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Someone told me that a 240V circuit does not require a neutral wire in the cable. Can anyone explain this phenomenon from the electricity perspective and generally explain why circuits do and do not need a neutral wire?

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Note: 240V in the US is split-phase and doesn't use the 120V neutral. 240V in the UK is single phase with one live wire, one neutral (and always one earth wire). –  RedGrittyBrick Oct 9 '14 at 8:17

6 Answers 6

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In a 120/240V single split phase system, the two ungrounded (hot) legs are actually connected to the secondary winding of the distribution transformer. The transformer actually steps down the voltage to 240 volts, so the two legs are a complete 240 volt circuit.

The grounded (neutral) conductor is connected to the center of the coil (center tap), which is why it provides half the voltage.

split phase transformer

Therefore, if a device requires only 240V, only two ungrounded (hot) conductors are required to supply the device. If a device runs on 120V, one ungrounded (hot) conductor and one grounded (neutral) conductor are needed. If a device needs both 120V and 240V, then two ungrounded (hot) conductors and one grounded (neutral) conductor must be used.

If you connect a load between the two ungrounded legs of the circuit, you can see how you have a complete circuit through the coil.

240V complete circuit

If you connect a load between one of the ungrounded conductors, and the grounded (neutral) conductor. You can also get a complete circuit, though it's only through half of the coil.

120V complete circuit
120V complete circuit

Since these circuits only include half the coil, the voltage is also half (Es = EpNs/Np).

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This gets me to about 90% of understanding the answer, but I can't quite get there. –  Evil Elf May 5 at 20:08
    
So Neutral is just the route returning the current to the source? –  Evil Elf May 6 at 12:31
    
Is the middle branch that completes either of the 120V circuits + or - ? Am I confused on how it can be hot AND netural? –  Evil Elf May 6 at 13:44
    
@EvilElf Ignore the + and - in the diagrams (I've removed them now), it's AC so there's no positive or negative. –  Tester101 May 6 at 14:06
    
That is the part I got stuck on. Thanks. Great diagram. –  Evil Elf May 6 at 17:56

For the second part: clothes dryers often have 240 V heaters and 120 V motors. Stoves use 240 V for the elements and 120 V for the light bulbs. These are both plug-in and need the neutral.

My new electric hot-water heater is 240 V, not plug-in, and uses the old 120 V wiring. The electrician doing the install marked the "old" neutral with black tape at each end to warn that it's now hot, and that there's no neutral......

In some wiring codes, each individual plug in a duplex outlet in a kitchen needs a separate breaker. They run a 240 V line to the plug, wire two hot lines each to the hot on a different plug, wire the one neutral line to both neutrals, and break off the tab connecting the two hots.

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AC current requires a return path, electricity goes out one way and back the other. With 120V wiring in the US, you have a center tapped transformer with two hots that total 240V. The neutral is that center tap, which combined with only one of the hots gives you 120V. Use both of the hots and you have 240V. The only need for the neutral is to get a 120V circuit. It's smart to wire a 240V circuit with a neutral conductor even if you don't need it for the current appliance since it's easier to run the extra wire once in case you need it in the future, but that's not required and electricians often cut these sorts of corners since that third conductor adds a lot to the wire cost.

Separate from the neutral is the ground, and this does need to be run on every circuit (though that wasn't always the case).

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Ground or earth is same together. In electrical systems wiring system is a conductor that provides a low impedance path to the earth to prevent dangerous voltages from looking on equipment. The terms "ground" (North American practice) and "earth" (most other English-speaking countries) are used synonymously here). Under normal conditions, a grounding conductor does not carry current. But Neutral is a circuit conductor that carries current in normal operation, which is connected to ground (or earth). Ref. wikipedia

