Err, we are talking single phase here, the 120V legs will not cancel each other out. They are at the same phase position, they cannot be different if there is only one supply phase, one would need to be the inverse of the other at the same moment in time which simply does not happen. If you take a volt meter and measure across the two hot legs you get 240, if you measure neutral to hot leg, you get 120V, regardless of what leg you test.



The only time you start canceling out voltages is when you start working with rectifiers, which would be entering the realm of DC.



But yes, your dryer and electric stoves will (most of the time) have 2 poles, neutral, and ground. Pole to pole for your heating elements (less current) and pole to neutral for controls. Ground for safety. If you only require 240V then you can tap off of the two poles and ground.

 

Trust me the two opposite phases of a 240V circuit most certainly cancel each other out. Just touch them together and see what happens. (don't really do that)

 

<------ notices we're heading right to the crapper, decides to remind everybody:



Try to keep it on topic - or at least/especially don't advise dangerous things, mmmkay?

 

I'd be glad to if the post I answered to wasn't completely wrong.



240V single phase is two 120v legs that are 180* out of phase. When one is 120v+ the other is 120v-. If the device is using the 240V there is no need for a neutral. It is only when one leg is used that a neutral is needed to ground the 120v+ to 0v.



Here's a pic of the 120V cycle. It goes + to - 60 times a second or at 60Hz.







On your transformer out side you have three wires coming in. One leg coming off the peak at +v and one leg coming off the opposite peak at - v; the third wire is the neutral which at the transformer is also tied to a ground to earth. These will read 240V when a voltmeter is placed across both legs. But only 120V when a voltmeter is placed across one leg to ground.

 

Please excuse me for noticing that ^ is a single phase 120v waveform. For the (center-tapped 2x 120V) 240v system in the USA's residential service I would expect to see THIS at the output of the transformer: 2x 120V, 180ยบ out of phase, adding at the point of use to 240 but requiring much less copper to get the power delivered . . .







This is why you hear people talk about "single phase" 230/240V as having "two hots and a ground" - there are two wires carrying voltage to the load, and the voltage is added by the load and clever wiring. You have a straight 230V 1phase motor, you need 2 hots. Throw in the neutral or ground wire (pick one at the outlet) for safety, tie it to the motor's metal frame. Done.



Here is yet another link saying similar things a different way. To the point, it answers the OP's question the long way 'round without me having to type it all out.

 

LOL, I just realized that we are basically thinking about the same thing, but just practical application vs theoretical application. I don't want to start a war, I just want to make sure we are all on the same page, because I wasn't! Yes, you both are right! As per the diagram, the X1 is - polarity and so is X3. Effectively voltages measured from X1 and X4 to the center tap will be 180 degrees out of phase.



I'll use this diagram as a visual aid, to myself and others lol.







Now hear me out, when I say practical I'm thinking from a transformer construction point of view. From my perspective, I see a 240V secondary winding with a center tap. That 240V is a result of stepping the primary voltage down by way of the transformer turns ratio. If you tap a point at the center of this secondary winding you effectively create two windings at double the turns ratio, meaning you will now measure 120V instead of 240V.



I get what you guys are saying, I'm just used to the practical side so when I hear PHASE I think of RWB, so when I hear someone say you have two phases in a single phase system my brain does cartwheels! When I read the posts I realized you two were talking about POLARITY, and thus is makes sense to me. Sorry for the confusion! FWIW, I enjoyed this conversation, as I haven't put my theory to test in a while, so thanks! As I am typing this I am mulling through my old textbooks and thinking to myself "Ah, I see what they are talking about!"



Sorry for the thread jack, total misunderstanding. At least we were still slightly on topic!

 

Now bump the voltage up to 277 single phase, add a third waveform so they are all 120* out of phase (360*/3) and you will 480V 3 phase.

 

Indeed! Sometimes I find 3phase easier to understand for some reason, maybe because its not split phase, and just phase to phase and phase to neutral, no center taps.



For the most part we only use 120/240 single phase, and 120/208, 347/600 three phase in Ontario. I rarely if ever see 277/480 or delta services. We have applications for other voltages, but we are trying to stay away from providing delta at any other voltage level other than what would be considered primary. I think its all part of standardization. The only time I have ever seen delta provided nowadays is with a 44kV direct feed.