energy usage

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gregwaits

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hi,
this may be a dumb question but i figured i would ask it..

if i run a device at 120 vac and it draws 10 amps...

would the same device draw 5 amps @ 240

or does it just split between legs at 240?

thanks

gw
 
In general you will always use the same amount of power (watts) to get the same amount of work done and the power company bills you based on watts not amps.

Imagine just a basic 100 watt lamp, I have seen them 12 volts to 250 volts, but they all used the same amount of power ...... 100 watts.

At 12 volts the 100 watt lamp would draw 8.3 amps, 100 watts divided by 12 volts results in 8.3 amps

At 120 volts the 100 watt lamp would draw .83 amps, 100 watts divided by 120 volts results in .83 amps

So regardless of the different voltage and current you still use 100 watts of power
 
gregwaits said:
hi,
this may be a dumb question but i figured i would ask it..

if i run a device at 120 vac and it draws 10 amps...

would the same device draw 5 amps @ 240

or does it just split between legs at 240?

thanks

gw
Click to expand...

Not always in the case of a resistive heater. If you took a 120 volt that pulled 10 amps and put 240 to it then it would draw 20 amps. It would however give the same heat at 5 amps 240 volt that it would 120v 10 amps. You did say "energy use" so then yes 120v at 10 amps is the same energy as 240 at 5
 
mikeames said:
Not always in the case of a resistive heater. If you took a 120 volt that pulled 10 amps and put 240 to it then it would draw 20 amps. It would however give the same heat at 5 amps 240 volt that it would 120v 10 amps. You did say "energy use" so then yes 120v at 10 amps is the same energy as 240 at 5
Click to expand...
This is a very good point that I not too long ago realized in a thread here. I can't remember what thread it was, but LarryFine and I exchanged posts and he made me realize what you just said, which in all of my years in the trade I never knew it or ran across it. But now I know, and knowing is half the battle.
 
090101-2322 EST

Going back to the original question in post #1.
---- if i run a device at 120 vac and it draws 10 amps...

would the same device draw 5 amps @ 240 ----
Click to expand...

If this device is a heater using nichrome wire and there is not a thermostat and blower motor, and it was designed for 120 V operation, then if you connect it to 240 V it will probably fairly quickly burn out. By saying "the same device" it implies that the resistance element is the same for the two different applied voltages.

A test heater I have is 10.6 ohms at 118 V, which is 118*118/10.6 = 1314 W. Assume the resistance does not change with 236 V applied, then the power dissipation is 4*1324 = 5254 W. This resistance element would get mighty hot with 236 applied.

Let's add to this heater a thermostatic controller using a phase shift circuit. This control monitors the temperature of the heater and maintains it at a constant value and thus the power input remains at 1314 W. Now the RMS input current remains constant to the resistor because the resistance has not changed because the heater temperature has not changed. This also means the RMS voltage across the resistor has remained constant at 118 V.

There will be more lost power in the 236 V source because of the higher line current than if the the heater resistance had been changed to 42.39 ohms which is the value to produce 1314 W with 236 V applied to the resistor and a full sine wave.

I have assumed sine waves except for the phase shift temperature controller. In that case the waveforms are a trailing portion of a half sine wave.

.
 
mikeames said:
Not always in the case of a resistive heater. If you took a 120 volt that pulled 10 amps and put 240 to it then it would draw 20 amps. It would however give the same heat at 5 amps 240 volt that it would 120v 10 amps. You did say "energy use" so then yes 120v at 10 amps is the same energy as 240 at 5
Click to expand...
Conversely, if you took a 240V 4800W construction heater (ignoring the fan motor at the moment)...

4800W / 240V = 20A
240V / 20A = 12 ohms

...and connected it to 120V:

120V / 12 ohms = 10A
120V x 10A = 1200W

You wind up with a functioning heater that consumes 1/4 the power. (without the messy fire that would follow usage of a 120V heater at 240V) Of course this neglects the fan motor, so don't try this at home...
 
It's easy to forget there's a big difference between simply applying different voltages to a given impedance, and designing a given piece of equipment to run on different voltages by altering its impedance.

To answer the OP directly, the "same device" would use twice the current at twice the voltage, resulting in a quadrupling of power.
 
mikeames said:
Not always in the case of a resistive heater. If you took a 120 volt that pulled 10 amps and put 240 to it then it would draw 20 amps. It would however give the same heat at 5 amps 240 volt that it would 120v 10 amps. You did say "energy use" so then yes 120v at 10 amps is the same energy as 240 at 5
Click to expand...

