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# Eb cvoltages[1]

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### Eb cvoltages[1]

1. 1. Electrical Breakfast Club 8/17/07 Voltages
2. 2. Determining Amperage Determining Amperage on a circuit: Power (Watts) = V (volts) x A(amps) Example: What is the amperage of 10 receptacles on a branch circuit? Per NEC a receptacle = 180 watts per device. Therefore (10) 180 watts = 120 volts x amperage. Amperage = 1800 watts/120 volts = 15 amps  Size circuit for a 20A/1 pole circuit breaker. 
3. 3. Determining Amperage Example: What is the amperage of 10 Kw electric heater – 120 volts single phase? Assume small fan and heating coil = 10500 watts total watts. Therefore 10500 watts = 120 volts x amperage. Amperage = 10500 watts/120 volts = 87.5 amps  Size circuit for a 90A/1 pole circuit breaker. Would need #3 AWG, copper conductors.  Very large branch circuit is there a better way?
4. 4. Three-phase power a. Three-phase electric power is a common method of electric power transmission. A three-phase system uses less conductor material to transmit electric power than equivalent single-phase, two-phase, or direct-current systems at the same voltage. b. In a three-phase system, three circuit conductors carry three currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electrical current. This delay between "phases" has the effect of giving constant power transfer over each cycle of the current.
5. 5. Three-phase power c. Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).
6. 6. Power Generation a. At the power station, an electrical generator converts mechanical power into a set of alternating electric current, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but offset in time to give different phases. In a three-phase system the phases are spaced equally, giving a phase separation of one-third one cycle.
7. 7. Voltage a. The line-to-line voltage of a three-phase system is √3 times the line to neutral voltage. Where the line-to-neutral voltage is a standard utilization voltage, individual single-phase utility customers or loads may each be connected to a different phase of the supply. Where the line-to-neutral voltage is not a common utilization voltage, for example in a 277/480 V system, singlephase loads must be supplied by individual step-down transformers.
8. 8. Why three phases a. The power plant produces three different phases of AC power simultaneously, and the three phases are offset 120 degrees from each other. There are four wires coming out of every power plant: the three phases plus a neutral or ground common to all three. If you were to look at the three phases on a graph, they would look like this relative to ground:
9. 9. Three Phases  In 3-phase power, at any given moment one of the three phases is nearing a peak. High-power 3phase motors (used in industrial applications) and things like 3phase welding equipment therefore have even power output. Four phases would not significantly improve things but would add a fourth wire, so 3phase is the natural settling point.
10. 10. Why √3  Three phase wye and delta
11. 11. Common Voltages     120/240 volt – 1 phase – 3 wire. Standard residential voltage 240 volt – 3 phase – 3 wire. Industrial voltage 120/208 Volt – 3 phase – 4 wire. Commercial voltage 277/480 volt – 3 phase – 4 wire. Commercial voltage
12. 12. Back to our 10 KW heater. Benefits of three phase: Example: What is the amperage of 10 Kw electric heater – 120 volts – single phase?  Assume small fan and heating coil = 10500 watts total watts.  Therefore 10500 watts = 120 volts x amperage.  Amperage = 10500 watts/120 volts = 87.5 amps  Size circuit for a 90A/1 pole circuit breaker. Would need (2) #3 AWG, copper conductors.
13. 13. Back to our 10 KW heater. Benefits of three phase:     Example: What is the amperage of 10 Kw electric heater – 208 volts – three phase? Assume small fan and heating coil = 10500 watts total watts. Therefore 10500 watts = 208(1.73) volts x amperage. Amperage = 10500 watts/360 volts = 29.1 amps Size circuit for a 30A/3 pole circuit breaker. Would need (3) #10 AWG, copper conductors.
14. 14. Back to our 10 KW heater. Benefits of three phase:     Example: What is the amperage of 10 Kw electric heater – 480 volts – three phase? Assume small fan and heating coil = 10500 watts total watts. Therefore 10500 watts = 480(1.73) volts x amperage. Amperage = 10500 watts/831 volts = 12.6 amps Size circuit for a 15A/3 pole circuit breaker. Would need (3) #12 AWG, copper conductors.