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I've always thought P = U * I (units watt, volt and ampere, respectively), but I ran across this discussion on a forum:

If you look in your panel there are 2 “hot wires coming in from the pole”. 1 is the 120v A phase and 1 is the 120v B phase. you need a hot leg from each one to make 240V. if you have a machine drawing 20 amps on 120V it is drawing it from 1 phase. If you have the same machine drawing 10 amps on 240V it is drawing 10 from the A phase and 10 from the B phase.

The easiest motor for me to read the name plate on is a 1.5 HP 3450 rpm Baldor motor. It was on the shelf. It states at 115V it draws 13.2 amps. next it states 230V it draws 6.6 amps.

“amps x volts = watts”

13.2×115 = 1518

6.6 x 230 = 1518

no matter what you do it will draw the same amperage total.

To me this explanation contradicts itself since it says that the motor draws both the same current and the same power. Is the motor actually drawing 6.6 A from each hot wire (for a total of 13.2 A), or is it drawing 3.3 A from each hot wire? If it's 6.6 A from each hot wire, it seems the classic "current x voltage = power" equation is misleading for 240 V "single-phase" circuits.

Update: to clarify, this question applies to 120 V and 240 V current in the US.

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Don't think of current as being "drawn from" a wire when working out what the current is through a system. Rather, imagine putting a mark on any wire and then ask the question "on average, how much charge per second is moving past this mark?" –  Eric Lippert May 2 at 20:39
    
@EricLippert I see you've gotten bored with C# questions :-) –  Jonathon Reinhart May 3 at 7:24
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5 Answers 5

up vote 12 down vote accepted

A 240 volt motor will only be connected to the two live wires, not to neutral, so your example draws 6.6 amps at 240 volts. If it is connected to operate at 120 volts, it will be connected between one live wire and neutral, and will draw 13.2 amps at 120 volts.

I think it is misleading to say that, in the 240 volt case, the motor draws 6.6 amps from each live wire. A more correct description would be that it draws 6.6 amps from one live wire, and returns that 6.6 amps through the other live wire.

Using the same terminology, in the 120 volt case, the motor will draw 13.2 amps from the live wire, and return that 13.2 amps through the neutral wire.

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I would suspect that you will be metered on both phases and the current you are charged for is the sum of currents in the hot wires. So although the physics says it's the same 6.6A current coming in one phase and going out the other on one circuit across 240V, the accountants charge for it twice.

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Aha! Maybe that's what he was getting at in his explanation. –  rob May 2 at 16:04
    
One question though--when you do the math, the accountants are still charging by the number of watt-hours, correct? So whether you look at it as 240V*6.6A or 2*(120V*6.6A), I'll still pay the same amount if I run the machine for 1 hour, right? Or is there some weird accounting magic that means I'll be paying for 2*(240V*6.6A)? –  rob May 2 at 16:36
    
@rob It should be the same number of Wh that is charged for. –  Pete Kirkham May 2 at 21:10
    
You will even save some power when using it on 240V since there will be less power loss in the conductors. (although 6.6A is not such a large load, so the effect will be marginal) –  sebleblanc May 3 at 4:36
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It is written in a confusing way, but well. The one phase motor has only one phase from which something is drawn, the other one is bound to ground. You use a one phase motor because you don't have three phases. That means using phase-phase voltage on it wouldn't make sense, you should be using a three phase motor instead.

The 120V vs. 240V is not related to phase to phase voltage(or live wire to live wire) as that would be \$120\sqrt(3)\approx 207V\$. It is a refference to the outlet voltage for different countries. You have:

  • 120V 60Hz (for example US)
  • 240V 60Hz (for example Brazil)
  • 240V 50Hz (for example Europe)

The other ratings you read were for people in other regions of the world that want to use the same motor.

You need a bit more of data in the question, and clarify it so that I really know what you are talking about. I'm now doing my best guess at what you really wanted to ask.

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Sorry, I should have mentioned both voltage options are for the US. Although US standard household current is 120V, we can also get 240V for commercial applications and machinery. –  rob May 2 at 16:00
    
Well I didn't know that. Nice to know, I mean it now makes sense that the voltages are 120/240 because they are from a single transformer. I see you already have a relevant answer from Peter Bennett. Good luck with it. –  Kurtovic May 2 at 16:09
    
Instead of "US", say "North America" - Canada uses the same electrical standards as the US. In homes, 240 volts will be used for high power loads like an electric stove or clothes dryer. –  Peter Bennett May 2 at 17:06
    
@PeterBennett Doesn't matter in this case, but industrial voltages are higher in Canada (600VAC vs. 480VAC 3-phase 60Hz). Commercial 208 = 120 * sqrt(3) is the same. –  Spehro Pefhany May 2 at 19:26
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Is the motor actually drawing 6.6A from each hot wire (for a total of 13.2A), or is it drawing 3.3A from each hot wire?

A phrase like "drawing from each hot wire" is unclear. For the 120V circuit, there is one hot wire and one neutral wire and the motor is between the two. The current is the same in both the hot and neutral.

For the 240V circuit, there are two (out of phase) hot wires and the motor is between the two. The current is the same in both hot wires.

Just as you do not add the currents in the hot and neutral for the 120V case, you do not add the currents in the two hots for the 240V case.

Perhaps a drawing will help.

schematic

simulate this circuit – Schematic created using CircuitLab

In the left hand circuit, the load "draws" current from 1 hot wire and the current "returns" through the neutral.

In the right hand circuit, the load "draws" current from 1 hot wire and the current "returns" through the other hot wire.

Assuming the power is the same for both loads above, the current in the 240V circuit is half that of the 120V circuit.

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You can think of it this way: the motor is using the same amount of power regardless of the voltage. From your formula, you know that since the voltage doubled, the amount of current flowing through the wires and motor must have halved, since the power is the same.

This is also the reason devices that are switchable for 120/240 volts use a fuse with half the current rating of the 120 volt setting when set for 240 volts.

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