Free NEC Code Questions and Answers — Questions and Answers

Question 1: A set of 450 kcmil feeder conductors is protected by a 400 breaker. With a 25 A over current device, a feeder fed from a 450 kcmil feeder disconnects. What is the smallest conductor size for these taps?

4 AWG
16 AWG
8 AWG (Correct answer)
6 AWG

Correct answer: 8 AWG

This scenario likely involves a continuous load calculation for the tap conductor. If the 25A overcurrent device is protecting a continuous load, the conductor must be sized for 125% of that load, which is 25A * 1.25 = 31.25A. According to NEC Table 310.16 (75°C column), 10 AWG copper is rated for 30A (insufficient), while 8 AWG copper is rated for 50A, making it the smallest suitable conductor.

Question 2: For a 4in * 4 in wireway, what is the maximum permitted conductor fill?

4.8 sq in
3.2 sq in (Correct answer)
16 sq in
8 sq in

Correct answer: 3.2 sq in

According to NEC Article 376.22(A), the sum of the cross-sectional areas of all conductors in a wireway shall not exceed 20% of the wireway's interior cross-sectional area. For a 4-inch by 4-inch wireway, the interior area is 16 square inches. Therefore, the maximum permitted conductor fill is 20% of 16 sq in, which equals 3.2 sq in.

Question 3: For a 4000 W, 240 electric water heater, what size branch circuit conductor is required?

14 AWG
10 AWG (Correct answer)
12 AWG
8 AWG

Correct answer: 10 AWG

First, calculate the current: I = P/V = 4000W / 240V = 16.67A. Since a water heater is a continuous load, the conductor must be sized for 125% of the load: 16.67A * 1.25 = 20.84A. According to NEC Table 310.16 (75°C column), 12 AWG copper is rated for 25A, but NEC 240.4(D) limits its overcurrent protection to 20A. Therefore, 10 AWG copper, rated for 30A, is required to safely carry 20.84A and be protected by a standard overcurrent device (e.g., 25A or 30A).

Question 4: What is the general lighting load of a hospital with an area of 80,000 sq ft after the demand factor is applied?

1,60,000 VA
42,000 VA (Correct answer)
82, 000 VA
58,000 VA

Correct answer: 42,000 VA

To determine the general lighting load, first calculate the total VA before demand: 80,000 sq ft * 2 VA/sq ft (from NEC Table 220.12 for hospitals) = 160,000 VA. Then, apply the demand factors from NEC Table 220.42: the first 50,000 VA at 40% (20,000 VA) and the remaining 110,000 VA (160,000 - 50,000) at 20% (22,000 VA). Summing these gives a total demand load of 42,000 VA.

Question 5: For a dwelling unit with a calculated load of 240 A, what size feeder conductors are required? The feeder conductors carry the whole load of the dwelling unit, and the service disconnect is rated for 250 A?

2/0 AWG
4/0 AWG (Correct answer)
1/0 AWG
3/0 AWG

Correct answer: 4/0 AWG

For dwelling unit feeder conductors, NEC Table 310.12 provides specific conductor sizes. While the service disconnect is rated for 250A and the calculated load is 240A, 4/0 AWG is a standard size for a 200A service when using aluminum conductors. This size is often considered sufficient for dwelling unit loads up to 240A, especially when applying the special allowances for dwelling unit services and feeders.

Question 6: For a constant load of 22 amperes at 750C, what size conductor should be used to protect against overcurrent?

12 THWN
10 THWN (Correct answer)
8 THWN
16 THWN

Correct answer: 10 THWN

A constant load implies a continuous load, requiring the conductor to be sized for 125% of the load. Thus, 22A * 1.25 = 27.5A. According to NEC Table 310.16 (75°C column for THWN), 12 AWG copper is rated for 25A (insufficient), while 10 AWG copper is rated for 30A. Therefore, 10 AWG THWN is the smallest conductor size that can safely carry a 27.5A continuous load.

Question 7: What is the needed conductor size for GEC if a service is fed by three parallel sets of 400 kcmil conductors?

2 AWG
2/0 AWG
4 AWG
3/0 AWG (Correct answer)

Correct answer: 3/0 AWG

To size the Grounding Electrode Conductor (GEC) for parallel service conductors, you sum the circular mil area of the individual conductors. Three sets of 400 kcmil conductors equal a total equivalent area of 1200 kcmil. Referring to NEC Table 250.66, for service conductors between 1100 kcmil and 1750 kcmil, the required GEC size is 3/0 AWG copper.

Question 8: In a 4 in. * 4 in. wireway, how many 500 kcmil THWN conductors can be installed?

