224
II only
Each transformer in the bank, shown in Figure 220.402, is rated to deliver 120/240 when rated primary voltage is applied. Which diagram shows the correct connections for a 120/208-volt, 3-ph, 4-wire secondary?
If the load connected to the secondary of a delta-connected transformer is balanced, the coil currents are equal in each of the three windings.
True
Where a delta bank of three transformers is installed to serve a load that is comprised mostly of 120-volt loads (balanced), each power transformer must supply ___ required by the 120-volt loads.
about a third of the current
When a 3Ø, 4-W supply system delivers 120/208 volts, the system is ___ connected.
wye
When a 120/240-volt, 3Ø, 4-W open-delta bank of transformers delivers power to a load, the AØ-to-CØ voltage is ___ volts.
240
When a 3Ø, 4-W supply system (typically a transformer) delivers 120/240 volts, the system has coil (or winding) voltages that are ___ volts.
240
A ___ output could be produced by connecting three individual 240 × 480 -120/240 volt single-phase transformers into a bank.
240 volt 3Ø, 3-wire or 120/240 volt 3Ø, 4 wire
A 480 - 240-volt, Δ- Δ transformer is connected to a circuit with a voltage of 518. If compensating taps are not used, the secondary line voltage will be ___ volts.
259
A wye-connected transformer secondary has a line-to-line voltage of 480 volts. The voltage across each winding is ___ volts.
277
___-connected primaries are more common.
Delta
When a 120/208-volt, 3Ø, 4-W supply system (typically a transformer) delivers power to a load, the coil (or winding) voltage equals ___.
EL ÷ 1.732
When a 120/240-volt, 3Ø, 4-W open-delta bank of transformers delivers power to a load, the line current equals ___.
Icoil
When a 277/480-volt, 3Ø, 4-W supply system (typically a transformer) delivers power to a load, the coil (or winding) current equals ___.
IL
In a 240-volt ungrounded delta, what is the voltage between B phase and ground?
0 volts
A 3Ø transformer supplies 120/240 volts to a 3Ø, 4-wire panelboard. From the breaker in space ___, there will be 208 volts to ground (or neutral).
10
A wye-connected transformer secondary has a line-to-line voltage of 208 volts. The voltage across each winding is ___ volts.
120
When a 120/240-volt, 3Ø, 4-W open-delta bank of transformers delivers power to a load, the AØ-to-neutral voltage is ___ volts.
120
In a 240-volt corner-grounded delta, what is the voltage between C phase and ground?
0 volts
A 480 Δ — 240-volt 3-wire Δ transformer is delivering power to a 3Ø load and has 75 amps of coil current flowing through each winding. The current flowing in each circuit conductor to the load is ___ amps.
129.9
A 3Ø transformer bank supplies 120/240 volts to a 3Ø, 4-wire panelboard. There is a 30 kVA 3Ø load connected to a 3-pole breaker in spaces 1-3-5. There is a 240-volt 15 kW load connected to a 2-pole breaker in spaces 2-4. There is a 120-volt 10 kW load connected to a 1-pole breaker in space 6.Refer to Scenario 220.21. If all the loads are operating and a clamp-on ammeter were placed around the AØ feeder conductor to the panel, there should be ___ amps of current flowing in the feeder conductor.
134.7c
An open-delta bank of transformers uses 100 KVA transformers. This bank can deliver a maximum of ___ kVA of power.
173
When a 120/240-volt, 3Ø, 4-W open-delta bank of transformers delivers power to a load, the BØ-to-neutral voltage is ___ volts.
208
When a 3Ø, 4-W supply system delivers 120/208 volts, the system has line-to-line voltages that are ___ volts.
208
A closed-delta bank of transformers delivers 120/240-volt power to several loads. The 1-phase loads are drawing 120 amps on AØ and 120 amps on CØ. The bank is also supplying power to a 3Ø load that draws 46 amps.Refer to Scenario 220.23. The approximate calculated size required for the "lighting" transformer(s) has a rating of ___ KVA.
