Lesson 216 - Single-Phase Transformers: Dual-Voltage, Fault-Current, Code Calculations

A 10 KVA transformer has a 120/240-volt secondary. Each of the two secondary windings can carry ___ amps without being overloaded.

a. 41.67

For a single-phase transformer with a dual-voltage secondary, the voltages induced in each secondary winding are equal.

a. True

Live parts to which an employee may be exposed shall be ___.

c. de-energized

A 20 KVA transformer feeds a 120/240-volt, 1Ø panel. Seven 120-volt lighting circuits, that will draw 11.2 amps each, are to be connected. Additionally, three 120-volt pieces of equipment are to be connected. Each equipment draws 7.8 amps. Which of these circuit assignments will most closely balance the transformer?

a.

Using an ohmmeter on the transformer-winding connection points, ___ would be read between X1 and X4. Refer to Figure 216.37.

a. (or OL)

Using an ohmmeter on the transformer-winding connection points, as shown in Figure 216.34, ___ would be read between X1 or X2 or H1 or H2 and the metal core.

a. (or OL)

Refer to Figure 216.31. Using an ohmmeter on the autotransformer, ___ ohms would be read between points 1 and 3.

a. 0

Refer to Figure 216.38. If the leads of an ohmmeter are placed between L1 & L2, the meter should indicate a reading in the range of ___.

a. 0.2 to 20

Refer to Figure 216.38. If the leads of an ohmmeter are placed between L2 & the shielding wire connection, the meter should indicate a reading in the range of ___.

a. 0.2 to 20

Refer to Figure 216.38. If the leads of an ohmmeter are placed between N & the shielding wire connection, the meter should indicate a reading in the range of ___.

a. 0.2 to 20

Refer to Figure 216.21. The transformer, feeding MDP, must supply ___ volts to the Main Distribution Panel.

a. 120/240

A 37.5 KVA, 240-Volt, 1Ø transformer loaded at 85% is delivering ___ amps to the connected loads.

a. 132.8

Refer to Figure 216.33. Using an ohmmeter on the transformer-winding connection points, ___ would be read between X1 and X2.

a. 2 to 20

A transformer is located in the rear corner of a residential lot. The transformer supplies four homes. It is marked 167 kVA, 120/240 volts, 2% impedance. The available line-to-line fault current at the load terminals of the transformers is ___ amps.

a. 34,792

A transformer feeds the MDP shown above in Figure 216.21. The transformer must have a minimum (calculated) rating of ___ KVA to serve the following loads:

a. 38.08

A transformer with a nameplate rating of 240/480-V pri — 120/240-V sec can be connected to a 2-pole breaker in a ___ panelboard and deliver nameplate-rated voltage to a load.

a. I or II or III

Your journeyman is using an ohmmeter to troubleshoot a single phase transformer that has only one primary winding and one secondary winding. He tells you that the "H" leads are leaking to the case. Your conclusion should be that the primary winding is ___.

a. bad

For a single-phase transformer with a dual-voltage secondary, the transformer power rating is the total power ___ can supply.

a. both secondary windings

It is the ___ responsibility to connect equipment grounding conductors to the metal enclosure of the transformer.

a. electrician's

When selecting a single-phase transformer, select one with a rating that is ___ to the capacity required by the load.

a. greater than or equal

When working with energized circuits, safety-related work practices shall be employed to prevent electrical ___.

a. shock

Using an ohmmeter on the transformer-winding connection points, ___ would be read between X2 and X3. Refer to Figure 216.37.

b. (or OL)

Refer to Figure 216.35. If the ohmmeter probes (leads) are placed between X4 and the metal core (not shown), ___ would be indicated on the meter.

b. 2 to 20

Using an ohmmeter on the transformer-winding connection points, ___ would be read between X3 and X4. Refer to Figure 216.37.

b. 2 to 20

A 120/240-volt, 1Ø, 3-wire panelboard is to be fed from a transformer. The minimum calculated (NOT STD) size of the transformer is ___ kVA where the following loads are connected:

b. 21.512

Employees working in areas with potential electrical hazards shall use ___ to protect specific parts of the body.

