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Unit 14
TRANSFORMERS
OBJECTIVES
After studying this unit, the student will be able to
- state the purpose of a transformer.
- explain the principle of mutual induction.
· • determine the output voltage of a transformer if the input voltage and turns ratio are
known.
- determine the full-load current of a transformer given the kVA and voltages of the
primary and secondary windings.
- identify the common types of transformers from their schematic diagrams.
- read transformer winding diagrams and connect a transformer for the desired primary
and secondary voltage.
- choose the proper transformer taps to obtain the desired output voltage.
- connect buck and boost transformers to obtain desired voltage for a single-phase appli-
cation.
- choose the correct transformer kVA for the application, given the voltage, current, and
phase requirement of a load.
- size overcurrent protection for dry-type transformers operating at 600 V or less.
- size the feeder conductor for the transformer and wires from the transformer to loads.
- properly ground a transformer, and the secondary electrical system produced by the
transformer.
The purpose of a transformer is to change electrical
voltage to a different value. For example, a farmer has a
large, 480-V, 3-phase motor powering a well. The motor
is in a building, and the farmer wants one 120-V circuit
for a few lights and a receptacle outlet. A transformer is
used to lower the voltage from 480 V to 120 V for the
lighting circuit, Figure 14-1. The controls for furnaces
and air-conditioning units are often operated at 24 V,
Figure 14-1 Dry-type transformers
Figure 14-2. A small transformer inside the equipment
lowers the line voltage to 24 V for the control circuit.
Transformers are frequently used inside electronic equip-
ment.
HOW THE TRANSFORMER
WORKS
A clear understanding of how transformers work is
nectssary in order to wire them properly in an electrical
system. Understanding input and output current and
grounding are particularly troublesome. A dual-voltage
transformer can be ruined when power is applied, if the
connections are made improperly.
An important property of electricity is that a mag-
netic field is produced around a wire in which electrical
current is flowing, Figure 14-3. The more current that
flows, the stronger is the magnetic field. An even
stronger magnetic field can be produced by winding the
wire into a coil. Now the magnetic fields of adjacent
wires add together to form one strong magnetic field.
The electrical current flowing in a transformer is al-
ternating current. The current flows first in one direc-
tion, stops, then reverses and flows in the other direc-
tion. The magnetic field around the winding is constantly
in motion. Figure 14-4 shows the magnetic field during
one cycle. Notice that the north and south poles of the
magnetic field reverse when the flow of current reverses.
Another property of electricity is important to the
operation of a transformer. When a magnetic field moves
across a wire, a voltage is induced into the wire, Figure
14-5. If the wire forms a complete circuit, current will
flow in the wire. If a second coil of wire is placed in a
moving magnetic field, then a voltage will be induced in
this second coil, Figure 14-6. This phenomenon is callec
mutual induction. Alternating current in one winding
produces a moving magnetic field that induces a voltag~
in a second winding. Electrical energy is converted into'
magnetic field and then converted back into electrica
energy in a second winding. The trick is to do this witt
little or no loss of energy.
The magnetic field loses strength quickly in air
therefore, a special steel core is used. The core is com
posed of thin sheets of a silicon-steel alloy. The mag
netic field is concentrated in the core, and energy losse
are reduced to a minimum. Figure 14-6 shows the tw<
windings separated. Most transformers have one wind
ing placed directly over the other to further reduce th(
loss of energy, as shown in Figure 14-7.
Steel core
ac supply
Primary winding
Secondary winding
Figure 14-6 The two transformer windings are on sepa- rate parts of the silicon-steel core.
Secondary winding
Figure 14-7 In most transformers, the two windings are placed one over the other to reduce energy losses.
Step-down transformer
Primary 240 v
Secondary
120 v
Turns ratio 2
VOLTAGE AND TURNS RATIO The input winding to a transfonner is called the pri- mary winding. The output winding is called the second-
ary winding. If there are more turns of wire on the pri-
mary than on the secondary, the output voltage will be lower than the input voltage. This is illustrated in Figure 14-8 for a step-down and a step-up transfonner. Notice that the winding with the greater number of turns has the higher voltage. In Figure 14-8, one winding has twice as many turns as the other. In one case the voltage is stepped down to half, while in the other the voltage is. stepped up to double. It is important to know the ratio of the number of turns of wire on the primary winding as compared to the secondary winding. This is called the turns ratio of the transformer, Equation 14.1. The actual number of turns is not important, just the turns ratio.
