240V circuit. Neutral unused?

[NetWareHead]

Wired up an electric cooktop for the in-laws and I was kinda puzzled about this. 240 volt appliance and I have a 6 gauge cable with 3 conductors connecting to the stove (2 hots, 1 neutral & 1 ground).

Instructions on the stove said to only use the 2 hots, cap the neutral and leave it unused in the box. This kinda goes against my understanding of electricity where the hot brings the current in and the neutral brings it out forming a complete path back to the panel/transformer. I followed instructions anyway and the stove works. Remembered when I wired for an electric dryer and I did connect the neutral as instructed. So my research says the neutral is used on dual voltage appliances like a dryer where the 240 is used for the heating elements and the 120 is used for the motor, timer and other various items. Popped open the water heater's electrical access cover and sure enough, 2 hots only.

What is exactly going on here that makes a pure 240 volt circuit work without a neutral wire? Something about 1 hot canceling the other hot out but this is where Im unsure of how this works... Would appreciate an explanation, thanks.

 

[NetWareHead]

Continued my reading and saw this applies further up in the circuit and at the panel itself concerning one hot bus vs the other. And specifically concerning balancing the load a panel sees. E.g. if the load is unbalanced and hot bus A is drawing 10 amps while B draws zero, 10 amps will return to the transformer via the neutral. But if A draws 10 amps while B draws 4, then the 4 on each side will cancel and only 6 will return via the neutral. Interesting.

 

[Rubycon]

Continued my reading and saw this applies further up in the circuit and at the panel itself concerning one hot bus vs the other. And specifically concerning balancing the load a panel sees. E.g. if the load is unbalanced and hot bus A is drawing 10 amps while B draws zero, 10 amps will return to the transformer via the neutral. But if A draws 10 amps while B draws 4, then the 4 on each side will cancel and only 6 will return via the neutral. Interesting.

Follow the manual. A neutral is only required on a two wire (240VAC) appliance if 120V is used, ex. oven lamp.
Dryers are two wire as well. Just L1/L2/G.

Draw is across L1/L2, i.e. 240V.
Neutral is the "center tap" of the transformer, hence getting 120V from neutral to either L1/L2.
Ground is for safety and is bonded to chassis so if an energized part comes in contact, the resulting fault current will cause the supplying circuit breaker to open. This is why omitting grounds (ground fault) is hazardous!

 

[NetWareHead]

Im having trouble understanding the mechanism whereby the cancellation occurs between current from L1 & L2. I have read about where it originates from in a split phase electrical system and the neutral is the center-tap but having trouble translating that into an explainable concept that I can understand easier. Nearly all of the reading I have done fails so far in helping me understand this.

 

[NetWareHead]

Assuming this graphic I am linking to here represents residential split phase power delivery.

If I understand this correctly, there are two resulting sine waves, each is a 120 V leg of the circuit? And they look 180 degrees offset from each other.

Im assuming when both legs are fed into the load, this is where the cancellation is done? If the appliance is designed correctly and spreads the load demand across both legs equally, this is where the need for a neutral wire disappears?

 

[Rubycon]

Nope, same sine wave just has its amplitude reduced.
When you have a load connected across L1/L2 there is no imbalance.
Imbalance can occur say in a panel where more (120V) loads are on one side of the box than the other. Electricians can use a clamp on ammeter at the top where the power is feeding the main and compare the amps and move breakers to balance things out. It's never constantly even however you don't want it too imbalanced either.

A 240V appliance like an electric dryer or water heater will always draw across the entire secondary winding of the step down transformer.

BTW that pic has the windings wrong. That actually indicates a step up transformer. The primary winding is always smaller with more turns and the secondary has larger wire with less turns. Minus core losses, the power is the same so 1A at 12kV = 50A at 240V, etc.

 

[Paperdoc]

You're getting it. The transformer output or Secondary winding has a centre tap. Actually, for several reasons - but this males it easier to grasp - the system connects that centre tap to a real Ground connection to the earth, BOTH at the transformer and at your breaker box. This becomes the Neutral line inside your home for all the power cables. Because it is Grounded, we can recognize its voltage at all times to be zero. So, compared to that reference point, the voltage at the two ends of the transformer secondary winding swings up to max and back to zero all the time, at the 60 Hz distribution frequency. Thus a meter from Neutral to either end will show 120 V AC, because that is the time-averaged value of the sine wave. BUT the trick is that, when the upper end of the secondary winding is rising to maximum + voltage, the lower end is dropping to max - voltage. That is, the sine wave of voltage on the two ends moves in opposite directions, as your diagram indicates. So at a particular instant in time, the voltage from Neutral to L1 (the top of the windings, may be + y volts, but the voltage from Neutral to L2 (the bottom ) is - y volts. Thus, the voltage difference between L1 and L2 (between the two ends) is 2y. Therefore, the time-averaged voltage difference between L1 and L2, as measured by a meter, is 240 V AC.

