical appliances - such as electric shavers, etc. - have been designed to run safely on different supply voltages whilst using and outputting almost the same amount of power. The appliance's rating plate will state the range of suitable voltages.

Such small, low-wattage, appliances or electronics items work in that way because they contain some built-in converter circuitry having semiconductors, transformers and resistors, etc. This is why you can plug such low-wattage appliances or electronics items into your wall and not have them "fry"... The built in converter circuitry converts the supply voltage to the right level.

A detailed answer
If you tried to use something that was designed for 240 volts on only 120 volts, or 110-120 volt equipment at only 60 volts, the result is not that you would get half the power output: you would get only a quarter!

That is because of Ohms Law, which says:

Amps = volts divided by resistance (ohms)

and the Power Law which says: Power (watts) = volts multiplied by amps

so it is also true to say:

Power = volts x ( volts / ohms ) or V2/ohms

If the appliance was designed to run on 240 volts, with a certain number of ohms of resistance, its power is given by the formula as:

Power = 240 x 240 / ohms = 57600 / ohms.

So, if the resistance of the appliance is 5 ohms its power consumption is 57600 / 5
= 11,520 Watts = 11.52 kilowatts

If we tried to use that same appliance on 120 volts, which is half of 240 volts:

The appliance's resistance would stay the same so the resulting power taken at 120 volts is:

= 120 x 120 / ohms = 14400 / 5 = 2880 watts

Another way to see it is: if V = 240 volts, and if you are plugging into only half of V: Power = (V/2)2 / ohms = (V2/4) / ohms
but as the appliance's resistance would still be the same, the resulting power = 1/4 x (240 x 240 / ohms) = 1/4 x 57600 / 5 = 2880 watts.

Anyway, however you calculate it, 2880 watts at 120 volts is only a quarter of the power which would be supplied at 240 volts, 11520 watts.

The story does not end here, however!

If you want to supply 11,520 watts of power at only 120 volts, the current required would be 11520 / 120 = 96 amps.

To allow such a current to be carried safely to the appliance without catching on fire would require extremely thick wiring to be used from the main breaker panel to the appliance. Also, it would not be possible to use any standard 120 volt breakers, plugs and socket outlets to carry such a current.

So no appliances for normal domestic home use are manufactured to operate at such high power outputs on 120 volts.

However, to supply a power of 11,520 watts at 240 volts, the current required would be 11520 / 240 = 48 amps. This is well within the normal specifications for the standard wiring sizes and outlets used for 240 volts supplies, to allow such a current to be carried safely to the appliance without catching on fire.

That is why high-powered appliances, such as clothes dryers and kitchen ranges, for normal domestic home use, are manufactured to operate at 240 volts.

For more information please see the answers to the Related Questions listed below.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.
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