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09-21-2011, 11:00 AM   #1
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## Just a general question on 220VAC

I was just wondering how 220VAC works. This is like the circuit to an electric range or air conditioner (not 3 phase). I understand that this is essentially two 110 VAC wires. Are these out of phase by 180 degrees or what?

09-21-2011, 11:02 AM   #2
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Quote:
 Originally Posted by garya505 I was just wondering how 220VAC works. This is like the circuit to an electric range or air conditioner (not 3 phase). I understand that this is essentially two 110 VAC wires. Are these out of phase by 180 degrees or what?
The two hot wires from the two respective sides of your 120/240 volt service work together as a pure 240 volt power source. Each of those hot wires relative to the neutral is 180 degrees out of phase from the other. If we take a moment in time when the alternating current is at plus 120 relative to neutral for one of the two hot wires, the other will be at minus 120 volts relative to neutral.

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Last edited by AllanJ; 09-21-2011 at 11:05 AM.

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09-21-2011, 11:13 AM   #3
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Quote:
 Originally Posted by AllanJ The two hot wires from the two respective sides of your 120/240 volt service work together as a pure 240 volt power source. Each of those hot wires relative to the neutral is 180 degrees out of phase from the other. If we take a moment in time when the alternating current is at plus 120 relative to neutral for one of the two hot wires, the other will be at minus 120 volts relative to neutral.
Ah, that's what I was thinking. The reason for my question is that I noticed there is no neutral on the connection to my AC condenser, just 2 hot wires for power and 1 ground. Since I assumed that the ground is just there for safety, the 2 hot wires would have to be out of phase by 180 degrees for this to work.

 09-21-2011, 11:17 AM #4 Member     Join Date: May 2009 Location: Easton MD Posts: 1,893 Rewards Points: 1,104 The current goes out one hot wire and returns on the other. Back and forth.
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 09-21-2011, 11:23 AM #5 Member   Join Date: Oct 2009 Location: Albuquerque NM Posts: 159 Rewards Points: 95 I was also curious how the 240 VAC power gets to the house, since power plants make 3-phase. Looks like local transformers convert 7200 VAC single-phase to the two 120 VAC lines, is that right?
09-21-2011, 11:55 AM   #6
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Quote:
 Originally Posted by garya505 I was also curious how the 240 VAC power gets to the house, since power plants make 3-phase. Looks like local transformers convert 7200 VAC single-phase to the two 120 VAC lines, is that right?
It depends on how you look at it. The 7200 is stepped down to 240 but since the secondary is center-tapped 120 volts is available. With respect to the center tapped neutral, the two 120 volt sides are 180° out of phase.
BTW, that 7200 is one leg of the three phase power that is distributed by the PoCo. It is 7200 volts to earth ground.

Last edited by a7ecorsair; 09-21-2011 at 11:58 AM.

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09-21-2011, 12:10 PM   #7
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Quote:
 Originally Posted by a7ecorsair It depends on how you look at it. The 7200 is stepped down to 240 but since the secondary is center-tapped 120 volts is available. With respect to the center tapped neutral, the two 120 volt sides are 180° out of phase. BTW, that 7200 is one leg of the three phase power that is distributed by the PoCo. It is 7200 volts to earth ground.
I was thinking about the 7200V. If residential 240 is taken from one leg of the 3-phase grid, how does that balance out. Seems like there would be current flowing in that leg.

 09-21-2011, 12:26 PM #8 I=E/R     Join Date: May 2010 Location: Minnesota Posts: 2,052 Rewards Points: 1,000 There will be current flowing in all the legs but the PoCo has to balance out the distribution network. My power lines come through an agricultural area and there are many 3 phase drops to support sprinkler systems. Along with that, there are several branches, some just one phase of the power and some two phases, that distribute power to different areas. It is a Wye system so a neutral is carried along with the power line. This neutral is grounded at every pole and step down transformer.
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09-21-2011, 12:29 PM   #9
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Quote:
 Originally Posted by a7ecorsair There will be current flowing in all the legs but the PoCo has to balance out the distribution network. My power lines come through an agricultural area and there are many 3 phase drops to support sprinkler systems. Along with that, there are several branches, some just one phase of the power and some two phases, that distribute power to different areas. It is a Wye system so a neutral is carried along with the power line. This neutral is grounded at every pole and step down transformer.
Ah, thanks. I meant to say "more current flowing in that leg".

Do they always take the residential 120/240V from the same leg?

 09-21-2011, 12:38 PM #10 I=E/R     Join Date: May 2010 Location: Minnesota Posts: 2,052 Rewards Points: 1,000 No, they use all the legs. Any leg can feed a step down transformer. The three phase lines for my network dead ends about 8 miles from my neighborhood. There is a single connection to one phase (7200 volts), and also the neutral of course, that travels through the woods down a high line cut to a power pole. At this point is a T junction. One line goes south and one goes north from this point and from these lines, several transformers are connected along the way to support the local residences.
09-21-2011, 12:50 PM   #11
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Quote:
 Originally Posted by a7ecorsair No, they use all the legs. Any leg can feed a step down transformer. The three phase lines for my network dead ends about 8 miles from my neighborhood. There is a single connection to one phase (7200 volts), and also the neutral of course, that travels through the woods down a high line cut to a power pole. At this point is a T junction. One line goes south and one goes north from this point and from these lines, several transformers are connected along the way to support the local residences.
Hmmm, so not all 120 VAC everywhere is in phase. Interesting

09-21-2011, 01:25 PM   #12
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Quote:
 Originally Posted by garya505 Hmmm, so not all 120 VAC everywhere is in phase. Interesting
True. There are up to 6 possible phase angles for a 120V leg, I think: The transformer could be fed from any of the three phases plus neutral (wye), or could be fed from any two phases together (delta), giving a total of 6 possible phase angles for the transformer's primary.

 09-21-2011, 06:20 PM #13 JOATMON     Join Date: Aug 2011 Location: S. California Posts: 11,481 Rewards Points: 2,116 Blog Entries: 2 Here is a little diagram that sort of shows how the two 120Vac phases give you 240....ignore the refereces to current flow on the neutral...not correct....it was drawn that way during a discussion with another guy..... __________________ Even if you are on the right track, you will still get run over if you just sit there. My 2-Story Addition Build in Progress Link ... My Garage Build Link and My Jeep Build Link
 09-21-2011, 06:42 PM #14 I=E/R     Join Date: May 2010 Location: Minnesota Posts: 2,052 Rewards Points: 1,000 Hummmmm I thought that is how 240 gave us 120
09-21-2011, 06:46 PM   #15
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Quote:
 Originally Posted by a7ecorsair Hummmmm I thought that is how 240 gave us 120

Yea, yea....I know....110, 111....120...what ever it takes....

I was in a rush when I did that a couple of years ago....just too lazy to fix it...

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