Second Year, Third Quarter Review - 2017NEC, Based on Lessons 219 to 226

1,500,000

1500 kV = ___ volts

177 I line▲ = I coil▲ x √3 = (102 x 1.732) = 176.67

A 3Ø load is connected to the delta-connected secondary of a transformer. The transformer has 102 amps of current flowing in each secondary winding. Between the load and the transformer, ___ amps of line current is flowing in the conductors.

167x3 = 501

If three single-phase, 167 KVA transformers are connected together to form a three-phase delta bank, what will be the total KVA rating?

wye

Refer to Figure 221.201. The transformer bank has a ___-connected secondary ("X" leads).

277

Refer to Figure 221.401. The winding (coil) voltage produced by the power supply in Winding A is ___ volts.

38.5 x [480 x 1.732] = 32kW

Refer to Figure 221.403. If Ammeter 2 indicates 38.5 amps, the motor is using ___ kW of power.

72.2

Refer to Figure 221.403. If the nameplate rating of the 3Ø motor is 60 KW at 480 volts, Ammeter 2 will indicate ___ amps at the applied voltage.

17200 ÷ 240 = 71.66

A 240-volt, 17.2 KVA single-phase load is fed from a circuit that originates in a 120/240-volt, three-phase, 4-wire panel. How much current will the load draw?

12

A 45 KVA, 480 to 208Y/120-volt dry-type transformer is located in front of a plywood telephone equipment board that is attached to the wall. The general rule in the NEC® is that the minimum clearance from the transformer to this combustible wall covering is ___ inch(es). NEC 2017 450.21(A)

engine or motor

A prime mover is the ___ component of a generator set.

226.13 I coil = I line = P 3Ø ÷ (1.732 x E line) 94000 ÷ (1.732 x 240) = 226.13

An open-delta bank of transformers uses 100 KVA transformers and supplies 120/240 volts to a 94 kVA 3Ø load. Each transformer in this bank has ___ amps of current traveling through the windings.

out from

Doors for transformer vaults shall open ___ the vault.

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? Obj 220.2 Worksheet

3

Individual dry-type transformers rated over 112½ kVA shall be enclosed in a fire-resistant room with materials having a ___-hour minimum fire rating.

all of these

Lifting generator sets during installation requires special care to avoid damage to ___. a. fuel tanks b. radiators c. shrouds d. all of these e. none of these

702

Optional standby systems are covered by NEC® Article ___.

open delta

Refer to Figure 220.301 where the secondary voltage is 240 volts. This is a(n) ___ transformer bank.

10

The maximum size fuse permitted, among those listed below, to protect the primary of a 480 to 240 V, 1Ø, 3 kVA dry-type transformer is ___ amps. The secondary conductors are 14 AWG copper THHN/THWN and overcurrent protection is not provided on this secondary. Although NEC® 450.3 permits a certain OCPD rating for transformer protection, don't forget to check NEC® 240.4(F) for secondary conductor protection requirements.

25.44

The secondary voltage from a buck-boost transformer with a nameplate rating of 1.0 KVA can be expected to be about ___ volts with the 24-volt load disconnected. The supply circuit voltage is 120 volts. OBJ 224

no moving parts

Transformers are electrical devices that contain ___.

additive

When buck-boost transformers are connected to boost the input voltage, the primary and secondary voltages are ___.

I

Which of the drawings, shown in Figure 224.101, is connected as an autotransformer?

4/0

A 225 KVA 480 - 120/208-volt,3Ø, 4-W transformer feeds a panelboard with a 225-amp main breaker. The length of the secondary conductors between the transformer and the main is 9 feet. These conductors must be at least ___ in size where the primary breaker is 300 Amps.

250 kcmil [Secondary ampacity] = [Epri line ÷ Esec line] x [OCPpri ÷ 3] [480 ÷ 240] x [300 ÷ 3] = 230.8amps => 250 kcmil versus 200 amp main breaker => 3/0

A 300-amp breaker supplies power to a 225 KVA 480 - 120/208-volt, 3Ø, 4-W transformer. The transformer feeds a panelboard with a 200-amp main breaker. The length of the secondary conductors between the transformer and the main is 13 feet. These conductors must be at least ___ in size. 240.21(C)(6)(1) OBJ 223.5 Worksheet

I line = P3ø ÷(E line x 1.732) = 59200 ÷ (240 x 1.732) = 142.4

A 3Ø 240-volt load is supplied power by a 100 KW generator. The nameplate rating on the load is 59.2 KVA. The line (circuit) conductors between the generator and the load will be carrying ___ amps.

