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2015-07-16 19:28:46
2015-07-16 19:28:46

First, it's 277/480 and 120/240. But you're trying to relate what I call in the trade 'high voltage' 3 phase and 'low voltage' single phase.

Let's make it easier, with 'low voltages' only: you want to know the difference between 120/208 (3 phase) and 120/240 (split single phase).

First, look at your power source. Commercial services are often 3 phase, meaning you are using all 3 phases of generated AC in the building service. (look up AC generation for more information).

Residential services are really only using 1 of those 3 originally generated phases. Let's say a subdivision is made up of 30 houses, there will be approximately 10 houses on each phase from the power company to keep the load balanced.

On the ground somewhere (if underground service) or on a pole (overhead service) there is a transformer that converts single phase to split single phase. Some people incorrectly call this two phase power. Keep in mind you only have one phase from the power company's generator.

Split single phase enters a residence as two 120 volt conductors, and a neutral conductor. This is done by the pole or ground transformer's center tapped secondary winding. (Google this for more info) This center tap is actually where the neutral conductor is connected, and the two ends of the winding are where the 120v conductors come from.

A measurement from hot wire to hot wire in a residence produces about 240 volts, the sum of the two 120 volt legs. In fact, this is how your large electrical appliances operate. Your receptacles are connected to either 120v leg and a neutral.

Commercial services are actually simpler to explain. The power company produces 3 phases of power at very high voltages (to aid transmission over long distances), and those voltages are periodically stepped down lower the closer they get to a building's service entrance. As with the example earlier, there will be a ground mounted or pole mounted transformer that does a final step down before the wires enter the building.

When they do enter the building, they are the same 3 phases generated miles away at the power company. The difference is they aren't stepped down as low as residential services. Commercial services come in at 277 volts phase to ground, or 480 volts from phase to phase. Inside the building, there are more Transformers that produce the 120 volt 'household' voltages for receptacles by stepping down even further.

Notice that in the residential example, the voltages are 120v phase to ground and 240 volts phase to phase. (Exactly double)

In the commercial example, the voltage is 277v phase to ground and 480v phase to phase, or 120v and 208v (depending on if you are talking high voltage or low). The multiplier here is not 2.0, it is 1.73, due to a different type of transformer used. I won't explain this in detail though. If you want to understand this, you need to do your homework.

Long winded, but if this question was easy to explain and understand, everyone could be an electrician overnight!

If you don't know what you are doing three phase will kill you dead in half a second, so don't mess with it, call a professional.

The 220V takes a phase from the 440V 3 phase, to neutral to give you single phase.

Say you had Red Yellow Blue, then take the Red phase (live) and a neutral and you will have half the 440V

To understand 277 3 phase and 440 3 phase you need a knowledge of how AC electricity actually works.

The two live power lines of single phase power in a house come from a single pole transformer. They are 180 degrees out of phase, due to the grounded center tap in the transformer winding.

The three phases are actually produced at the generator. They are sent down a set of three live wires and one neutral wire. Those are the wires you see on high tension power lines. There is always a multiple of three wires on big insulators, plus one ground wire. Three transformers are needed to step down the power for distribution, and three pole transformers feed the industry the three phases.

The three phases are 120 degrees apart from each other. This makes the sum of the currents in the three hot wires equal to zero. The currents flowing in one direction exactly equal the currents flowing in other direction in other hot wires. If the three phase system is properly loaded, no current flows through the neutral.

Three phases are the perfect source for a heavy motor. The three phases naturally provide a rotating magnetic field. A single phase motor needs a capacitor or shading coils to make the magnetic field rotate. And to reverse a three phase motor, a switch trades two of the phase wires.

The 120 degree difference in timing between the phases is why the voltages of two phases do not add to twice the voltage. Due to the phase difference, two 120 volt phases produce 208 volts, and two 277 volt phases produce 480 volts.

The 440 or 480 confusion exists because there is also 440/760 three phase power available.

ANSWER:You need to think of the relationship between the voltages in terms of vectors. Start at a point in the center of your paper and draw 3 lines originating from that point 120 degrees apart (Like a clock with one hand at the 12, another at the 4 and a third at the 7 - Approx). Each line should be the same length and end with an arrow pointing out. Now draw three dotted lines connecting the three arrow heads together. These dotted lines represent the potential (voltage) between each hot leg of the 3 phase system... So pick one (480V, 240, 208, [400 Europe]) and label your dotted lines with this voltage. The dot in the middle of your drawing = ground and we are interested what the voltage to ground is going to be.

Time for some trig! We know the angle between our 3 arrow lines is 120 degrees and we know the line opposite that angle = (Your voltage) mine is 208V. We are looking for the voltage of the adjacent line to my 120 degree angle, so we use the TAN function to give us the ratio between these two sides TAN of 120 degrees = -1.732. Since we don't care about the vector location, I can eliminate the "-" sign and call it 1.732. Since TAN = Opposite / Adjacent I can write the equation: 1.732 = 208 / Adjacent then 1.732(adjacent) = 208 then adjacent = 208 / 1.732 or adjacent = 120.09V. As you can see, 208 3phase was chosen for the availability of our common 120v single phase.

If you do the same calculation on 480V, you end up with 277V to ground. This is why you see so many light ballasts rated for 208/277V. If you run the calculation at 400V you get 240V to ground - the common European single phase voltage.