Mike, I live in a real world, I do not connect 120 equipment to 240, but thanks for adding confusion to the thread.:D
 
Rampage_Rick said:
Conversely, if you took a 240V 4800W construction heater (ignoring the fan motor at the moment)...

4800W / 240V = 20A
240V / 20A = 12 ohms

...and connected it to 120V:

120V / 12 ohms = 10A
120V x 10A = 1200W

You wind up with a functioning heater that consumes 1/4 the power. (without the messy fire that would follow usage of a 120V heater at 240V) Of course this neglects the fan motor, so don't try this at home...
Click to expand...

Its like getting back to running your water heater on a small generator using 120 volts instead of 240. Takes a while to heat up but a warm shower is better than a cold shower.
 
There are "constant power" devices, motors generally being in this category. Double the voltage, and you will reduce the current by half, and the power will remain the same (not a surprise, since we are talking about constant power devices).

There are "constant impedance" devices, resistance heaters and incandescent lights being in this category. Double the voltage, and you will double the current, and the power drawn will go up by a factor of four.

Double the voltage on any device that is not rated for the higher voltage, and you may very well destroy it.

There are no dumb questions, except perhaps the ones not asked.
 
charlie b said:
There are "constant power" devices, motors generally being in this category. Double the voltage, and you will reduce the current by half, and the power will remain the same (not a surprise, since we are talking about constant power devices).

There are "constant impedance" devices, resistance heaters and incandescent lights being in this category. Double the voltage, and you will double the current, and the power drawn will go up by a factor of four.

Double the voltage on any device that is not rated for the higher voltage, and you may very well destroy it.

There are no dumb questions, except perhaps the ones not asked.
Click to expand...

Excellent explanation Charlie.

So back to the OP, if the device were rated to operated at the same output on either voltage there would be no real difference (except maybe for voltage drop) from using 120 or 240 volts.
 
A bit tangential to the original post but nonetheless...noobie question..

Will the energymeter in a 3 phase supply ( for simplicity phase to phase is 208 volts and phase-N 120 volts) will read same units consumed in one hour,

a) if it has six 100 watt bulbs of 120 volts on one leg ( say A phase -N)

b) If it has two 100 watt of 208 volts ( total six) on each phase ( ie A-B B-C C-A)

When a three phase energy meter records energy consumed,.. does it average out the three phases?

In case "a" all the load ( ie 600 watts) is on one leg where as it is balanced in case "b".

cheers.
 
chaterpilar said:
Will the energymeter in a 3 phase supply . . . read same units consumed in one hour. . . .
Click to expand...
I don't know how energy meters work. But the two configurations you described will, in fact, consume the same amount of energy during any given hour. That is because your two loads are the same amount of watts, with a watt being the rate of using energy.

Please note, however, that the two cases will have significantly different currents in the various wires. I'll leave that calculation to you, as a "homework assignment." ;):grin:
 
090103-1009 EST

infinity and charlie b:

Slight correction on lamps. A tungsten filament lamp is not a constant resistance device relative to voltage.

A sample 75 W lamp measured:
106 ohms @ 30 V and 0.28 A
142 ohms @ 60 V and 0.42 A
169 ohms @ 90 V and 0.53 A
193 ohms @ 120 V and 0.62 A
205 ohms @ 140 V and 0.68 A

The current changed by a ratio of 2.43 vs 4.67 for the voltage.

If the device is two resistors of equal resistance and includes a switch to change from series to parallel connection, then by changing the switch position it can be a dual voltage device and produce the same power output for either input voltage. The RMS current changes by a factor of two between the switch positions. There is no distortion of the sine waves.

If the device is an automatic adjuster for input voltage, then within design limits the device can produce constant output power for different input voltages.

One illustration is a servo motor driven Variac to maintain constant output voltage and no distortion of the sine wave. This can produce equivalent results to the switched resistors. The input RMS current will decrease as the input voltage increases.

Another illustration is the phase shift controlled current limiter I described in post #6. In this case the input RMS current remains constant as the input voltage increases.

Many switching regulator power supplies today are designed to operate from input voltages of about 90 V AC to 270 V AC, and produce a constant output voltage.

.
 
090103-1054 EST

chaterpilar:

KWH meters are designed to measure energy used, whether balanced or unbalanced. There may be some assumptions made in the meter design, but they come close to doing energy measurement.

I believe that the typical rotating disk single phase residential KWH meter uses a single voltage coil connected across the two hot lines. As an approximation this is really just a scaling factor and is part of the inherent calibration of the meter.

.
 
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