Turns ratio Eq. 14.
Number of turns on the pwumy Number of turns on the secondary
The step-down transfonner of Figure 14-8 has 14 turns on the primary, and 7 turns on the secondary; there- fore, the turns ratio is 2 to l, or just 2. The step-up transformer has 7 turns on the primary and 14 on the
secondary; therefore, the turns ratio is 1 to 2, or 0.5. If
one voltage and the turns ratio are known, the other volt- age can be determined with Equations 14.2 or 14.3.
Primary voltage Eq. 14.
= Secondary voltage x turns ratio
Step-up transformer
Primary 120 v
Secondary 240 v
Turns ratio = 0. Figure 14-8 Schematic diagrams of step-down and step-up transformers.
Primary voltage Secondary voltage = Eq. 14. Turns ratio
Use these equations to verify the voltages in Figure 14-8.
Problem 14-
A step-down transformer has a turns ratio of 4 to 1 or
4. If the transformer secondary voltage is 120 V, deter-
mine the primary voltage.
Solution Use Equation 14.2 to solve for the primary voltage.
Primary voltage = 120 V X 4 = 480 V ,
The turns ratio tells us that the primary voltage is four times as great as the secondary voltage.
TRANSFORMER RATINGS Transformers are rated in volt-amperes (VA) or kilo- volt-amperes (kVA). This means that the primary and the secondary winding are designed to withstand the VA or kVA rating stamped on the transformer nameplate. The primary and secondary full-load currents usually are not given. The installer must be able to calculate the primary and secondary currents from the nameplate information. When the volt-ampere (or kilovolt-ampere) rating is given, along with the primary voltage, then the primary full-load current can be determined, using Equation 14. (for a single-phase transformer) or Equation 14.5 (for a 3-phase transformer). Single phase:
or
VA rating Full-load current = ---=- Voltage
kVA X 1 000
Full-load current = -----
Voltage
Three phase:
VA rating Full-load current= ____.::...__
1. 73 x Voltage
Eq. 14.
Eq. 14.
or
kVA X 1 000 Full-load current = ------
1. 73 X Voltage
Problem 14- A single-phase transformer with a 2-kVA rating has a 480-V primary, and a 120-V secondary. Determine the primary and secondary full-load currents of the trans- former.
Solution Use Equation 14.5 to solve for both primary and sec- ondary currents.
Primary full-load current
2 kVA X 1 000
480 v
= 4.17 A
Secondary full-load current
2 kVA X 1 000
120V
= 16.67 A
It may seem strange at first, but the transformer current
will be higher in the winding which produces the lower voltage. This concept is important to understand in order to avoid transformer or conductor overloading. The pri- mary and secondary transformer full-load currents are also related by the turns ratio, as shown in Equa- tion 14.6.
Primary full-load current Eq. 14.
Secondary full-load current Turns ratio
TYPES OF TRANSFORMERS Transformers are of the dry type or oil filled. From 2% to 5% of the electrical energy is lost in a transformer, mostly due to the resistance of the windings. Large trans- formers circulate oil through the windings to remove the heat. Dry transformers use air for cooling. Heat is moved from the windings to the case by conduction in smaller sizes of the dry type. Large dry-type transformers actu- ally allow air to circulate through the windings, Figure 14-9. Oil-filled transformers are used by the electric util- ity, and for industrial or large commercial applications.
Control transformers are designed to withstand short- duration overloads with minimal output voltage drop. Motor starter solenoid coils draw six to eight times as much current when they are closing as is required to hold them closed.
Constant output voltage transformers or voltage reg-
ulating transformers produce a nearly constant output voltage, even though the input voltage may not be con- stant. The voltage supplied by the utility typically will fluctuate up and down a few percent during the day. This voltage fluctuation is of little concern except for certain equipment, such as electronic computers. Installing a constant output voltage transformer to supply sensitive equipment will eliminate undesirable voltage fluctua- tions. Special filters can also be added to these trans- formers to eliminate voltage spikes and electrical noise caused by other equipment operating on the electrical system, Figure 14-12.
