Chapter 10 The Service Entrance
181
Transformer Rule
Mathematically stated, the total voltage
in the primary coil is to the total voltage in
the secondary coil as the number of turns in
the primary is to the number of turns in the
secondary. This is the transformer rule and it
can be expressed as:
Voltage of Primary
Voltage of Secondary
Turns of Primary
Turns of Secondary
=
Power into and out of a transformer is
(ideally) the same. Since voltage changes within
the transformer, so must amperage. Remember,
power equals volts × amperes. For this reason,
the amperage must change proportionately but
inversely, as does the voltage. This can be stated
mathematically as:
Primary Voltage
Secondary Amperage
Secondary Voltage
Primary Amperage
=
For example, suppose the primary voltage
entering a transformer is 12 volts and the
primary amperage is 20 amperes. What will the
secondary amperage be if the secondary voltage
is 120 volts?
Using the equation just given:
12 V
x
120 V
20 A
=
by cross-multiplying
120 V
20 A
=
12 V
x
120 x = 240
x = 240/120
x = 2 Amperes
Since a transformer changes voltage, it can
either increase or decrease the primary voltage.
One that increases the primary voltage is called
a step-up transformer. One that decreases the
primary voltage is called a step-down trans-
former, Figure 10-30. Directions for selecting the
proper transformer to match the electrical load
are found at the end of this chapter.
Service Designation
The power delivered by the power company
is routed through a chain of transformers,
Figure 10-31. The last transformer is near the
transformer rule: The total voltage in the primary coil is
to the total voltage in the secondary coil as the number
of turns in the primary is to the number of turns in the
secondary.
Primary
Secondary
AC In
AC Out
Magnetic flux
Core
Figure 10-29. Application of alternating current causes
a magnetic fl ux within the transformer core. This
magnetic fl ux creates a current in the secondary side
(outgoing) of the transformer.
Step-up
transformer
Step-down
transformer
10 V
1 A
10 VA
20 V
1/2 A
10 VA
20 V
1/2 A
10 VA
10 V
1A
10 VA
10 Turns
5 Turns
5 Turns
10 Turns
Figure 10-30. Step-up transformers increase the voltage
and step-down transformers decrease the voltage.