b. PPE

Generally, transformer lugs and the installation hardware for these ___ shipped with the transformer.

b. are not

Transformers with ventilating openings shall be installed so that the ventilating openings are not blocked by walls or other obstructions. The required clearances shall be ___.

b. clearly marked on the transformer

Your journeyman is using an ohmmeter to troubleshoot a single phase transformer that has only one primary winding and one secondary winding. He tells you that there is continuity between the between the two "H" leads. Your conclusion should be that the primary winding is ___.

b. good

The impedance of a transformer ___ shown on the nameplate.

b. is

Refer to Figure 216.22. The transformer nameplate rating is 37.5 KVA with these voltage ratings: 240 x 480 primary volts - 120/240 secondary volts. Where the loads are as listed below, the transformer is ___.

b. operating within its ratings

Using an ohmmeter on the transformer-winding connection points, as shown in Figure 216.34, ___ would be read between X1 or X2 or H1 or H2 and the metal transformer enclosure.

c. (or OL)

Refer to Figure 216.35. If the ohmmeter probes (leads) are placed between X3 and the metal transformer enclosure (not shown), ___ would be indicated on the meter.

c. 0

Refer to Figure 216.31. Using an ohmmeter on the autotransformer, ___ ohms would be read between points 1 and 4.

c. 2 to 20

Refer to Figure 216.33. Using an ohmmeter on the transformer-winding connection points, shown below, ___ would be read between H1 and H2.

c. 2 to 20

An existing transformer is to be replaced with a new one. The specifications require that a new transformer be sized at 125% of the existing load to allow for additional loads in the future. The minimum calculated size of the transformer is ___ kVA. The existing load connected to the existing transformer is 79.2 kVA.

c. 99

___ shall be used to warn employees about electrical hazards that may endanger them.I. Safety signs II. Accident prevention tags

c. either I or II

Lugs for transformers can be made of ___.

c. either a or b

Some transformers are provided with two secondary windings (coils). When the current through each of these two windings is equal, the transformer is ___.

c. electrically balanced

When working near exposed energized conductors, each employee shall use ___.

c. insulated tools

To allow for future expansion, the specifications limit transformer loading to 85%. The maximum load that can be connected to a 150 KVA transformer is ___ kVA.

d. 127.5

When fully loaded, a 480-volt pri/240-volt sec 37.5 KVA 1Ø, 3-wire transformer can deliver ___ amps to a 240-volt load.

d. 156.25

A 240-volt fused safety switch is fed from a 100 KVA transformer that has Z = 1.8% on its nameplate. If the two 1Ø conductors going to the 240-volt switch are "shorted" together, about ___ amps of current will flow through the transformer winding. The fuses in the safety switch should have an AIC rating of at least this many amps.

d. 23,148

Open-type transformers are permitted to be installed in a ___. One of these transformers is shown at the right.I. stainless steel control cabinetII. fiberglass combination starter enclosureIII. sheetrock-covered furdown (soffit)

d. I or II only

A 75 KVA 1Ø transformer has a 120/240-volt secondary. The existing load is 193 amps on L1 and 187 amps on L2. Load(s) ___ could be added without overloading the transformer.

d. either I or II, not both

Nine-foot lengths of 1/0 AWG copper THHN/THWN conductors are installed from a 120/240-V, 1-ph, 3-wire panel to the secondary terminals on a 50 KVA transformer. The maximum rating of the main breaker in the panel is ___ amps. The breaker terminals are rated at 75ºC.

e. 150

A 150 KVA 120/240-volt, 1Ø, 3-wire transformer has Z = 2.07% on its nameplate. The available line-to-line (bolted) fault current on the secondary is ___ amps. The main breaker in the panelboard that this transformer feeds should have an AIC rating of at least this value.

e. 30,193

The load to be connected to a new transformer is 88.2 KVA. The specifications require that the transformer be sized at 120% of the connected load to allow for future expansion. The minimum calculated size of the transformer is ___ kVA.

e. none of these