I= 21000 ÷ (240 x 1.732) = 50.5

A 3Ø load will consume 21 kVA. The circuit is fed from breakers in a 120/240-volt 3-ph, 4-W panelboard. The line current drawn by the load is ___ A.

164.6 AØ = BØ = CØ = 84+18+34+62=198 amps 198amps x 480v x 1.732 = 164.614

A 480-volt, 3-phase panel has nine 2-pole breakers that supply a total parking lot lighting load of 252 amps. The lighting load is equally divided among the 2-pole breakers so that each phase is carrying 84 amps. The panel also has three 3-pole breakers that supply three-phase loads of 18, 34, and 62 amps respectively. What is the total load in KVA?

5678

A 45 KVA transformer is fed from a 3-pole breaker in a 277/480-volt panelboard. The transformer then supplies power to a 120/208-volt, 3Ø, 4-wire panelboard. The transformer has Z = 2.2% on its nameplate. The available line-to-line (bolted short-circuit) fault current on the secondary is ___ amps. Any OCP installed on the secondary of this transformer should have an interrupting rating of at least this value. Use ISC = VA ÷ (Z x ESEC x 3)

I rated pri = 150000 ÷ (7200x1.732) = 12.03 = OCPD = 3x12.03 = 36.1 = 35 amps

A 7200 to 4160-volt, 3Ø, 150 kVA transformer is installed in a supervised location and is supplied from a feeder tap that is protected with a 3Ø fused safety switch. The transformer supplies power to a circuit breaker in an enclosure. As limited by NEC® 450.3, the maximum size fuses permitted to be installed in the safety switch is ___ amps. The transformer Z = 7%.

P single xfmr = (P3Ø load ÷ 3) = (69÷3) = 23 kva

A bank of transformers will require that three transformers with ratings of at least ___ KVA each be used to supply enough power at 120/240 volts to a 69 KVA 3Ø load.

both a and b

K-Rated non-linear load isolation transformers have Faraday shields that ___. a. help eliminate electrical noise b. help eliminate voltage spikes c. both a and b d. neither a nor b

4.92 E=Vd= I X R = (9.8)(2.01 ohms/1000 feet)(150 +150) =5.9094 volts

Voltage Drop Scenario #1 For a load, the conductors consist of two 12 AWG THHN/THWN solid coated copper conductors connected to the neutral bar and a 1-pole breaker in a 120/208-volt, 3Ø, 4-wire panelboard. The circuit serves a load that draws 9.8 amps. The distance between the panel and the load is 150 feet. Refer to Voltage Drop Scenario #1. Use NEC® Table 8 resistance and Ohm's Law to find the voltage dropped in the conductors. The voltage dropped in the circuit conductors is ___%.

Vd = (1.732 x K x I x L) ÷ (cma) (1.732)(12)(12.9)(164) ÷ (10380) = 4.24volts ÷ 240volts= 1.77 %

Voltage Drop Scenario #7 A load draws 12.9 amps and the distance between the load and the panel is 164 feet. The circuit to the load consists of three 10 AWG THHN/THWN stranded uncoated copper conductors that are connected to a 3-pole breaker in a 120/240-volt, 3Ø, 4-wire panel. Use K = 12, as required in any calculations. Refer to Voltage Drop Scenario #7. The voltage dropped in the circuit conductors is _____%. Use Vd = ( √ 3 × K × I × L) ÷ (cma)

3

Where a transformer is installed in a vault on the 3rd floor of a high-rise building and is not protected by an automatic fire suppression system, the minimum fire rating for the floor is ___ hour(s). NEC 450.42

III

Which connection diagram in Figure 225.107 should be used where the Supply Circuit is from 2-pole breaker in a 277/480-volt, 3-ph, 4-W panelboard? Due to voltage drop, the available voltage at the load is only 438 volts. However, the load is rated to operate at 480 volts. The transformer is a 240 × 480 - 24/48 volt.

I

Which connection diagram, shown in Figure 224.108, should be used where the Supply Circuit is from a 1-pole breaker 120/240-volt, 3-ph, 4-W panelboard? The load is rated to operate at 16 volts. The transformer is a 120 x 240 — 16/32 volt. Obj 224.2 Worksheet

II only

Which of the illustrations shown in Figure 210.1 would satisfy the Code requirements of 240.22(1)?