26
A transformer has a 120/208-volt wye secondary. The transformer is delivering 75 amps of line current to a 3Ø load. This 3Ø transformer must be rated for at least ___ KVA to avoid being overloaded.
27.020
When a 3Ø, 4-W supply system (typically a transformer) delivers 277/480 volts, the system has line-to-neutral voltages that are ___ volts.
277
The primary of a 112.5 KVA transformer is 480-volt delta. The secondary is 240-volt delta and is delivering 100 amps to a 3Ø load. Given these conditions, ___ amps of current are flowing through each primary winding.
29
The delta-connected secondary of a transformer is delivering a line current of 54 amps. What is the phase (coil or winding) current? The secondary voltage is 240 volts line-to-line.
31.2 amps
An open-delta bank of transformers delivers ___ power.
3Ø
If the output of a 3Ø generator is 208 volts, line-to-line, and it has a neutral connection, this is a ___ system.
3Ø, 4-W
A bank of three 100 KVA transformers is delivering 120/240 volts to a building service. This bank can deliver ___ amps of current to the service if one of the transformers goes out.
417
A 3Ø, 45 KVA load will be supplied power by a 3Ø, 480 Δ to 120/208-volt "Y" transformer bank. This transformer bank must have a 3Ø rating of at least ___ KVA.
45
Three-phase loads of 16, 42, and 75 amps are fed from a 208-volt panel. This total 3Ø load is ___ KVA.
47.9
Without exceeding its rating, the maximum line current a 500 KVA 480-volt, three-phase delta secondary transformer can deliver is ___ amps.
601
A 300 KVA transformer has a 480-volt delta primary and a 240-volt secondary. The delta secondary is delivering a total line current of 150 amps to several 3Ø loads. Primary and secondary conductors are the minimum size required to serve the load. Refer to Scenario 220.52. The primary line conductors must be sized to carry at least ___ amps.
75
A 480 to 120/208-volt wye transformer is delivering power to a 3Ø load and has 75 amps of coil current flowing through each winding. The current flowing in each circuit conductor to the load is ___ amps.
75
A 120/208-volt panel has 80 amps of 120-volt loads connected to AØ 1-pole breakers, 80 amps of 120-volt loads connected to BØ 1-pole breakers, and 80 amps of 120-volt loads connected to CØ 1-pole breakers. The panel also has three sets of 3-pole breakers that supply three-phase loads of 20, 60, and 80 amps.Refer to Scenario 221.13. The smallest calculated size 480 Δ - 208/120 3Ø, 4-W wye transformer that can supply this panel is a ___ KVA.
86.4
The secondary of a 300 KVA transformer delivers power to a 180-amp, 3Ø load. This is a 480 — 240 volt Δ-Δ transformer. Primary and secondary conductors are the minimum size required to serve the load.Refer to Scenario 220.53. The primary line conductors must be sized to carry at least ___ amps.
90
I only
Each transformer in the bank, shown in Figure 220.402, is rated to deliver 120/240 when rated primary voltage is applied. Which diagram shows the correct connections for a 120/240-volt, 3-ph, 4-wire secondary?
When a 120/240-volt, 3Ø, 4-W open-delta bank of transformers delivers power to a load, the line voltage equals ___.
Ecoil
When a 120/208-volt, 3Ø, 4-W supply system (typically a transformer) delivers power to a load, the line voltage equals ___.
Ecoil (or winding) × 1.732
When a 120/240-volt 3Ø, 4-W supply system (typically a transformer) delivers power to a load, the line current equals ___.
Icoil × 1.732
open delta
Refer to Figure 220.301 where the secondary voltage is 240 volts. This is a(n) ___ transformer bank.
delta
Refer to Figure 220.401. The transformer bank has a ___-connected secondary ("X" leads).
series
Refer to Figure 220.401. The transformer bank has each of its secondary ("X") windings connected in ___.
delta-delta
Refer to Figure 220.401. The transformer bank is a ___ connected bank.