CONNECTING TRANSFORMER
WINDINGS
Transformer wiring diagrams are printed on the transformer nameplate which may be affixed to the out- side of the transformer or printed inside the cover to the wiring compartments. The lead wires or terminals are marked with the letters Hand X. Those lettered Hare the
Noise~
Figure 14-12 Voltage spikes and noise distort the normal alternating-voltage sine wave.
primary (high-voltage) leads, and those lettered X are the secondary (low-voltage) leads. Some transformers have two primary and two sec- ondary windings (as shown in Figure 14- I 3) so they can
be used for several applications. These are called dual-
voltage transformers. Connections must be made cor-
rectly with dual-voltage transformers. If connected im- properly, it is possible to create a dead short that will usually ruin the transformer when it is energized. Consider a dual-voltage transformer rated at 240/ 480 V on the primary, and 120/240 V on the secondary. Each of the two primary windings is, therefore, rated at 240 V. Each secondary winding is rated at 120 V. The transformer must be connected so that each primary winding receives the proper voltage. In Figure 14-13, the transformer is shown with the primary windings con- nected in series, with HI and H4 connected to a 480-V supply. The voltage across HI and H2 is 240 V and the voltage across H3 and H4 is 240 V. Each winding is receiving the proper voltage. With each primary winding receiving the proper 240 V, each secondary winding will have an output of 120 V. Connecting the secondary windings in series produces 240 V across XI and X4. Now consider a case where the primary voltage available is 480 V, but the desired output is 120 V. In this case, the primary windings are connected in series, as in Figure 14-13. The secondary windings are, how- ever, connected in parallel, Figure 14-14. This is accom- plished by connecting Xl to X3, and X2 to X4. If this is not done properly, a 240-V dead short will occur. A voltmeter can be used to make sure the connection is correct. Connect X 1 to X3, and then connect a voltmeter between X2 and X4. Energize the primary and read the
H X
240 v (^120) v
480 v
240 v
240 v 120 v
X H
Primary Secondary 240/480 v 120/240 v Figure 14-13 The windings are connected in series to ob- tain the higher of the rated transformer voltages.
480 v
480 v
Primary 240/480 v
H
H
H
H
X
X
Secondary 120/240 v
X
X
120 v
Figure 14-14 The secondary windings are connected in parallel for an output of 120 V. A voltmeter can be used to make sure the transformer is connected properly.
voltmeter. If the connection is correct, the voltmeter will
read zero. If the voltmeter reads something other than
zero, check all primary and secondary connections to make sure they are connected exactly as indicated by the manufacturer. The primary on the example transformer has two windings; therefore, it can also be connected for a 240-V supply. The primary windings must be placed in parallel by connecting HI to H3 and H2 to H4. If this is not done as indicated on the transformer nameplate, the magnetic fields created by each winding will oppose each other. The magnetic fields work together when the windings are properly placed in parallel.
THREE-PHASE TRANSFORMERS Changing the voltage of a 3-phase system can be done with a 3-phase transformer or with single-phase transformers. Three-phase transformers are generally designed and constructed for specific voltages. For ex- ample, a transformer may have a 480-V delta primary and a 120/208-V wye secondary. A typical nameplate for this type of transformer is shown in Figure 14-15. The 3-phase transformer has one core with three sets of windings. A primary and a secondary winding are
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Figure 14-15 Nameplate of a 3-phase transformer
placed one on top of the other on each of the three legs of the core, Figure 14-16. The secondary windings are con- nected in either wye or delta, as required by the load to be supplied. The primary is connected in wye or delta, depending upon the type of electrical system available. Common 3-phase transformer connections, listing pri- mary windings first, are: delta-delta, wye-delta, and delta-wye. A wye-wye connection is usually not recom- mended. In a wye-wye connection, a third harmonic cur- rent may occur, causing possible current overloading and damage to the primary neutral wire. A delta-wye trans- former can usually be substituted. Always be sure to consult the transfo-rmer manufacturer before installing a wye-wye connection.
Problem 14- A building is supplied with a 480-V, 3-phase electri- cal system. Many 120-V circuits are needed; therefore, it is decided to use a 3-phase transformer to step down the voltage to supply a 100-A, 120/208-V panelboard. Which of the following transformers is suitable for this application: a delta-delta, a wye-delta, or a delta-wye?
Figure 14-16 Three-phase transformer construction
Primary A
Secondary
A (^) B c
.-----120 v ----~Pt.,.____ 208 v ---Jo>i (^) Secondary N A (^) B c
Figure 14-18 Dual-voltage, single-phase transformers with 240/480-V primary windings, and 120/240-V secondary wine ings are shown connected to form a 480-V delta to 120/208-V wye, 3-phase, step-down transformer bank.
