How does 120, 240 and 440 circuits work?

  1. Aug 10, 2006 #1
    I am looking for a web site that explains how 120, 240 and 440 volt circuits work. I am refereeing to the ones you have in your house and such. I am trying to figure out how they get 240 volts from 120 and such.
  2. jcsd
  3. Aug 10, 2006 #2
    Somebody might tell you here, but if you search two things on google you'll be alot more prepared to understand any of their answers:

    First, search "voltage amperage resistance"

    Second, search "transformers"

    You'll need the first to understand the second.
  4. Aug 10, 2006 #3
    A great site to start with would be to visit They have great articles on power grids ond household circuits. Do a search on electricity at the top of the page.

    For your last question...

    Most houses in residential areas have 240 volt, 200 amp service that is transformed down from a very high voltage from the power company. Power companies use the high voltage for power transmission because it is more efficient. From that you are able to supply devices in your house with either 120 volts or 240 volts. The 240 volt service contains two 120 volt wires, a neutral wire and a ground. To get 240 volts to an appliance, you connect both 120 volt wires to get a 240 volt potential. To get 120 volts to say a lightbulb, you would connect a single 120 volt lead and the neutral wire (which is at ground potential).
  5. Aug 10, 2006 #4
    I am well aware of what the four main electrical properties are. I am a car audio installer / SPL competitor. Anyway I know that the main lines in your house come on two 120-volt hot wires and one neutral wire. I am looking for a little more “in depth” or advanced explanation then that. I am looking to know how they get like 208, 220, 440 and 480-volt lines. Also regarding the two hot 120VAC lines that come into the house, how do they get 240 out of them? In the automotive industry we can simply series electrical circuits to double the final voltage of the circuits. Like for example we can series two 12-volt battery to get one 24-volt battery. Obviously that’s not how its done in houses because you can not really series two positively charged lines. Also I am interested in how then get those weird voltage levels, such as 230 volts and 208 volts and whatever other ones there are.

    I cannot find any articals on about this. :(
  6. Aug 10, 2006 #5


    User Avatar

    Staff: Mentor

    Keep in mind that all the voltages you are talking about are AC voltages. So to get 240Vrms from the two Hot 120Vrms lines that go into houses, the two AC voltage waveforms are 180 degrees out of phase.

    And for some of the other voltages that you are mentioning like 208, those are from 3-phase power distribution systems. Here are a couple hits from a google search I did just now on something like tutorial ac mains voltages 208 220 240: -- Single & 3-phase power converters -- Some info on house power
    Last edited: Aug 10, 2006
  7. Aug 10, 2006 #6


    User Avatar

    Staff: Mentor

    As triden indicated, start with the subject "electricity" at, and go through a few pages until you get to how the electrical utility grid works (from generation to homes). Pretty good long tutorial, including 3-phase issues, starting about here:
  8. Aug 10, 2006 #7


    User Avatar
    Science Advisor
    Homework Helper

    Actually, you can think of it this way.
    The transformer has 2 120v windings hooked in series for a total of 240v.
    The connection between the windings, in this case is called the neutral, elsewhere it might be called a centertap.
    Any other ways commonly used to describe this are largely a matter of semantics.
    As a practical mater, you can open the transformer, cut the link between the windings and have two independent 120v circuits.

    It's just differently constructed transformers.
  9. May 31, 2009 #8
    If the supply voltage is single phase 240VAC plus ground, is it acceptable to use one wire of the 240VAC and the ground to obtain 120VAC supply voltage? Will this be ok or will it be breaking the electrical code?

    For your last question...

    Most houses in residential areas have 240 volt, 200 amp service that is transformed down from a very high voltage from the power company. Power companies use the high voltage for power transmission because it is more efficient. From that you are able to supply devices in your house with either 120 volts or 240 volts. The 240 volt service contains two 120 volt wires, a neutral wire and a ground. To get 240 volts to an appliance, you connect both 120 volt wires to get a 240 volt potential. To get 120 volts to say a lightbulb, you would connect a single 120 volt lead and the neutral wire (which is at ground potential).
  10. May 31, 2009 #9


    User Avatar
    Science Advisor
    Homework Helper

    It's worth noting that the split 120v is a north American thing, Europe uses 230V single line to run everything. The UK uses 240V, this makes it a little more dangerous and means you can't have a TV in the bathroom but you do get kettles that can make a hot cup of tea in a reasonable time.
  11. May 31, 2009 #10


    User Avatar
    Science Advisor

    Thanks MGB.

    Australia uses a system much like Europe's. Maybe it is the same.

    There are high voltage (330 KV and lower ) lines that eventually feed street transformers.

    The output of those is 3 phases that are all 230 volts relative to a neutral point but 120 degrees out of phase with each other. Many houses have just one phase but, by paying extra, you can have all 3 phases brought into the house.

    This would be done if you had an electric oven or hot water system that needed 2 or 3 phases.

    So, we have 3 phases and some of the wall power outlets are on different phases to others.

