The 200 amp, 277/480 volt, 3-phase service has a capacity of 166.3 kVA (200 amps x 480 volts x 1.732). At 120/208 volts, 3-phase, this amounts to 461.5 amps. To convert the whole 200 amp, 277/480 volt, 3-phase service to 120/208 volt, 3-phase, a 225 kVA transformer is required. A 230 kVA transformer will also be sufficient.
A transformer's size is based on two factors: voltage and current. The higher the voltage, the smaller the transformer should be. The same thing goes with current; if you increase the amperage, a smaller transformer will do the job.
In your case, since the voltage is constant at 277/480 volts, a smaller transformer will do. A 230 kVA transformer will suffice.
Transformer ratings vary but usually fall in the range of 25 kVA to 500 kVA. Transforms from 100 kVA up are available but they are not commonly used. Transforms from 250 kVA and above are custom made and rarely seen in the market.
Your best option would be to contact an electric company and ask them what size transformer they use in this situation. They should be able to tell you exactly what you need.
Or you can look online for transformer sizes.
If you want a 240V 3-phase variation, you must supply your own transformers. I am now dealing with a school that has a 240/120V 3P4W that the utility is converting to 208Y/120V since they are moving to a 3-phase padmount transformer instead of single-phase units in a vault. The old ones will not work with the new breaker panel so they are getting new ones. It's very expensive ($12K-$15K each) but it's their school district and they can do what they want with it.
The other option is to get a double-conversion transformer. These were common in North America up until about 1990. They cost less than custom-made units and provide only 120V service from the utility. The downside is that they cannot handle load changes like a regular three-phase unit can. For example, if you have two separate circuits loaded with appliances that both switch on at the same time, a triple-conversion unit will break one circuit while still providing power to the other. A double-conversion unit would still give everything off at once because it cannot distinguish between different types of loads.
Finally, there is no reason why you should not be able to use a single-phase transformer for a 3-phase system. All that means is that you will only have access to 1/3 of the voltage when something is plugged into all three slots, which isn't really any safer or more efficient than standard 2-wire operation.
Transformers are not necessary in the home because most homes in the United States are furnished with two 120-volt service lines that may be combined to produce 240 volts. Three-phase power service comes into its own when bigger loads and more power are required. Three-phase electricity is used in the majority of commercial electrical systems.
The word "transformer" comes from the fact that it changes one voltage into another. A transformer takes power from one circuit or channel and distributes it to another. The term single-channel power supply refers to any device that produces electricity from one circuit only; that is, it can operate on either a 110-volt or a 220-volt circuit. Most household appliances require two circuits to function, so they must be plugged into a split-system wiring diagram. For example, if you want to use a hair dryer and a vacuum cleaner at the same time, you will have to put out two sets of wires because they need separate circuits. A three-way switch allows you to choose which outlet supplies power to each appliance.
The amount of power being drawn by your appliances determines how many amps should be carried by their respective circuits. If you plan to use other appliances while you're away from your house, make sure they can handle these currents without burning up. Heating elements and air conditioners tend to be heavy users of power, so they usually need their own independent circuit. Appliances that work on batteries also need their own circuit since they cannot carry current indefinitely.
The primary side voltage will be 480 volts, and the secondary side voltage will be 208 volts/120 volts. On the primary side of this transformer, a three-phase fused disconnect with 300A fuses will be fitted. Copper wires of size 1/0 AWG will be placed from the fused disconnect to the transformer's primary side. These wires are called hot wires because they carry power from the fuse to the transformer.
On the secondary side of the transformer, copper wires of size 2x4 AWG or larger should be used as conductors. The black wire is the hot conductor while the white wire is the neutral conductor. The red wire is the protective ground.
The total current carried by the hot, white, and red wires is called the carrying capacity of the transformer. The transformer must be large enough to carry this current without overheating. A common rule of thumb is that the transformer should be twice as large as needed to handle the maximum load. For example, if you want to be able to supply 200 watts to a load, then the transformer should be at least 400 watts. This way you have some margin in case someone wants to use more than 100 watts of power from the outlet.
In our example, with a carrying capacity of 300 amps, a standard 20 ampere transformer would be sufficient. But if we wanted to be able to supply 500 watts to the load, then a 40 ampere transformer would be needed.
We briefly discussed the 240 volts delivered by the electricity provider to your home. The single-phase power from the utility provider is separated into three wires at the transformer: two line wires and a ground. This is referred to as a single-phase three-wire system or split-phase system. The third wire of the transformer is connected to the metal case of the transformer.
The voltage on each of these lines is 120 volts. The full 360 degrees of rotation of the rotor will result in two rounds of 120 volts each being delivered through the primary coil. Thus, the total voltage across the primary coil will be 240 volts. This energy is then passed on to the secondary coil where it is transformed back into the low-voltage wiring system used within your house for lighting, appliances, and other uses.
The amount of energy that can be transmitted through a conductor is proportional to the square of the current flowing through it. Therefore, if the current through the conductor is increased by a factor of four, the potential energy available for use is increased by a factor of sixteen. In other words, a transformer can increase the voltage applied to its secondary side while keeping the current through it constant. This is why high-voltage transformers are typically larger than low-voltage ones; they need to handle more energy.
A magnetic core is used inside most transformers to guide the flow of magnetic flux through the winding structure.