In order to get an output of 240 V with an input of only 444 V, the turns ratio will have to be changed to 1. 85 to
- The purpose of the tap connections, usually on the primary, is to change easily the transformer turns ratio. A typical single-phase transformer nameplate with pri- mary taps is shown in Figure 14-19.
Winding taps each make a 2 1 /2% change in the volt-
age. A transformer will often have two taps above nor- mal voltage and four taps below normal voltage. A trans- former usually comes preconnected for normal voltage.
If an abnormal voltage is present, it is up to the installer
to change the tap connections.
Problem 14-
A 25-k VA, single-phase transformer is used to sup- ply 120/240 V, 3 wires, to a 100-A panelboard. The pri- mary voltage is only 450 V instead of the normal480 V. Show how to connect the transformer of Figure 14-19 to compensate for the low input voltage.
Solution
The primary windings must be connected in seric because the normal voltage should be 480 V. Therefon check the transformer nameplate for series tap conne< tions. Move down the voltage column until the actu: input voltage fits between two numbers. Usually, th
higher voltage on the chart is used. If the lower numbt
is used, the output will be greater than 240 V. Tr proper connection is shown in Figure 14-20 with
jumper wire placed between taps 5 and 6. If the input he
been 480 V, the jumper would have been between taps and 4. Assume that the input is 225 V and the desired ou put is 120 V. This requires the primary windings to l. placed in parallel. This time, the parallel high-volta! tap connection chart on the nameplate is used. Mark tl
location on the chart where the actual input voltage fit
Then take the next higher tap connection on the chm Figure 14-21 shows the proper connection.
s
tl
e :r
e
a
d 3
t- ~e :e te
s.
t.
TRANSFORMER ONE PHASE DRY TYPE kVA I 5 I H.V.j240/480J RISE oc I 50 I (^) HZ IL___---1 I
Series H.V. Connect Volts 1-2 (^504) 2-3 492 3-4 480
Parallel H.V. Connect Volts
~~
Xl~X
1 120 if7^3
4-5 468 5-6 456 6-7 444
Hl-2 H2-1 252 Hl-4 H2-3 240 H1-6 H2-5 (^228) 7-8 (^432) '-------'-------'--------' '1 20/240 ' r1 X~2 X~
Hl-8 H2-7 216
Figure 14-19 Nameplate of a single-phase transformer with primary taps.
BUCK AND BOOST TRANSFORMERS A buck and boost transformer is an insulating trans- former· which can be connected as an autotransformer. The buck and boost transformer is used to make small adjustments in voltage either up or down. For example, a machine has an electric motor which requires 208 V, but
the electrical supply is 240 V. If ordering the machine
with a 240-V motor is expensive, a less costly solution to
t-ool----- 450 v ----~
Series H.V. Connect Volts 1-2 504 2-3 492 3-4 480 4-5 468 5-6 (^456) ~ 6-7 444 450 v 7-8 432
t-ool---- 240 v -----IP-f
Figure 14-20 Single-phase transformer showing proper tap connection for an input of 450 V, and a 3-wire 120/ 240-V output.
the problem may be to buck the voltage from 240 V down to 208 V with a buck and boost transformer. Low voltage resulting from voltage drop can be cor- rected with a buck and boost transformer, although this is not a good practice except in unusual circumstances. Voltage drop on wires is wasted energy and should be avoided. Buck and boost transformers for single-phase appli- cations have a dual-voltage primary rated at 120/240 V. A choice of two sets of secondary voltages is available, depending upon the amount of boosting or bucking re- quired: 12/24 V and 16/32 V. Three-phase applications from 380 V to 500 V requires the use of a buck and boost transformer with a 240/480-V primary and a 24/ 48- V secondary. A typical buck and boost transformer is shown in Figure 14-22. A buck and boost transformer, when used as an auto- transformer for bucking or boosting, can supply a load which requires several times the k VA rating of the trans- former. The maximum k VA rating of the load supplied depends upon the full-load current rating of the trans- former secondary and the operating voltage of the load. Each manufacturer supplies load current and k VA data for buck and boost transformers for all combinations of input and output voltages. The manufacturer also supplies complete wiring dia- grams for both single- and 3-phase applications. The
I.
l-
e l-
J
The next standard transformer size larger than this mini- mum kVA requirement is chosen. In this example, a 25- kVA, single-phase transformer will be used. This could be the same as the transformer shown in Figure 14-19. Common transformer k VA ratings are shown in Table 14-1.