If it helps, we'll draw an analogy with a DC circuit, although this is not exactly the same. Suppose you have two car batteries, which each can deliver 12 V DC to your load. We connect the two of them in series by running a short cable between the + terminal of one and the - terminal of the other, and we also connect to this short cable an output lead ending in a terminal we'll call Neutral. Just to make it more similar, we will also connect this Neutral terminal to a true Ground, so now that point is always at zero volts compared to Ground. If we now connect a headlight bulb from that Neutral terminal to the unused + terminal of one battery, it gets a 12 VDC supply and lights up. Now take a second headlight of the same type and connect it from the Neutral terminal to the - terminal of the other battery. It also gets 12 VDC and lights up. Now we go back and completely disconnect the two headlamps from the Neutral terminal, but leave in place the connecting line between the two batteries. We also leave in place the separate wire connecting the two headlamps together in series and each is still connected to the end + and - terminals of the batteries. Now we have two headlights in series connected to two batteries in series, and the two lights have a total of 24 VDC supplying them. They still operate exactly as the did before, because each of them experiences a drop of 12 VDC across it, and the same current flows through it. In fact, the exact same current flows through both of them. But note that there is NO current flowing from the common connection between the two headlights to the Neutral terminal we used. We disconnected that link and the whole thing still works!

 

[NetWareHead]

I do understand your example where we measure the voltage from the 3 available points. We get 240 if measured across the 2 ends of the transformer (line 1 and line 2) and we get 120 if measured from one line to the center tap.

I understand how the line to center tap works as that fits well into my understanding of power delivery using 1 hot to deliver and a neutral to return it forming a complete circuit.

I also have a question about your battery example. While it does a great job of explaining how to aggregate 2 legs and how the result increases from 12 to 24 volts, we also have the polarity of the terminals that helps me to understand that current leaves one terminal and flows back to the other. The path is easy for me to envision as electrons flowing from negative to positive whether that be from one battery or both.

I dont think there is a polarity when dealing with AC and thats where my logic is faltering. The concept of + & - are DC territory so how are we getting that current back to the transformer for a complete circuit when drawing from both legs?

BTW, awesome info here, thanks for all of your explanations and examples.

 

[Paperdoc]

Polarity is still essential with AC circuits. The problem is that it is changing constantly, but in a predictable manner, whereas the polarity in a DC circuit is fixed.

The way to deal with that is to think in terms of a frozen moment in time, rather than a continuous fixed picture. Let's say, for example, that we pick the specific time along one cycle of the voltage sine wave where the voltage relative to Neutral is actually +120 V at the L1 end of the winding. At that frozen time, the voltage at L2 relative to Neutral is - 120 V. Thus, if we have connected a load from L1 to Neutral it is experiencing 120 V across it and an appropriate current is flowing with electrons flowing from the Neutral through the load to the L1 terminal. At exactly the same time if we were to connect an identical load from Neutral to L2, it also would experience 120 V across it and a corresponding current flow. But in this device the electron flow would be from L2 to Neutral. At the Neutral terminal, exactly the same number of electrons per microsecond would arrive from the L2 side load as leave going to the L1 side load. In other words, there actually would be no net current flowing between our external Neutral terminal and the centre tap of the transformer.

Now, suppose that the load we wish to connect really is two heating elements each designed to operate on 120 V, and connected in series so that we get the heat of both heaters coming off. We might do this for the heating system inside an electric water heater, or for a stove-top element. But now we realize that we really are not using the connections between each heater and the Neutral terminal. We can eliminate that connection entirely, and just keep a connection between the ends of the two heaters. That actually frees us to re-design the system so it's only ONE heating element twice as long as each of the original units, and thus able to work properly if supplied with 240 V across it. Electrically this new design will function exactly like our old two-elements-in-series design, but it will be simpler and it will NOT require any Neutral connection at all.