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I'll try to clarify things. With AC current, the voltage changes direction 60 times per second (60 hertz) in the US and 50 hertz in Europe. The hot wire (black or red, typically) is the "pump" that pushes or pulls. The neutral is a path for the "pushed or pulled" current to return. So although the neutral carries current, it is not supposed to have a voltage other than zero. Think of it like a piston water pump pulling or pushing water through a tube into a can full of water. The pump provides the pressure (voltage) and the can (neutral) provides a place for the water to go and come from. This is not a completely accurate analogy because unlike electricity, water doesn't need a return path to the pump, but otherwise the principles are very similar. If the circuit is well insulated from the environment, the circuit can begin to build a voltage of it's own, just because it is exposed to voltages on the wires. After all, without a reference, how does the circuit "know" where zero volts is? This voltage can get quite high and can shock you pretty badly so it needs a reference at zero to drain off the extra charge that trickles into the circuit and one that "pulls it to zero". So we attach a bare copper wire to the neutral bar in the circuit breaker box to drain off any residual charge and pull the bar to the same voltage of the ground, which is zero. Think of the earth as an ocean of electrons. You can add to it and draw from it and it doesn't make a bit of difference in the charge just as you could pump water into or out of the ocean all day and not change its level. We tap into this immense reservoir of zero voltage by driving a metal rod into the earth and attaching the other end of that bare copper wire to it. Then the neutral bus is always at zero voltage and you don't have to worry about getting shocked when you touch your stove or refrigerator. So, you see, under normal operating conditions, the bare ground wire only has a voltage of zero with no current flow. The neutral is also zero, but carries all the current the circuit uses for running the hair drier or other appliance (because all the current that goes out on the black wire, must come back on the white neutral wire since it has no where else to go). The black and red wires cycle back and forth from -120 to zero to +120 and so on. The black and red wires (may also be two black wires) are out of phase by 180 degrees so when one is -120, the other is +120. Therefore, the difference between them changes from 240 in one polarity to zero to 240 in the other polarity, etc. A friend of mine wired his own house but didn't understand these principles. He wired the black wires to one phase and the white wire to the other phase (instead of the neutral bus). This made all of his outlets 240 volts instead of 120. When he turned on the power, he ruined ever appliance he'd turned on and had to replace most of his new appliances. Hope this helps!

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All electric circuits require 2 "sides" or "legs" of power regardless of voltage or polarity be it a 12 v DC circuit in a car or a 120 v AC wall outlet or a 220 v dryer outlet. 1 hot leg is 120 volts, 2 hot legs is 240 volts across both legs with a 120 volt circuit we only use 1 hot leg, so what is the 2nd leg going to be if not the other side of power , it being a hot leg? We use the "neutral" a neutral is earth, earth is ground literally the dirt of the planet earth is earth. There are only 3 legs or wires entering your home and 2 of them are hot legs, 120 volts each or 230 volts across both. The reason we cannot merge ground and neutral after the service drop is because ground is supposed to be an alternative path not a concurrent path to earth. It negates its purpose to merge them after the service drop. So basically we need 2 sides or legs of power in any circuit so if you don't need both hot legs than neutral is your only option. Grounds were never around in homes outlets until after the 1950's

ground is an escape ramp for errant voltage to use rather than use our bodies! should we get in a circuit.

All electric circuits require 2 sides of power,let's call them L 1 and L 2 It can be 2 hot legs of 120 volts as in a 230 volt circuit or 1 hot leg of 120 volts and a neutral leg for the opposite side of power. Earth is Ground and it is also Neutral, in Europe they refer to Earth as Ground. Either way all electric circuits require 2 sides or legs of power be it 1 hot leg or 2, if one that requires the Neutral leg to complete the circuit. Ground and Neutral are Earth, Earth is what it sounds like it is the soil/dirt of our planet. Older systems had no grounds , grounds are for safety only and as a result is not to be merged with the Neutral beyond the service drop as it is to be an alternative path to Earth/ Ground, if they are merged ground is no longer an alternative path just a concurrent path to earth. Earthing- Assuring the voltage between equipment and EARTH will remain non-lethal even during non-typical conditions such as faults or lightening.

Ground- [Assuming you meant ground CONDUCTOR.] The conductor intended to accomplish the earthing of a piece of equipment. Usually done via a bare conductor. It is done because the earth has an inconsistent and often high impedance. I.e. it's a poor conductor.

Neutral- Conductor designed to carry current during normal and abnormal operation. Is usually connected to local power system's ground at the point of supply only, no where else. Thus, is "typically" at a low potential and safe to touch.

During NORMAL operation these can APPEAR quite similar. But the differences are quite significant and should not be trivialized.

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you lost me at We use the "neutral" a neutral is earth, earth is ground literally the dirt of the planet earth is earth –  amphibient Apr 22 at 15:32
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The part about earth and dirt is completely incorrect. The dirt outside has NOTHING to do with the ground wires run with circuits. The grounding electrode system in a home has nothing to do with fault clearing. Says this clearly in the NEC. Besides, current is NOT seeking ground or earth, it is seeking it's source. You completely ruined a decent answer with this edit. –  Speedy Petey Apr 28 at 11:04
    
Ground or earth is same together. In electrical systems wiring system is a conductor that provides a low impedance path to the earth to prevent dangerous voltages from looking on equipment. The terms "ground" (North American practice) and "earth" (most other English-speaking countries) are used synonymously here). Under normal conditions, a grounding conductor does not carry current. But Neutral is a circuit conductor that carries current in normal operation, which is connected to ground (or earth). Ref. wikipedia –  Ceaser Oreo Apr 28 at 12:14

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