NCED Electrical Systems & Wiring 3 — Questions and Answers
Question 1: Ohm's Law states that voltage equals current multiplied by which quantity?
- Resistance (Correct answer)
- Power
- Capacitance
- Frequency
Correct answer: Resistance
Ohm's Law: V = I × R, meaning voltage (V) equals current (I) in amperes multiplied by resistance (R) in ohms.
Question 2: What is the function of a circuit breaker compared to a fuse?
- A breaker can be reset after tripping; a fuse must be replaced (Correct answer)
- A breaker responds faster than a fuse to overcurrent
- A breaker protects against voltage surges; a fuse protects against overcurrent
- A breaker is used only in commercial buildings; fuses in residential
Correct answer: A breaker can be reset after tripping; a fuse must be replaced
The key difference is that a circuit breaker is a resettable mechanical switch, while a fuse must be replaced once its element melts.
Question 3: What voltage is standard for most 240V appliance circuits (dryers, ranges) in US homes?
- 240 V (two 120V hots + neutral) (Correct answer)
- 120 V (single hot + neutral)
- 208 V (three-phase delta)
- 480 V (industrial standard)
Correct answer: 240 V (two 120V hots + neutral)
US 240V circuits use two hot legs each at 120V with respect to neutral, providing 240V between the two hots for large appliances.
Question 4: Which NEC rule governs the maximum number of conductors allowed in a junction box?
- Box fill calculation based on cubic inch capacity (Correct answer)
- Maximum 12 wires per box regardless of size
- Wire count must not exceed the breaker amperage rating
- Conduit fill percentage applies to junction boxes too
Correct answer: Box fill calculation based on cubic inch capacity
NEC requires a box fill calculation: each conductor, device, and fitting occupies a certain cubic-inch volume, and the total must not exceed the box's rated capacity.
Question 5: What does the term 'service entrance' refer to in residential electrical work?
- The point where utility power enters the building and connects to the main panel (Correct answer)
- The main breaker inside the sub-panel
- The outdoor disconnect for HVAC equipment
- The grounding electrode system buried in the yard
Correct answer: The point where utility power enters the building and connects to the main panel
The service entrance is the set of conductors and equipment (meter socket, service head, main panel) that bring utility power into a building.
Question 6: In parallel circuits, what is true about the voltage across each branch?
- Voltage is equal across all branches (Correct answer)
- Voltage divides proportionally among branches
- Voltage is highest in the branch with least resistance
- Voltage sums to the total supply voltage
Correct answer: Voltage is equal across all branches
In a parallel circuit, every branch is connected directly across the voltage source, so all branches share the same voltage.
Question 7: Which grounding electrode is required by NEC when a building has metal water pipes?
- The metal underground water piping system (if at least 10 ft buried) (Correct answer)
- A concrete-encased electrode only
- A driven ground rod installed 8 ft deep
- A plate electrode buried 24 inches below grade
Correct answer: The metal underground water piping system (if at least 10 ft buried)
NEC 250.52 requires the metal underground water pipe as a grounding electrode when at least 10 feet of it is in direct contact with earth.
NCED Electrical Systems & Wiring 3
Ohm's Law states that voltage equals current multiplied by which quantity?