Problem 14- A farm grain-drying and storage center is supplied with a 277/480-V, 3-phase system. Two 20-A, 120-V circuits are required for lights and receptacle outlets. Determine the minimum kVA rating of the single-phase, 480- V -to- I 20-V step-down transformer.
Solution Two 20-A circuits are required for a total load re- quirement of 40 A at 120 V. The minimum k VA require- ment is determined by using Equation 14.7.
Table 14-1 Common transformer kVA ratings
Transformer kVA Single phase Three phase
120 V X 40 A
Single-phase kVA = = 4.8 kVA 1 000
From Table 14-1, choose a 5-kVA, single-phase trans- former.
Problem 14-
A machine has a 480- V, 3-phase electrical motor as an integral part of the machine. The total machine load
requirement is 10 A at 480 V. If the building has a
240- V, 3-phase electrical system, determine the mini- mum-kVA 3-phase transformer required.
Solution The load requirement is 10 A, 480 V, 3 phase; there- fore, use Equation 14.8.
1.73 X 480 V X 10 A
-phase kVA = -------- 1 000 = 8.3 kVA
From Table 14-1, choose a 9-kVA, 3-phase, 240-V-to- 480- V step-up transformer. Three single-phase transformers can be used to sup- ply a 3-phase load. This is frequently the case when low kVA rating are required, such as in Problem 14-6. When single-phase transformers are used, the rating of each transformer must be not smaller than one-third the 3- phase kVA required. In Problem 14-6, the 3-phase load requirement is 8.3 kVA. Therefore, the single-phase transformer rating must be at least 2. 8 k VA.
8.3 kVA 3
= 2.8 kVA
The next larger common size single-phase transformer is 3 kVA. Therefore, three single-phase, 3-kVA, 240-V- to-480- V step-up transformers can be connected to form a 3-phase transformer bank. The transformer bank will have a 3-phase rating of 9 kVA.
OVERCURRENT PROTECTION Wiring a transformer circuit is one of the most diffi- cult of wiring tasks, unless the installer understands transformer fundamentals. This unit deals with dry-type transformers operating at 600 V and less. Rules for siz-
ing overcurrent protection for this type of transformer are
covered in NEC Section 450-J(b). It must be noted that
these rules apply only to the transformer itself, and not necessarily to the input and output circuit wires. Sizing and protecting transformer input and output wires is cov- ered in the next section. Three methods of providing overcurrent protection
for transformers is covered by the National Electrical
Code. Both the primary and the secondary windings
must be protected. The procedure begins by calculating the primary and the secondary full-load current, using Equation 14.4 for single-phase transformers, and Equa- tion 14.5 for 3-phase transformers. A transformer can be protected by one overcurrent device on the primary side rated at not more than 1. (125%) times the primary full-load current, Figure 14-
- This overcurrent device can be a set of fuses in a panelboard, a fusible switch, or a circuit breaker. Consider that a 25-kVA, single-phase transformer has a 480-V primary and a 120/240-V, 3-wire secondary. The primary full-load current is 52 A.
Primary full-load current 25kVAX =52 A 480 v Maximum ~iz~ = 52 A X 1.25 = 65 A overcurrent device
The maximum size overcurrent device is 65 A. Check-
ing a fuse catalog or NEC Section 240-6, 65 A is not a
Primary
Disconnect ~
Maximum size / 125'Yc, of primary current (see exceptions in text)
Primary protection may be placed here to also protect feeder wire
125'Yo of primary current
Transformer
2-wire secondary
Transformer
2-wire secondary
Figure 14-24 A transformer may be protected with one overcurrent device on the primary, and sized at not more than 125% of the primary full-load current
standard size. However, Exception No. 1 to NEC Section
450-J(b)(l) permits the next higher standard size over-
current device to be chosen. Therefore, the maximum size overcurrent device permitted for this situation is 70 A. A smaller size overcurrent device could have been used; for example, 60 A. In fact, the overcurrent device can be as small as desired as long as it is large enough to satisfy the load requirements. If the primary full-load current is less than 9 A, the primary overcurrent device is not permitted to exceed
1.67 (167%) times the primary full-load current, NEC
Section 450-3( b )(I), Exception No. 1. If the primary cur-
rent is less than 2 A, the overcurrent device is not per- mitted to exceed 3.0 (300%) times the primary full-load
current, NEC Section 450-J(b)( 1).
