This document discusses three-phase power systems. It explains that three-phase systems transmit power using three circuit conductors with currents that reach peak values 120 degrees out of phase. This allows for constant power transfer over each cycle. Common three-phase voltages used include 120/208V, 240V, and 277/480V. The document provides an example showing how a 10kW heater would require a smaller breaker and wire size if powered by three-phase 208V or 480V rather than single-phase 120V.

1. Electrical Breakfast
Club
8/17/07
Voltages

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. 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. 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. 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. 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. 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. 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. 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. Why √3

Three phase wye and delta

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. 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. 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. 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.