NEC Practice Test (Electric)
The full-load current rating, in amperes, of a 150 kVA single-phase transformer with a secondary voltage of 120/240 installed in a multi-family dwelling is _____.
To find the full-load current rating of the single-phase transformer apply the single-phase current formula as shown: I = (150 kVA x 1000) / 240 volts = 15,000/240 = 625 amperes
Conductors with _____ marked on the insulation are not allowed to be used in wet locations.
As shown in Table 310.104(A), conductors with THHN insulation are approved for use in dry and damp locations only and conductors with THWN, THHW, and THW insulation are recognized for use in dry and wet locations.
The MINIMUM length of the free conductor must be left at each junction box for splicing conductors or connecting to luminaires or devices is.
As per Section 300.14 at least 6 in. of free conductor is to be left at each junction box for splices or the connection of luminaires or devices. The intent of this rule is to ensure enough conductor slack for the terminal connections or splices to be made easily.
At least one (1) single-phase, 125-volt, 15- or 20-ampere receptacle must be located not less than _____ from and not more than _____ from the inside wall of a permanently installed outdoor swimming pool on residential or commercial properties.
The provisions in Part II of Article 680 apply to the installation of wiring methods and equipment in or adjacent to permanently installed swimming pools. As confirmed by the requirements given in Section 680.22(A)(1), each permanently installed swimming pool is to be provided with at least one (1) single-phase, 15- or 20-ampere, 125-volt, receptacle placed not closer than six (6) feet from the pool and not more than twenty (20) feet from the pool. This requirement allows ordinary appliances such as a radio, to be safely plugged in and used near the pool, but avoids the need for extension cords in the pool vicinity. Be advised the receptacles must be provided with GFCI protection. [680.22(A)(4)]
Which of the following should not be placed over a stairway step?
Part II of Article 240 explains the permitted location requirements for overcurrent devices. Section 240.24(F) tell us "overcurrent devices shall not be located over steps of a stairway." This requirement address a serious safety issue. No one should be required to stand on different treads on a stairway to replace, repair, maintain, install, troubleshoot, or reset overcurrent devices in a panelboard or switchboard located in a stairway.
The NEC specifies that a galvanized eye-bolt be installed NOT less than _____ above finished grade when used as a point of attachment for electrical service.
The intent of Section 230.26 is to provide adequate vertical clearance of the conductors and prevent physical damage to the conductors and prevent persons from making accidental contact with the conductors which may create a shock hazard or electrocution. In no case shall the point of attachment of the service-drop conductors to be less than 10 ft. above finished grade.
Receptacle outlets for countertop surfaces must be located above, but not more than _____ above, the countertop or work surface in dwelling units.
In compliance with Section 210.52(C)(5), receptacle outlets installed for countertop or work surfaces in dwelling units may be located on or above, but not more than 20 in. above the countertop or work surface. The intent of this rule is to eliminate the use of unsightly extension cords (which reduces the amount of usable countertop space) on the countertop or work surface to supply cord-and-plug connected appliances. Also, receptacles located more than 20 inches above countertops or work surfaces may interfere with the cabinetry to be installed.
Which of the circuit breakers listed below does not have a standard ampere rating?
Table 240.6(A) illustrates the standard ampere ratings for fuses and circuit breakers. A circuit breaker with an ampere rating of 75 amperes is not a standard ampere rating.
Overhead service-drop conductors must not be smaller than _____ in general.
Section 230.23(B) requires overhead service-drop conductors to be not smaller than 8 AWG copper or 6 AWG aluminum or copper-clad aluminum. Do not get service-drop conductors, the overhead conductors between the utility electric supply system and the service point, confused with the service-entrance conductors, the conductors from the service point from the utility company to the service disconnecting means
When lighting units, general-use receptacles, or other appliances are supplied via a branch circuit, the total marked rating of a 120-volt, cord-and-plug-connected room air conditioner shall not exceed _____ of the branch circuit's rating, according to the NEC.
Part VII of Article 440 outlines the special provisions for room air conditioners which are (1) rated not over 250-volts ac, single-phase and (2) which are cord-and-attachment plug connected. In order to prevent overloading the branch circuit, as per Section 40.62(C), the total rating of the A/C unit shall not exceed 50% of the rating of a branch circuit where lighting units, general-use receptacles or appliances are also supplied.
Given: The local utility company provides a 120/240 volt, single-phase electrical system to a one-family dwelling unit with a demand load of 200 amperes. What is the MINIMUM size of aluminum THW conductors required for this residence's service-entrance conductors?
Section 310.15(B)(7)(1) permits one-family dwelling 120/240 volt, single-phase, service-entrance conductors to be sized at an ampacity of not less than 83% of the service rating. Therefore, 200 amperes x 83% = 166 amperes Table 310.15(B)(16) indicates size 4/0 AWG THW aluminum conductors with an allowable ampacity of 180 amperes should be selected for this dwelling.
Which of the following machines is the most efficient?
Electrical Transformer will have higher efficiency as there is no rotating parts and airgap in the flux path involved in it. Generally efficiency of the Transformer is of the order of 95% to 99% during full load operation
The ampacity adjustment factors for more than three (3) current-carrying conductors in the raceway are not required to be applied when conductors are installed in a conduit or tubing nipple of _____ or less in length.
Section 310.15(B)(3)(a)(2) clarifies that adjustment factors for more than three (3) current-carrying conductors in raceways shall not apply where the raceway is not more than 24 inches in length. Where more than 3 current-carrying conductors are installed in short lengths of conduit or tubing, the heating effect caused from the additional conductors is insignificant therefore, the allowable ampacity of the conductors is not greatly reduced.
During no-load operation, an induction motor's power factor will be in the range:
Induction motor during no load condition will draw high magnetizing current. This magnetizing current will not do any useful work (rotating the load). Therefore power factor will be low during no load in Induction Motor
Why is it required for the metallic conduits to have electrical continuity?
Electrical continuity is required for metallic conduit to establish an effective ground-fault path and facilitate the operation of the overcurrent protective device. Reference Section 250.4(A)(4) and (5).
The voltage waveform will lag behind the current waveform in a circuit supplying electricity to capacitive loads:
In Capacitor Circuit Voltage waveform lags current waveform and has leading power factor. Inductive circuit will be exactly opposite