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Related Questions


135 A at 120 v single-phase is 16.2 kVA. With a 208 v three-phase supply you get three single-phase 120 v supplies, so the same kVA is produced with a balanced load of 45 amps on each phase.


Split phase or single phase 120/240 volts.


The standard nominal voltage in Canada for a single-phase residential supply is 240/120-V split-phase supply.


Single phase or three phase? 120/208 would be measured hot to ground for 120 and hot to hot (phase to phase) for the 208 reading. Any hot to ground will read 120 and any two phases together will read 208.


Yes, 120 volts is single phase, being one leg to ground (neutral) of a 208 volt three phase wye connected distribution system. These connection are used in apartment buildings in North America. The service is stated as 120/208 volts. The 120 volts is derived from the three phase leg to leg voltage, 208/1.73 = 120 volts.


126 volts sounds very close to a standard home voltage of 120 volts, which is single phase. 208v is a three phase voltage, and is the lowest I have seen. 240 is single phase (usually center tapped, so in your house you have +120, and -120 referenced to the neutral which provides the normal 240 for dryers, stoves, etc.).


Very unlikely. Its 3 phase for a reason, it needs a large voltage/current to power it, single phase won't provide that.


120/240 volts single phase to a single family dwelling. On some new apartments 120/208 volts three phase is now being used.


The answer that three phase motors are more efficient than single phase motors and require less power to perform the work load required. To put it in simple terms, the poles in a single phase motor are 180 degrees apart to match the sign wave of single phase current. The poles in a three phase motor are only 120 degrees apart. Therefore, the motor only needs to turn 120 degrees before it receives its next pulse of electricity whereas the single phase motor must turn 180 degrees which requires more initial energy.


If you have a 208Y/120 volt 3 phase system, you can use any one of the three phases (hot or ungrounded conductor) with the neutral for 120 volt load or you can use any two phases of the three phase for a single phase 208 volt load. If you have a 120/240 volt 3 phase system, you can use any two phases of the three phase for a single phase 240 volt load or you can use either the A phase or C phase with the neutral for a 120 volt load. You can not use the B phase with a neutral (you would have 208 Volts!).


208 volts is the phase to phase voltage on a three phase low voltage system. In a split single phase service, the voltage is 120/240. In a three phase service, the voltage is 120/208. This is due to using a different transformer configuration. In the above, the first number is the phase to neutral reading, and the second is the phase to phase reading. For example, 120/240 and 120/208 both provide 120v on standard receptacles, but when using a dryer, or hot water heater, for example, that appliance would be using 240v or 208v, respectively.


A service drop rated 3-phase, 120/208V is used in small commercial applications where the main voltage in use is either 120V single phase, or 120/208V Network.


Three phase is typically only in commercial applications and used with industrial grade devices such as large motors. The outlets you find in a house will be single phase 120 or 240 volts.


The difference between a single phase and a three phase motor is the amount of power conductors that feed the device. As to the other part of the question a three phase motor will not start or run on single phase. The phase angles on three phase are 120 degrees apart on a single phase system they are 180 degrees apart.


Phasing, or the number of phases in an electrical system has nothing to do with the voltage. A single phase and three phase supply could have a voltage supply of any given voltage from virtually 0 to infinity. Most single phase power supplies in the US are 120/208-240 volt. Three phase power supplies are typically 120/208 to 277/480 volt.


Yes - But only if the three 1 phase supplies are all different phases. if they are the same phase - NO. In other words, to use single phase supplies to supply three phase current you must be able to phase shift the single phase supplies so they are 120 degrees out of phase with each other. If your supply is not able to do this, you would need to purchase some phase shifting transformers. In general, such a method seems very atypical, non ideal, and problematic to me.


Three phase output means three single phase outputs, so to "convert" three phase output to single phase, simply use just one phase or two phases depending on what voltage is supplied and what voltage you want. For example, a 120/208V three phase generator will give you 120V single phase-to-neutral or 208V single-phase (line-to-line) using one or two phases respectively.



One uses single phase AC current (120/240), the other uses 3 phase AC current (most households will not have 3 phase in their home) One uses a 3 phase motor the other a single phase motor. 3 phase is usually used in industrial applications while single phase is used in home or small business apps.


In the United States, 208v can be either. It is generally the voltage supplied by a 120/208v 3 phase system. However, using 2 legs of this 3 phase system is called single phase. I won't go into the reasons for that but just understand that 208v can be either single phase or 3 phase.


No, in North America most homes are wired to a single phase transformer. That transformer is connected to a three phase power grid. Some of the newer apartment buildings and condominium complexes have three phase systems in them. Their voltages are 120/208 as compared to a homes single phase connection of 120/240 volts


(120 X hertz ) divded by the numbver of poles


The number of Volts in a residential service drop in the US is either 120V 2 Wire, 120V-208V Network, or 120-240V 3 Wire. A 120-208V Network service is not single phase, but its 120V portion is.


Is there still 240V 3 phase around? Take 2 legs of the three phase 240V and use them as primary side of a 240 to 120 transformer. This will give you 120V single phase on the secondary side of the transformer. Make sure that you know the load current on the 120V side and size the transformer accordingly.


There are many voltages between what are classed as single phase systems. Any two legs of a three phase system is classed as single phase system. On wye connections there are 120/208, 240/415, 277/480, 347/600, 120/240 just to name a few on the lower voltage scale. On delta systems the leg combinations are L1 - L2, L2 - L3, L3 - L1. Any of these leg combinations are classed as single phase wiring.



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