Now, back to our frozen moment in time. But first, I have to correct an error in your drawing from Monday, in the way you depicted the sine waves on the Output side. I am assuming you meant that the highest positive voltage at the top end of the winding is where that upper (blue line) sine wave peaks, and it returns to zero when the input side sine wave also reaches zero. But after that, as the input sine wave continues into the negative region, the output voltage at the top terminal keeps on going down until it reaches a maximum negative voltage equal to the previous positive voltage. Similarly, the red line indicating the voltage on the bottom end of the winding does the same thing, except that its voltage at all times is exactly the opposite polarity of the one at the top. The voltages at each end do not swing only from max to zero. In each case they swing the full span from zero, up to max positive, down through zero again, down further to max negative, and back up through zero again to continue. Its just that the two lines are mirror images of each other and cross each other at zero twice each cycle of the wave.

With that in mind, let's pick a different time moment to freeze the view. In particular, let's pick the moment exactly one half of the period of the sine wave later than our first moment. In a 60 Hz power distribution system, this will be exactly 1/120th of a second later. Now we find that the voltage on the top end (L1 terminal) has changed to -120 V, and the voltage on the bottom end (L2) is + 120 V. Again, each terminal has a voltage difference from Neutral of 120 V, but this time the polarity is opposite. For the hypothetical heating element load I described, that makes no difference at all. However, if it had been an AC motor, that would make a BIG difference, which is why AC motors are designed quite differently from DC ones.

We could go on and choose several points in time for a series of frozen views. At each time we would see a different voltage at L1 and L2 with respect to Neutral, but they always would be the same in magnitude, but opposite in polarity.

 

[NetWareHead]

Polarity is still essential with AC circuits. The problem is that it is changing constantly, but in a predictable manner, whereas the polarity in a DC circuit is fixed.

Of course! DC there is no sine wave. Its just a straight line and the polarity is set. We expect the current flow to follow a predictable path from negative to positive and not oscillate

Part of helping me to understand this was to understand exactly what is happening during each cycle of AC. There is a reversal of polarity and this switches at 60 Hz. So if I understand this correctly, L1 and L2 each serve as the "positive" and "negative" but at intervals of 60 Hz. When one is pushing, the other is pulling.

We dont need a neutral wire because the leg the is at negative polarity at that instant in time is the "neutral".

 

[Rubycon]

A neutral is required if the appliance needs 120V. Using the ground as a current carrying conductor isn't permitted.
This isn't as crucial with modern appliances now as they can have their own power supplies to provide whatever is needed. Most of the lighting is LED with the exception of ovens.

 

[PowerEngineer]

Rubycon and Paperdoc have done a very good job of explaining how this wiring works. I will just belatedly chime in on the idea of the "neutral".

Besides being the center tap into the low-side 240 volt transformer winding (thereby providing two 120 volt options L1-neutral and L2-neutral), the neutral is also grounded (essentially to the actual ground). Doing this means the highest voltage you can experience between either L1 or L2 to anything in your house (e.g. the sink, the metal side of your washing machine, the puddle of water on the floor to the knife you stick into your toaster) is going to be around 120 volts (RMS AC). So the "neutral" becomes in essence the grounded zero volt reference for your house's electrical circuits. If the neutral were not grounded, then the actual voltage difference between L1 or L2 and ground could become much greater and pose a serious safety hazard.

The way to look at (single phase) AC circuits and DC circuits is that you need to have a complete loop for the current to flow out from and back to the source (e.g. transformer winding or battery). For the 120 volt circuits in your house, the "neutral" serves to complete the loop for current flowing from either L1 or L2 (L1-neutral, L2-neutral). For the 240 volt circuits, the loop is formed by L1 and L2 (L1-L2).

I hope this helps a little...

 

[Paperdoc]

Rubycon has reminded us of an important fact. Most household devices (such as dryers or stoves) have components that need a 120 VAC supply as well as the 240 VAC supply. For example, in a dryer the drive motor normally is a 120 VAC device, as are the tiny motor on the timer and the illumination light bulb inside the door. The main heating element is the item that needs 240 VAC only. On many older stove designs the temperature selection system for each cooking element is a multi-choice selector (pushbuttons or click-stop rotary knobs). That arranges for several different ways to supply the two voltages to parts of each cook element. The stove also has lights on the top and inside the oven that usually run on 120 VAC. Other devices, like an electric water heater tank, have only one power-consuming component, the heater element, and thus requires only 240 VAC with no Neutral connection.

I am a little surprised to read that OP's cooktop unit requires only 240 VAC with no Neutral. However, I certainly can see how that could be designed, and I'm not really up to speed on the latest cooktop systems.