120VAC vs 240VAC: Energy cost comparison.

[GoldDigger]

THANK YOU! This makes sense.

PeterJ

Glad to be of assistance. Pass it along someday.

 

[templdl]

THANK YOU! This makes sense.

PeterJ

The only advantage that I think of is starting a motor. I am inclined to believe that there is less of a voltage drop when starting a 240v motor than 120v. Because of the lower amperes of the 240v motor the 240v motor is more likely to excellerate faster while under the same load and the voltage would be stiffer for 240v.
As such there would be a negligable but it is of my opinion that a 240v motor would provide somewhat better performance based upon lower line losses due to voltage drop.

 

[GoldDigger]

The only advantage that I think of is starting a motor. I am inclined to believe that there is less of a voltage drop when starting a 240v motor than 120v.

True, for the same power motor. The absolute voltage drop would be 1/2 and the percentage voltage drop, which is more important to performance, would be only 1/4 as large.

 

[Smart $]

No, I'm not trying to steal from my electric company, I just want to understand this. I've been told by many instructors over the years that 240VAC is cheaper to operate than 120VAC. ...

Your instructors' statements have little or nothing to do with the meter... though that's where the difference will present itself.

Typically such statement is based on the same load on the same size wire and I?R losses... i.e. the heat generated by the wiring as a result of its resistance. That amounts to energy consumed. It may not be a loss inside a heated building during cold weather, but presents a double whammy when using energy to cool the same area where the wiring heat is dissipated.

That said, there is bit of fallacy to the instructors' claim because we typically adjust wire size (thus its resistance, and thus the I?R loss) for equivalent loads at different voltages.

 

[kwired]

The only advantage that I think of is starting a motor. I am inclined to believe that there is less of a voltage drop when starting a 240v motor than 120v. Because of the lower amperes of the 240v motor the 240v motor is more likely to excellerate faster while under the same load and the voltage would be stiffer for 240v.
As such there would be a negligable but it is of my opinion that a 240v motor would provide somewhat better performance based upon lower line losses due to voltage drop.

Would entirely depend on if there is a (significant enough) voltage drop in the first place. It is still going to take the same amount of power to accelerate the load, the only difference would be what losses actually occur. If power supply is stout enough the voltage drop may be negligible on both a 120 and a 240 volt motor of same output rating. But even so, just how much loss are we talking about during the short time likely involved here? Now if that motor is started pretty frequently it may start to become something that will be noticeably different between the two scenarios.

 

[kwired]

Sticking with single phase 120/240 as three phase metering gets more complex - some meters don't actually see 240 volts they have a single pick up coil with each line passing through from opposite directions. Everything is 120 volts to this kind of meter and even though a 240 volt load draws half the current of the same load at 120 volts - it actually passes through the pick up coil twice - which would double the seen 120 volt values.

 

[GoldDigger]

That said, there is bit of fallacy to the instructors' claim because we typically adjust wire size (thus its resistance, and thus the I?R loss) for equivalent loads at different voltages.

The one clear economy possible is in the cost of wire for the initial installation. Roughly a factor of two in copper weight. Not so much of an effect on operating costs, as you state.

 

[Smart $]

The one clear economy possible is in the cost of wire for the initial installation. Roughly a factor of two in copper weight. Not so much of an effect on operating costs, as you state.

Yes, initial costs would differ. My post was only meant to address...

PeterJ][/COLOR]I've been told by many instructors over the years that 240VAC is cheaper to operate than 120VAC.

 

[templdl]

Would entirely depend on if there is a (significant enough) voltage drop in the first place. It is still going to take the same amount of power to accelerate the load, the only difference would be what losses actually occur. If power supply is stout enough the voltage drop may be negligible on both a 120 and a 240 volt motor of same output rating. But even so, just how much loss are we talking about during the short time likely involved here? Now if that motor is started pretty frequently it may start to become something that will be noticeably different between the two scenarios.

