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I think this question is appropriate here in order to help DIYers (me) to understand why a neutral isn't needed on a 240 volt outlet.
I understand that two 120 volt power sources that are 180 degrees out of phase can be combined to provide 240 volts and I believe they accomplish this by using each others off phase to complete the circuit.
My Question
If all AC electrical power completes its circuit by returning to earth, how is that accomplished in a 240 volt circuit with no neutral?

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Power doesn't want to return to earth. It wants to return to source.

For natural power, ESD and lightning, yeah, source is earth. However, for human power, source is the transformer or battery. So hot wants to get back to neutral or the other hot.

As it happens, there's an equipotential bond to keep the three voltages (240V and neutral in the middle) from getting too high compared to earth. This bonds neutral to earth. As such, wayward current can work its way through the grounding system, the neutral-ground equipotential bond and back to neutral. But power would prefer neutral or the other hot.

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  • 2
    @Harper - Reinstate Monica : Thanks, your first line helped a lot - "Power doesn't want to return to earth. It wants to return to source"
    – HoneyDo
    Mar 3 '20 at 23:17
  • 240V and neutral in the middle ... it is only that way because the middle was chosen as neutral ... you could just as easily have 240 V and neutral with 120 V in the middle (of course that arrangement would not be as useful)
    – jsotola
    Mar 4 '20 at 0:43
  • @jsotola yes, you can set your N-G bond anywhere you please when you have your own transformer. I could see a reason to do that if you have both Euro and USA appliances in the same house. Use a CH or Square D board insulated for 277V, it'd work great. Mar 4 '20 at 3:39
  • For a picture : 240V split phase
    – J...
    Mar 4 '20 at 14:04
  • @jsotola, Re a 240-120-0V system vs. a 120-0-120V system: I'm not sure that arrangement would be any less useful than what we (in the U.S.A.) have now, but it might be a little less safe in the event of a fault condition or an accident. Mar 4 '20 at 15:38
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I think the fundamental issue you have is that AC power doesn't "return to earth". To get 120v and 240v power, we stuck neutral in the middle and decided it would be a good idea for that center point to be tied to earth, but it's not a requirement.

DC power (a battery) has a positive wire and a negative to complete the circuit without any reference to earth. AC power works the same way - you just need the two wires, and earth doesn't have to be involved.

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  • Yeah - Harper indicated that "it wants to return to source" which helps a lot. But you used DC as an example - isn't DC different in that the circuit is completed in the appliance/light? Probably showing my ignorance here but trying to learn.
    – HoneyDo
    Mar 3 '20 at 23:15
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    @HoneyDo, not sure what you mean by circuit is completed in the appliance with DC. Wiring and switches for AC and DC are very similar so I don't think what you're getting at is true. The main difference in AC and DC is that in DC polarity matters much more because the flow is always from positive to negative. Devices have to be hooked up to the right wires. Since the + and - switch rapidly with AC, the polarity doesn't matter (Except for safety). Almost any appliance/light will work fine if the black and white wires are switched (but that might be a shock hazard).
    – JPhi1618
    Mar 4 '20 at 3:44
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You partially answered your own question. Each leg is 180 out of phase so that's where the 240 comes from. The 240 volt circuit is across both phases and doesn't require a neutral to accomplish that. The Earth doesn't provide part of the circuit.

Also, you might be confusing "returning to Earth" with grounding.

The grounded neutral conductor provides a path back for 120 volt circuits and the bonded equipment grounding is to provide a path for errant currents to safely pass back to neutral/bonded ground which we keep at ground potential (no voltage between neutral and ground). Hope this helps.

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The AC current in a typical house circuit does not flow through earth. For 120 volt circuits, it flows through one of the 120 volt wires (normally black) and returns through the neutral wire (normally white). For 240 volt loads, the current flows from one of the 120 volt lines to the other 120 volt line. There is no current through the neutral for such loads. For safety reasons the ground (earth) wire (normally green) is connected to the neutral wire at the main electrical panel. However, if there are no faults, this ground wire does not carry any load current.

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Adding to the answers above, the 240v power in your house is called split phase electric service. It is created by tapping the house side winding of your service transformer in the middle. This center tap is bonded to ground and used as neutral in 120v circuits. Current from these circuits travels between the hot sides and center tap. https://en.wikipedia.org/wiki/Split-phase_electric_power

Formally, according to Kirchhoff's current law all current entering/leaving a circuit node must total zero. In other words current is conserved. The center tap is a node with three wires, the two sides of the transformer coil and another going into the house. All current leaving the transformer coils must be replaced by the wire into the center tap.

The ground bonding and ground wire use earth to conduct fault current. Since holding certain things at ground potential (like the metal cases of appliances) is important for safety, this provides a backup for the power service neutral. Imagine that a natural disaster damaged your electrical service and only broke the center tap wire, and somehow also smashed an appliance such that the metal case was attached to a hot wire. Usually the protective ground would flow that current to the service neutral (probably blowing a breaker) but with the neutral wire gone it will just sit at high voltage potential. Ground rods provide a backup place for current to flow and guarantee the assumption that the white wire will never shock you. They work in concert with other ground bonds throughout the power grid.

Some very old or extremely remote power systems actually use the earth for return current on the high voltage power grid, but there are lots of disadvantages. In these systems return current from the house would still flow throw the transformer center tap. https://en.wikipedia.org/wiki/Split-phase_electric_power

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  • Thanks. I saw you wrote, "They work in concert with other ground bonds throughout the power grid." The same was in Wiki. Does that imply that all power in a system is grounded at the transformer or other part of the power grid? Or is that only on "Some very old or extremely remote power systems"?
    – HoneyDo
    Mar 4 '20 at 22:40
  • @HoneyDo All power systems are bonded to the ground, it is really important for safety. It happens all over the place, transformers, houses, power poles, substations, power plants. The difference between modern systems and the one wire ones is that in new systems current only travels through the earth in exceptional/bad circumstances whereas in one-wire systems the earth basically replaces a wire! Mar 4 '20 at 23:29
  • I do understand they are bonded to ground. I was curious if all electricity returned to earth and I guess the answer is...no! As Harper stated above - "Power doesn't want to return to earth. It wants to return to source". It sounds as if you are saying the same. Thanks for the education.
    – HoneyDo
    Mar 5 '20 at 0:09

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