    The voltage between phases is about 400 volts.

    The voltage is dangerous but the rules about who can do house wiring are strict and the installer has to be licensed. You don't get bricklayers or plumbers doing electrical work unless they have double qualifications. Electrocutions are rare because safety standards are high.
  12. Jan 23, 2011 #11
    This might help everyone understand 120/240/440 volt circuits a little bit better. I have a degree in electronic engineering degree from Arizona State University. This doesn't make me any smarter, but it does allow me to understand how electricity works. They taught us about single phase and three phase transformers in school, but I didn't really understand things until I started my own consulting business and had to find out how things work for myself.

    It's really quite simple. The power company generates 3-phase electricity (3 separate voltage Outputs) from a generator. It is called 3-phase because each output has the same voltage and current capability but it is 120 degrees out of phase with the other two outputs.
    If you were to look at all three outputs on a scope, you would see three distinct sinusoidal waveforms. Since the generator is operating at 60Hz (60 cycles per second), one complete sinusoidal wave would be output in 1/60th of a second. If you were able to set your scope to 60Hz/division (1/60th of a second), then you would see three separate wave forms per division, each of the three wave forms would be 120 degrees out of phase or 1/3d of a waveform from peak to peak of each wave.

    It really doesn't matter what voltage the power company puts out, because they immediately use a 3-phase step up transformer to increase the voltage to 110,000 volts or more. The higher the voltage, the lower the current flow needed for power consumption. Lower current flow means smaller gage wire and less power loss in the lines due to resistance. What is important is that it be at exactly 60Hz, in order to synchronize with all other generators on the transmission grid.

    Before it gets to you, the home consumer, it is stepped down several times in substation transformers. That transform you see on the pole by your home is stepping down 16KV or 4KV to 120 volts not 240V as most people would believe. There is one primary coil and two secondary coils wrapped around the core of the transformer. The two secondary coils are tied together in the middle, but one of them is wound in the opposite direction. This means that both coils will produce 120 volts but that voltage output between the two of them will always be 180 degrees out of phase.

    If you measured the output between the two secondary transformers on a scope, you would actually see a 60Hz sinusoidal wave with a crest/peak at +170 volts and trough at -170 volts. That is because the peak voltage of a 60Hz sinusoidal wave is 1.41x the RMS voltage of 120 volts. You are not measuring RMS voltage on the scope you are measuring actual voltage from peak to peak.

    The center of the two secondary coils is tied to earth ground. It is the same earth ground that you are required to have at your home. If you look inside your main electric service panel, you will usually see two black wires and one white wire coming from the output of your meter. The Black wires come from each side of the two secondary windings of the transformer for your home. The white wire is actually connected to the grounded center wire coming from your transformer. This white wire is called neutral inside your home because it always has a voltage of zero with respect to the two black wires. It is always connected to the ground wire inside your service panel via the common ground/neutral bus.

    So earth ground and neutral are basically the same wire, but they serve two purposes. Neutral is only used for 120 volt circuits. If you think of electricity in terms of the current flow, then it is the return path for this current. It is returned to the center of the two, oppositely wound, 120 volt, secondary transformers windings. This simply means that 120 volt current flows from one black wire (hot) to neutral.

    The ground wire is there to protect you in case of a short to the chassis of the equipment in use. If the hot (black wire) inadvertently shorted to a metallic chassis, then it would cause a circuit overload and trip the breaker. If the chassis wasn't tied to ground then you might get electrocuted when you picked up the equipment.

    Under the right conditions, your body can conduct electric current from the shorted chassis of that device through you to ground. Your body is not a pure conductor. It has resistance to that flow of current, just like a light bulb has resistance. A light bulb glows and the breaker doesn't trip because the resistance of the light bulb limits the amount of current flow. Your body can do the same thing. It is possible for you to turn into a light bulb and the circuit breaker will not trip. Basically you fry to a crisp and die. That may seem like overkill, but that is what ground protection is intended for.

    Your service panel actually has a separate isolated 120volt buss for each of the two hot wires coming in from the transformer. The two busses have several legs. Each leg lies between the legs of the other bus. This means that the voltage between any two legs that are beside each other will always be 240V (RMS)

    240 volt current flows between the two legs of the two separate busses. It is not returned to neutral. That is why 240V breakers are usually twice the size of 120Vbreakers. They must connect to two legs, one on each side of the buss.

    Older 240 volt appliances have a three wire plug. Two of those wires are hot. They come from each branch of the opposing 120V busses. The ground wire is only there for safety purposes should there be a short to the chassis of the appliance.

    Newer 240 volt appliances have a four wire plug. Two hot wires, one neutral, and one ground. The neutral is there because many 240 volt appliances also use 120 volt sources such as light bulbs, digital displays, etc. You don't really need the neutral wire because the appliance would still work using ground as a return path for neutral. Remember, neutral and ground are really the same wire. They simply serve two different purposes.