Consider the case of a 3-kVA transformer stepping down 480 V to 120 V to supply one 20-A single-phase circuit. The primary full-load current is 6.25 A.
Primary current
3 kVA X 1 000
480 v
= 6.25 A
The overcurrent device is sized at 125% of the primary full-load current.
Overcurrent size = 6. 25 A X 1. 25 = 7. 8 A
The next standard size overcurrent device larger thar
- 8 A may be used as long as it does not exceed 167% o· the primary current.
6.25 A X 1.67 = 10.4 A
A 1 0-A time-delay fuse is the maximum size permittee
to protect the 3-k VA transformer. Time-delay fuses an used when the overcurrent device is sized at less thar 15 A. The overcurrent device protecting the primary of < transformer is permitted to be sized as large as 2. 5( (250%) times the primary full-load current, provided th< transformer secondary is also protected, Figure 14- The transformer secondary overcurrent device is sized a 1.25 (125%) times the secondary full-load current. Th' next size larger overcurrent device is permitted excep where the secondary current is less than 9 A. In thi case, the overcurrent device is not permitted to be large than 1. 67 (167%) times the transformer secondary cir cuit. Consider again the 3-kVA, single-phase, 480-V -tc
Not greJter thJn 6 times primJry current
protection
Not greater than 4 times primJry current
protection
Transformer impedance less thJn 6%
t---"-T-- 1 2 5 '!(, of secondJ ry current (see exceptions in text)
Transformer impedance greater than 6% but less than 1 0%
t-'T- 1250AJ of secondary current (see exceptions in text) Figure 14-27 Overcurrent protection for a transformer with factory-installed thermal protection on the primary
WIRE SIZE AND PROTECTION
Electrical wires must be adequate to supply the load
and protected from overcurrent, according to the wire
ampere rating as given in NEC Table 310-16. Therefore,
wire ampere rating must also be considered when sizing
transformer overcurrent devices. An example will help
to illustrate the procedure of sizing feeder wires and
overcurrent protection.
A 1 00-A, 120/208-V, 3-phase panel board is supplied
from a 480-V, 3-phase system through a 37.5-kVA step-
down transformer, Figure 14-28.
The first procedure is to determine the primary and
the secondary full-load currents.
Primary full-load current
37. 5 k VA X 1 000
------- = 45 A
1.73 X 480 V
Secondary full-load current
37. 5 k VA X 1 000
104 A
1.73 X 208 V
The I 20/208- V panel board in the example is rated at
100 A; therefore, it must be protected at not more than
100 A, NEC Section 384-16(a). An overcurrent device
must be provided on the secondary side of the trans-
former because the transformer is 3 phase and the feeder
consists of more than two wires, NEC Sections 384-
16(d) and 240-3, Exception No.5. A fusible disconnect
switch is installed adjacent to the transformer. If 100-A
fuses are installed in the disconnect switch, a main
breaker will not be needed in the 100-A panelboard. The
secondary feeder wire must have an ampere rating of a
least 100 A. No. 3 AWG copper with THWN insuiatior
is adequate.
The primary overcurrent device is not permitted tc
exceed 250% of the primary full-load current.
45 A X 2.5 = 112 A
But, there is no reason to protect the primary at 100 A
which is the largest size fuse that does not exceed 112 A
If the fuses are sized at 110 A, then the wire must b(
rated for 110 A. In this example, find a copper THWl'
wire from NEC Table 310-16 which is rated for at leas
45 A. No. 8 AWG copper THWN wire is rated at 50 A
Therefore, install 50-A fuses in the 480-V panelboard
The disconnect switch in Figure 14-28 must be lo
cated so that the wire from the transformer is not mor
than 25 ft (7 .62 m) in length, NEC Section 240-21, Ex
ception No. 8. This tap conductor must have an amper'
rating sufficient to supply the load; in this case, at leas
100 A. It must end at a circuit breaker or set of fuse
with a rating not greater than the ampere rating of the ta
conductor. The tap conductor must be enclosed in race
way.
