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I=E/R
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This is just to respond to WillK and is not directly part of this thread.
So for 240 appliances they use both hots and the potential difference between the two is 120 + 120, so it's equivalent to single phase 240 volts.
Yes, in a sense, it is 120 VAC 2 phase power. The problem is that nothing uses 2 phase 120V AC. Show me a single device that requires two 120 volt connections that are 180 deg out of phase and requires a common.
If you eliminate the common you loose the difference in phase angle between the two hot leads.
 

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Scared Electrician
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remember that a neutral is not a zero. It is a center tap. The only reason we talk about it being a zero reference point is because we ground it. If allowed to float it would have 120v, and it still does in theory, but be cause it is bonded to earth, there is no difference in voltage between and grounded metal object, therefore we say its a zero volts.
 

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Semi-Pro Electro-Geek
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This is just to respond to WillK and is not directly part of this thread.
Yes, in a sense, it is 120 VAC 2 phase power. The problem is that nothing uses 2 phase 120V AC. Show me a single device that requires two 120 volt connections that are 180 deg out of phase and requires a common.
If you eliminate the common you loose the difference in phase angle between the two hot leads.
Most ranges and dryers do. They have both 240V and 120V loads internally. This is a 120/240V connection - two hots, neutral, and ground. This is NOT the same thing as two phase power. Two phase does exist and was used in some very early systems. It consists of either two or four hots with 90-degree phase angles.

http://en.wikipedia.org/wiki/Two-phase_electric_power

The U.S. residential power system is "split phase" - a single phase source with a center tap. The real test is this: can you derive the power from a single phase connection using a transformer, or not? If you can, then it is single phase. If not, then it's polyphase. 120/240V split phase power is derived from a transformer with a single-phase primary and a center-tapped secondary. Thus, it's single phase.
 

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Actually, I didn't expect to come out of this any less confused about it. To my pleasant surprise, I think it actually makes more sense to me now. Thanks!
 

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I want to change the plugs to 50 amp plugs and also change the breakers to 50 amp - I read somewhere that as long as I marked the outlets "For Welder Use Only" it was acceptable by code. Is this ok?
You can use a breaker that is larger than the normal breaker for a specific wire size for a welder that has a limited duty cycle. The outlet does have to be labeled "For Welder Only". See NEC 630 for additional details.

You should calculate the current at max output AND the duty cycle at that output in order to decide if your wire is large enough to run the welder at full output. See NEC 630.11 (A). Due to the difference in the design of welders, it would be best to ask the manufacturer for the correct breaker size. As stated above the breaker size can be larger than the normal breaker for a specific size wire, but you must see NEC section 630 for the calculations for duty cycle and breaker size limitations. AND THE OUTLET CAN ONLY BE USED FOR THE WELDER IT WAS DESIGNED FOR.
 

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I=E/R
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Most ranges and dryers do. They have both 240V and 120V loads internally. This is a 120/240V connection - two hots, neutral, and ground. This is NOT the same thing as two phase power. Two phase does exist and was used in some very early systems. It consists of either two or four hots with 90-degree phase angles.

http://en.wikipedia.org/wiki/Two-phase_electric_power

The U.S. residential power system is "split phase" - a single phase source with a center tap. The real test is this: can you derive the power from a single phase connection using a transformer, or not? If you can, then it is single phase. If not, then it's polyphase. 120/240V split phase power is derived from a transformer with a single-phase primary and a center-tapped secondary. Thus, it's single phase.
A dryer or electric range is not a single device. These units have motors, elements, clocks, etc. none of which individually require the two hots and the neutral to operate. In theory it can be looked at as 120 two phase but in application we only use it single phase.
 
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