    So what about 208V systems. You only find 208V wiring in commercial/industrial settings, where three phase wiring is being used. There are two types of 3-phase transformer configuration; delta (as in a triangle), and "Y" (as in the letter). This represents how the different windings of the transformers are connected together. I'm not going to go into transformer theory here. You will have to read about that separately, but I will sum it up.

    The output voltage across any single phase of a 3-phase delta step-down transformer is 240V or 480V depending on the needs of the end user. 480V would simply be stepped down two 240 as described earlier. Neutral and ground are tied together at the service panel. It only requires three wires to be run to the point of service. Each phase of the transformer can be used for a home. That means that one transformer can provide 120/240 volt service to three different locations. The step-down transformer for your house is set up in a delta configuration because that is what makes it possible to get both 120 and 240 volts out of any one phase or any two wires connected across the transformer.

    If you needed 3-phase wiring at your home then the power company would have to put in a special transformer just for you at a very high cost, because each of the three legs of their transmission line are undoubtedly already being used in single phase 120/240 delta transformer configurations.

    Y- Transformer configurations are used in commercial/industrial settings because one transformer can service several clients. That means it is cheaper for the utility company. The output across any two phases of a 3-phase-Y transformer is 208V. One leg of each coil in the transformer is tied together like a "Y". The voltage across any leg to this common center point is 120V. This is quite convenient, because in the US, every commercial/industrial 3-phase user still needs 120V service. The power company is able to kill two birds with one stone. The end user gets 3-phase, 208V electric service and 120V single phase service for normal appliance use.

    All 240Volt single phase devices are designed to work using 208V in the US. So if you have a 240V single phase A/C unit or motor, it will work just fine using 208V. It will simply use more current at that voltage. 3-phase motors and A/C units are much cheaper to design. They also require less maintenance and can do more work. They last longer. That’s why commercial/industrial areas use 3-phase electricity.

    I'm sure most commercial/industrial users would prefer to have 3-phase 240V delta power if they could. But they don't want to pay for additional transformers to be installed. It would take a long time to recoup the money. This only occurs when extremely large amounts of power are required or when a company purchases a large amount of land, then develops it themselves. The average commercial storefront user is stuck with 208V because of the lower capital equipment cost required when power is initially brought in to an area.
  13. Mar 27, 2011 #12
    SPL Tech, It sounds like you may be trying to figure out a similar problem to what I have. If you have any information that can help me on the following problem that would be appreciated.

    I purchased a Samsung Dryer in the US and brought it back to Australia with me. The dryer is 120v/240v. I originally though that this meant that the dryer recieved 240v from the electrical outlet. As I have since learnt, the dryer takes two 120v inputs and then somehow uses them provide 240v. What I'm trying to figure out si how can I power this unit from the Australia single phase 204v circuit? I'm not sure wether the dryer takes the two 120v circuits and converts them to a single 240v output via a transformer or converts them to two 240v outputs. I was thinking that there may be a way to bypass the transformer(s) and supply single phase 240v direct to the output side of the transformer(s). Any thoughts or advise would be appreciated.
  14. Mar 27, 2011 #13


    User Avatar
    Science Advisor

    This thread started in 2006, so the original poster may not be around any more.

    You may be able to download a service manual for your dryer from Samsung in the US and you should do this if possible.

    The heater would certainly use 220 volts, but there would also be a motor to turn the clothes and this would be 120 volt 60 Hz.

    Also, there would be a timer and control module which would be low voltage but derived from a 120 volt 60 Hz transformer.

    There would also be a fan to blow air through the clothes. This would also be 120 volts 60 Hz.

    120 volts is not a problem, but getting it at 60 Hz could be difficult. If you need someone to rewire it for you, this could get very expensive as most electricians would not want to do this.

    I suppose you are in too deep now to back out and throw it in the bin, but economically, that might still be a good option. Dryers are not very expensive to buy new.
    Simpson Model Number: 39P400M $335
  15. Mar 27, 2011 #14
    Thanks for the response. The Samsung model is a 10kg dryer and would cost about $3000 in Australia. It only cost $1200 to get it from the US. If there is any way I can us it in Australia I will. Any thoughts on how to do this?
  16. Mar 28, 2011 #15


    User Avatar
    Science Advisor

    It would help if you posted the model of the dryer in question--there may be a manual somewhere out there for us to look at. It'd also probably be worth calling Samsung themselves and see if they can give you a better answer.
  17. Mar 28, 2011 #16
    The model number is DV448AE. I have contacted Samsung and awaiting a response.
  18. Mar 28, 2011 #17


    User Avatar
    Science Advisor

    It looks like it's a 3 or 4-wire model: you need the neutral between the split phases. Unfortunately, it looks like Samsung Australia doesn't even sell stand-alone dryers!

    In any case, this discussion may be relevant to you:

    Getting an appropriately-sized transformer and shipping the dryer (along with any other appliances you might have) to Australia may cost you more than selling your dryer and buying a new one.
  19. Mar 28, 2011 #18
    I already have the dryer in Australia.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Similar Discussions: How does 120, 240 and 440 circuits work?