The problem with this as well as other discussion is that they are bases upon opinion as is mine. To provide a reply that would have credibility would have to take into every item from the point where there power is generated to the motor when is impractical at best. But considering the POCO xfmr service drop and the branch circuit conductor with consideration that a 240v motor requires 1\2 the current as a 120v motor the wire size supplying either would have to be considered for voltage drop. It is of my opinion that a 240v motor would a marginally greater torque performance as a result of less voltage drop. But saving money it would not be measurable.

 

[kwired]

The problem with this as well as other discussion is that they are bases upon opinion as is mine. To provide a reply that would have credibility would have to take into every item from the point where there power is generated to the motor when is impractical at best. But considering the POCO xfmr service drop and the branch circuit conductor with consideration that a 240v motor requires 1\2 the current as a 120v motor the wire size supplying either would have to be considered for voltage drop. It is of my opinion that a 240v motor would a marginally greater torque performance as a result of less voltage drop. But saving money it would not be measurable.

But there would be a considerable difference in the losses if you were comparing a 5 hp motor to a fractional hp motor at both voltages, then you can also throw in compensation for voltage drop by increasing conductor size bringing the performance of lower voltage closer to the performance of the higher voltage. The shorter the circuit length is the closer the performance of either may become for light to moderate loads. Also a load that takes time to accelerate will have increased starting current at either voltage as compared to one that accelerates easily. I run into fans on grain storage facilities that have pretty heavy propellers and it may take 10 seconds to accelerate to full speed, where a conveyor motor reaches full speed nearly instantly (from a human perspective anyway).

 

[templdl]

But there would be a considerable difference in the losses if you were comparing a 5 hp motor to a fractional hp motor at both voltages, then you can also throw in compensation for voltage drop by increasing conductor size bringing the performance of lower voltage closer to the performance of the higher voltage. The shorter the circuit length is the closer the performance of either may become for light to moderate loads. Also a load that takes time to accelerate will have increased starting current at either voltage as compared to one that accelerates easily. I run into fans on grain storage facilities that have pretty heavy propellers and it may take 10 seconds to accelerate to full speed, where a conveyor motor reaches full speed nearly instantly (from a human perspective anyway).

All things being equal I agree that there would be no noticeable difference. But I would suggest that there would be less of a voltage drop because there is 1/2 the current at 240v which would be benificial during seating. Of course this would be dependent upon the conductor size selected based upon the motor HP. You may be reasonably limited to the selection of #10, #12 or #14 where you may start with #10 required for @ 120v motor and are able to use #12 or #14 for a 240v motor. If the motor was 20fla@120v it would be 10a @ 240v where one would be #10 and the other #14.
As such Any benefit of 240 over120 is a guess at best.
I still would be inclined to opt for 240v because of the available of better torque under load due to the possibility of less voltage drop under load.

 

[broadgage]

The utility meter measures kilowat hours delivered.
It does not "know" if the load is all one 120 leg, spread between both hots, or is a straight 240 volt load. KWH is what it measures.

With typical household wiring there might be a slight gain by the use of a load balanced between the two hots as this will reduce or eliminate the losses in the neutral.
Consider an example

1) a 240 volt , 2.4KW heater that uses 10 amps, on a 20 amp branch circuit with 3% voltage drop at full load. Since the circuit is 50% loaded the loss will be 1.5 % or 36 watts.

2) a pair of 120 volt , 1.2KW heaters used on the same* 20 amp branch circuit with 3% voltage drop at full load. Since the circuit is fully loaded, the losses will be 3% or 72 watts.

3) A pair of 120 volt, 1.2KW heaters each used on its own 20 amp branch circuit that was designed for 3% voltage drop at full load. As each circuit is 50% loaded, there will 1.5% loss in each circuit, or 18 watts loss per circuit, or 36 watts in total.

It may therefore be seen that in some cases nothing is saved by use of 240 volts, and that IF there is a saving, that it will be very small.
And presuming that the wiring is internal to the structure being heated, then the losses are not really lost at all as the warmth from the wires is added to the output of the heaters.

* one should not really load a 20 amp branch circuit to 20 amps continually, as is likely with heaters. However in the real world, in cold weather it does happen.