5 conductors
4 conductors (Correct answer)
6 conductors
7 conductors

Correct answer: 4 conductors

According to NEC Article 376.22(A), the maximum conductor fill for a wireway is 20% of its interior cross-sectional area. For a 4 in. x 4 in. wireway, the interior area is 16 sq in, so the maximum fill is 3.2 sq in. From NEC Chapter 9, Table 5, a 500 kcmil THWN conductor has an approximate area of 0.7073 sq in. Dividing the maximum fill by the conductor area (3.2 sq in / 0.7073 sq in/conductor) yields approximately 4.52 conductors, which rounds down to 4 conductors.

Question 9: A 45 KVA three phase transformer with a 115/230 delta configuration. What is the transformer secondary's high-leg voltage?

230 volts
208 volts
399 volts
199 volts (Correct answer)

Correct answer: 199 volts

In a 3-phase, 4-wire delta system (high-leg delta), the voltage from the 'high leg' to the neutral (grounded conductor) is calculated by multiplying the phase-to-phase voltage by the square root of 3 (approximately 1.732) and then dividing by 2. For a 230V phase-to-phase system, the high-leg voltage to neutral is 230V * (sqrt(3)/2) = 230V * 0.866 = 199.18V, which is approximately 199 volts.

Question 10: Under a pool or within _____ horizontally of the inside wall of a pool, underground wiring is not allowed:

3 ft
4 ft
5 ft (Correct answer)
6 ft

Correct answer: 5 ft

NEC Article 680.10(A) prohibits underground wiring from being installed under a pool or within 5 feet horizontally from the inside wall of a pool. This safety regulation aims to prevent accidental contact with electrical wiring during excavation or if the pool structure shifts, thereby minimizing electrical hazards in and around pool areas.

Question 11: A pool, spa, hot tub, fountain, or the current tidal high water mark should not be located within _____ horizontally of audio system equipment fed by a branch circuit:

3 ft
4 ft
5 ft (Correct answer)
6 ft

Correct answer: 5 ft

NEC Article 680.22(A)(5) mandates that audio system equipment fed by a branch circuit must not be located within 5 feet horizontally of the inside wall of a pool, spa, hot tub, fountain, or the current tidal high water mark. This separation distance is a critical safety measure designed to reduce the risk of electrical shock in wet environments.

Question 12: Type AC cables need to be supported and fastened at spaced no more than :

3 ft
4.5 ft (Correct answer)
5 ft
6 ft

Correct answer: 4.5 ft

According to NEC Article 320.30(B), Type AC cables must be supported and fastened at intervals not exceeding 4 1/2 feet (4.5 ft). Additionally, they are required to be secured within 12 inches of every box, cabinet, or fitting. These requirements ensure the cable is properly installed, supported, and protected from potential damage.

Question 13: The wires connecting the final branch circuit overcurrent device to the service equipment, an independently derived system, or another power source

Branch circuit
Service conductor
Feeder (Correct answer)
Service drop

Correct answer: Feeder

As defined in NEC Article 100, a 'feeder' refers to the circuit conductors located between the service equipment, the source of a separately derived system, or other power supply, and the final branch-circuit overcurrent device. Feeders are responsible for carrying electrical power from the main source to various distribution points within a building or system.

Question 14: Continuous load is defined as a load when the maximum current is anticipated to last for __ hours or more:

two
three (Correct answer)
four
six

Correct answer: three

NEC Article 100 defines a 'Continuous Load' as a load where the maximum current is expected to continue for 3 hours or more. This definition is crucial for electrical design, as continuous loads require conductors and overcurrent protection devices to be sized at 125% of the load to prevent overheating and ensure safety.

Question 15: If the overcurrent device rating is not specified, the overcurrent protection device of non-motor equipment shall not be greater than :

15 A
20 A (Correct answer)
25 A
30 A

Correct answer: 20 A

According to NEC Article 240.4(D), unless specific exceptions apply elsewhere in the Code, 12 AWG copper conductors must be protected by an overcurrent device rated at no more than 20 amperes. This rule is a fundamental safety measure to prevent smaller conductors from overheating due to excessive current, which could lead to fire hazards.

Question 16: For 12 AWG copper conductor, the maximum overcurrent protection is:

15 Amperes
20 Amperes (Correct answer)
25 Amperes
30 Amperes

Correct answer: 20 Amperes

As per NEC Article 240.4(D), 12 AWG copper conductors, unless otherwise permitted by specific exceptions, must be protected by an overcurrent device rated at no more than 20 amperes. This is a fundamental safety rule designed to prevent overheating and potential fire hazards in electrical circuits by ensuring adequate protection for smaller conductors.

Free NEC Code Questions and Answers

A set of 450 kcmil feeder conductors is protected by a 400 breaker.
With a 25 A over current device, a feeder fed from a 450 kcmil feeder
disconnects.
What is the smallest conductor size for these taps?