Preguntas de práctica de ejemplo

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1. 1. Branch Circuits & Feeders

   A 208-volt, single-phase branch circuit supplies a continuous load of 24 amperes. What is the minimum standard overcurrent device rating permitted?

   • a.30 amperes
   • b.25 amperes
   • c.24 amperes
   • d.40 amperes

   Respuesta: a

   Explicación: An overcurrent device supplying a continuous load must be rated at not less than 125 percent of that load: 24 A x 1.25 = 30 A, which is a standard rating in 240.6(A). The 24 A and 25 A choices ignore or under-apply the 125 percent factor, and 40 A jumps past the next standard size above the calculated value.

   Fuente: 2023 NEC §210.20(A)

2. 2. Services, Grounding & Bonding

   A service is supplied by two parallel sets of 250 kcmil copper conductors per phase. What is the minimum size copper grounding electrode conductor?

   • a.4 AWG
   • b.2 AWG
   • c.1/0 AWG
   • d.3/0 AWG

   Respuesta: c

   Explicación: With parallel conductors the equivalent area is the sum of the areas in one phase: 2 x 250 kcmil = 500 kcmil, which falls in the over 350 through 600 kcmil row and requires 1/0 copper. Using 2 AWG treats a single 250 kcmil conductor as the whole service, and 3/0 corresponds to services above 1,100 kcmil.

   Fuente: 2023 NEC Table 250.66

3. 3. Wiring Methods & Materials

   A short raceway nipple no longer than 24 inches connects two enclosures. What fill percentage is permitted?

   • a.31 percent
   • b.40 percent
   • c.53 percent
   • d.60 percent

   Respuesta: d

   Explicación: Notes to Chapter 9, Table 1 permit a nipple not exceeding 24 inches in length to be filled to 60 percent of its total cross-sectional area, because heat buildup and pulling friction are both minimal over such a short run. The 40, 53 and 31 percent values are the normal fills for three or more, one, and two conductors respectively.

   Fuente: 2023 NEC Chapter 9, Table 1, Note 4

4. 4. Motors & Equipment

   A continuous-duty motor over 1 horsepower has no marked service factor and no marked temperature rise. What is the maximum overload device rating as a percentage of nameplate full-load current?

   • a.140 percent
   • b.125 percent
   • c.115 percent
   • d.175 percent

   Respuesta: c

   Explicación: Motors that carry neither a service factor of 1.15 or greater nor a marked temperature rise of 40 degrees C or less are limited to 115 percent of nameplate current, because there is less thermal margin available. The 125 percent value applies to the better-rated motors, 140 percent is the permitted increase when the motor will not start, and 175 percent is a fuse sizing percentage.

   Fuente: 2023 NEC §430.32(A)(1)

5. 5. Branch Circuits & Feeders

   A dwelling has a single household electric clothes dryer with a nameplate rating of 4.5 kilowatts. What value must be used for the dryer load in the service calculation?

   • a.4,500 volt-amperes
   • b.4,000 volt-amperes
   • c.5,000 volt-amperes
   • d.3,600 volt-amperes

   Respuesta: c

   Explicación: The load for a household electric clothes dryer is 5,000 volt-amperes or the nameplate rating, whichever is larger, so a 4,500-volt-ampere nameplate is calculated at the 5,000-volt-ampere floor. Using the nameplate 4,500 value violates the minimum, and the other figures apply factors that do not belong here.

   Fuente: 2023 NEC §220.54

6. 6. Theory & Safety

   As defined in Article 100, ampacity is best described as which of the following?

   • a.The maximum current a conductor can carry continuously under the conditions of use without exceeding its temperature rating
   • b.The current that will instantly melt the conductor
   • c.The rated current of the overcurrent device protecting the conductor, taken directly from the marking on the breaker or fuse without regard to installation conditions
   • d.The current at which voltage drop reaches 3 percent

   Respuesta: a

   Explicación: Article 100 defines ampacity as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. It is a property of the conductor and its environment, not the rating of the protective device ahead of it.

   Fuente: 2023 NEC Art. 100

7. 7. Branch Circuits & Feeders

   A 480-volt three-phase feeder carries 30 amperes 300 feet on 4 AWG copper (41,740 circular mils). Using K = 12.9, what is the approximate voltage drop?

   • a.4.8 volts
   • b.9.6 volts
   • c.5.6 volts
   • d.2.8 volts

   Respuesta: a

   Explicación: Three-phase VD = (1.732 x K x I x L) / CM = (1.732 x 12.9 x 30 x 300) / 41,740, about 4.8 V. Using 2 instead of 1.732 gives the 5.6-V answer. See NEC Chapter 9, Table 8.

8. 8. Services, Grounding & Bonding

   A service uses two parallel sets of 600 kcmil copper conductors per phase. Using the 12.5 percent rule, what is the minimum copper main bonding jumper?

   • a.2/0 AWG
   • b.4/0 AWG
   • c.3/0 AWG
   • d.250 kcmil

   Respuesta: c

   Explicación: Where copper service conductors exceed 1,100 kcmil, the main bonding jumper must be at least 12.5 percent of that area: 2 x 600 = 1,200 kcmil, and 1,200 x 0.125 = 150 kcmil. 2/0 (133.1 kcmil) is too small; 3/0 (167.8 kcmil) qualifies. See NEC 250.28(D)(2).

9. 9. Wiring Methods & Materials

   A 15-ampere, 125-volt receptacle is installed in a damp location such as under a roofed open porch. What cover is required?

   • a.A cover that is weatherproof when no attachment plug is inserted
   • b.A cover that is weatherproof whether or not a plug is inserted
   • c.No cover is required in a damp location
   • d.A standard indoor wall plate

   Respuesta: a

   Explicación: In damp locations a receptacle needs an enclosure weatherproof when a plug is not inserted; the stricter in-use cover, weatherproof with a plug inserted, is required in wet locations. See NEC 406.9(A).

10. 10. Wiring Methods & Materials

    Type NM cable entering a single-gang nonmetallic box that has no cable clamps must be secured within what distance of the box?

    • a.24 inches
    • b.8 inches
    • c.3 feet
    • d.4 and 1/2 feet

    Respuesta: b

    Explicación: Where a single-gang nonmetallic box without internal clamps is used, the NM cable must be secured within 8 inches of the box. See NEC 314.17(C) Exception and 334.30(A).