120 vs 240 1 Phase efficiency

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Curtlre

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I have been an electrician for 38 years and am retired now. I am totally second guessing myself and need some input. To me watts are watts weather a load is 120 volts at 10 amps or the load is 240 volt 1 ph at 5 amp you are consuming 1200 watts and the efficiency of the equipment is the same. Seems very simple to me but again I am second guessing myself since a neighbor's pool guy him it's more efficient to run his pool pump and heater at 240. Thanks in advance for your help.
 
Because voltage drop is only dependent on current. Raise your numbers a bit. Try 60a at 120v vs 30a at 240v.

Insulation is cheaper than conductor. Which would you rather (pay to) wire for?
 
Thank you for your reply. Yes I do understand your possible scenario with the conductor size difference. Since our distance is not an issue and either way we would be running #12 AWG so the install material and labor should be the same and efficiency KWH consumption should also be the same at either operating voltage I believe.
 
Within the load itself, theoretically, there's no difference; externally, there is. If "efficiency" includes getting as much of power out of the load as you can for the electricity you're paying for, the higher voltage wins hands down.

With an all-other-things-being-equal scenario (which is what a theoretical discussion is) the higher voltage has half the voltage drop, so more of the power consumed is transferred to the load, and less toward heating the wire.
 
There's an I x I x R, (I squared x R) power loss for the resistance R of the wire. I is half at double the voltage for single phase.

The guy running his pool pump and heater at 240 V is correct, it is marginally more efficient. The heater does not care any about the voltage drop on the run, it will just run longer on the aquastat. The motor does care about the increased Voltage drop (resulting in both marginally reduced torque and marginally reduced motor efficiency, at the less than nameplate Voltage.

The wire does care a lot about the 2 x I current. The increased power loss is I x I x R.
 
Here is a second order effect for you, the operation during starting at 240 V for typical capstart 120/240 V motors is more efficient at 240 V.
Why? 90+% of 120/240V capacitor start motors are wound with only ONE start winding, meaning at 120V the start winding is only at half voltage so starting time is longer with longer inrush current, thus more power loss during acceleration.

Take a common motor like a 120/240 V CS motor on say a radial arm saw. Put on a heavy blade. Connect for 120Voperation and the;n 240 V operation - very obvious that time to speed is about 4x at 120V than 240 V.

During run as others have said, less volts drop of feed line at 240 V, so motor closer to full design voltage.
 
If the conductors are sized properly, the efficiency differences of the system will be negligible between the two operating voltages. I think that’s what the OP is getting at. As others have stated, there are other considerations that favor operating at the higher voltage.

It’s surprising how many people connected to the industry believe there is a huge difference in operating costs between 120 and 240V equipment. I had a senior plant electrician argue once that “a 110 air conditioner costs twice as much to run as a 220”.
 
As others have mentioned, you must first define how the word efficiency is being used.

My guess is the plumber is simply equating current to power. As those schooled in KW and KWH calculations know, that is not correct.

However, for pool and spa heaters there can be a big difference in actual power consumed versus the voltage the system is configured. At 240V my spa is wired with a 50A circuit, at 120V it only requires 20A. The biggest difference is the wattage of the heaters and therefore how long they need to run to maintain temperature.
 
I agree with the above.

For a motor, watts is watts. The motor itself is essentially a rotating transformer, and if the winding is appropriately adjusted for the different voltages will have essentially the same efficiency at those different voltages.
It is very common to have motors designed for either 120 or 240 V depending on terminal connections.

Where higher voltage is more efficient is in the wiring feeding the loads.

Jon
 
Curtlre said:
Seems very simple to me but again I am second guessing myself since a neighbor's pool guy him it's more efficient to run his pool pump and heater at 240.
Click to expand...
The last electric pool heater I wired was a heatpump type unit and only ran at 240 and I think took a 30 or 40 amp.
1200 Watts sounds like the pump only.
What does electricity cost in your area? 20 cents per kWH?
As others have said there is 12XR losses;
If you figure the pump runs what 6 months a year 5 hours a day? = 900 hours a year?
And say your #12 conductor is what an average of 100 foot run max to the pool panel?
So say the resistance+reactance of the run is 0.1984 Ohms and we ignore powerfactor for a moment I come up with 18 cents a year difference:


VA​
1200​
1200​
Volts​
120​
240​
Amps​
10​
5​
12XR​
1.984​
0.992​
Watt hours per year​
1785.6​
892.8​
kWh per year​
1.7856​
0.8928​
.20 Cents per kWH​
0.36​
0.18​

If it was a continuous load, or longer run on smaller wire it might start to make a difference.
Cheers
 
Tortuga, do your numbers take into account the current increase caused by the reduced voltage at the motor terminals, which causes more voltage drop, which causes lower voltage, etc., until a new equilibrium is reached? :unsure:
 
tortuga said:
The last electric pool heater I wired was a heatpump type unit and only ran at 240 and I think took a 30 or 40 amp.
1200 Watts sounds like the pump only.
What does electricity cost in your area? 20 cents per kWH?
As others have said there is 12XR losses;
If you figure the pump runs what 6 months a year 5 hours a day? = 900 hours a year?
And say your #12 conductor is what an average of 100 foot run max to the pool panel?
So say the resistance+reactance of the run is 0.1984 Ohms and we ignore powerfactor for a moment I come up with 18 cents a year difference:


VA​
1200​
1200​
Volts​
120​
240​
Amps​
10​
5​
12XR​
1.984​
0.992​
Watt hours per year​
1785.6​
892.8​
kWh per year​
1.7856​
0.8928​
.20 Cents per kWH​
0.36​
0.18​

If it was a continuous load, or longer run on smaller wire it might start to make a difference.
Cheers
Click to expand...
The kWh per year is the same for both, not twice or double.
 
Right its trivial, unless you have a load thats running a lot of hours, and its a longer run, and you have a big facility, like parking lot lighting.
Over 20 years you'd make up the cost of the 2 pole breaker :ROFLMAO:
LarryFine said:
Tortuga, do your numbers take into account the current increase caused by the reduced voltage at the motor terminals, which causes more voltage drop, which causes lower voltage, etc., until a new equilibrium is reached? :unsure:
Click to expand...
No LOL double check my numbers, I just went off tables in chapter 9.

0.054​
Reactance Total (Z) per 1000 FT​
1.93​
Dc Resistance (X) per 1000 FT​
1.931​
Impedance SQRT[(Z*Z)+(X*X)] Ohms​
0.0019​
Ohms Per Ft (divide by 1000)​
100​
Round trip Distance FT​
0.1931​
Resistance Of Run Ohms​
​
​
​
​
900​
Load Hours Per Year​
1.0​
PF​
1200​
VA​
 
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