.SO-A fuses for
Transformer Pri. 480 V Sec. 120/208 V
- .5 kVA
transformer circuit
480-V fusible panel LNo.8 AWG copper THWN
Tap conductor must not exceed 2.5 ft (7.62 m) in length
No. 3 AWG copper THWN
100-A, 120/208-V Panel
L__ 45ft __j I~ .... (13.72 ml I L 3oft _ I~ (9.14 ml ~I
Figure 14-28 A 3-phase transformer supplying a 120/208-V, 1 00-A panelboard
If the 1 00-A panel board of Figure 14-28 is within
25 ft (7 .62 m) of the transformer, the disconnect switch
can be eliminated, and a set of 100-A main fuses or a
1 00-A main circuit breaker can be installed in the panel-
board, as shown in Figure 14-29. The minimum size
secondary wire would be No. 3 AWG copper THWN.
A 2-wire secondary from a single-phase transformer
is permitted to be protected by the transformer primary
overcurrent device, NEC Section 240-3, Exception No.
5. However, there is a caution. The primary overcurrent
device must be sized so that the secondary wire cannot
become overloaded. Equation 14.9 can be used to deter-
mine the maximum size primary overcurrent device rat-
ing.
Transformer 37.5 kVA Pri. 480 V Sec. 120/240 V 50 A overcurrent device
No. 8 AWG wire copper THWN
Maximum primary fuse rating Eq. 14.
Secondary voltage
Primary voltage
X Secondary wire ampacity
An example will help to illustrate how a primary fuse
can protect the secondary circuit wire. Consider a 2-
kVA, 480-V-to-120-V, single-phase step-down trans-
former supplying one circuit for lights and receptacle
outlets.
First, determine the primary and the secondary full-
load current for the 2-kVA transformer.
Tap not more than 25 feet (7 .62 m) long No. 3 AWG wire copper THWN
Panelboard with 100 A main
Figure 14-29 The panelboard is kept within 25 ft (7.62 m) of the transformer and the fusible disconnect is eliminated. A 1 00-A main breaker is installed in the panel board.
Unit 14 Transformers 2B
Transformer
Use metal r·aceway or run grounding wire
.,__----l- Equipment bonding
Must run grounding wire if metal raceway is not used
Transformer
n
Ji§'
Must run grounding wire if metal conduit is not used
lug
Grounding electrode
Grounding electrode _ building metal frame,
- ..,.___ metal water pipe or other electrode
Bonding jumper
Grounding electrode
conductor -
Grounding electrode building metal fr·ame, metal water pipe - or other electrode Figure 14-31 Methods of grounding and bonding a separately derived system from a transformer
CONTROL TRANSFORMERS
Transformers used only for the purpose of supplying power to control motors and equipment come under the
category of a Class 1 circuit, NEC Section 725-/1. The
installation of these transformers is covered by NEC Ar-
ticle 450, as discussed in this unit. Control transformers
are available with a fuseholder on the primary side to
provide transformer protection, as required in NEC Sec-
tions 725- JJ, 725- I 2, and 430-72.
A grounding electrode and grounding electrode con-
ductor are not required for these transformers as long as
the transformer is rated at not more than I 000 VA
(1 kVA), Exceptions toNEC Section250-26. Groundir
can be accomplished by bonding the wire to be groundt
to the metal frame of the transformer or enclosure.
REVIEW I
Refer to the National Electrical Code when necessary to complete the following
review material. Write your answers on a separate sheet of paper.
1. Explain the purpose of a transformer.
2. As electrical current flows through a wire, is a force field built up around the wire?
3. Describe what will happen if a magnetic field is moved across a wire.
4. A transformer operates on the principal of what kind of induction?
5. A transformer has a primary to secondary turns ratio of 15 to I. If the primary is rated
at 240 V, then the secondary voltage is:
a. 120 V b. 24 v c. 16 v d. 12 v
6. A transformer has a primary rated at 240 V and a secondary rated at 120 V. The
primary to secondary turns ratio is:
a. 4
b. 2
c. 0.
d. 0.
7. The primary full-load current of a 3-phase, 480-V-to-120/208-V step-down trans-
former with a 30-kVA rating is:
a, 36 A
b. 45 A
c. 62 A
d. 83 A
8. A control transformer has a secondary rated 10.4 A at 24 V. The primary is rated
480 V, and the primary to secondary turns ratio is 20 to 1. Of the following, which is
the primary full-load current?
a. 2.08 A
b. 1.22 A
c. 0.80 A
d. 0.52 A
9. Of the following diagrams A, B, C, and D, which represents an autotransformer?
