CME System Design and Installation Questions and Answers 1 — Questions and Answers
Question 1: A 3-phase, 480V motor has a full-load current (FLC) of 65A per Table 430.250. The branch circuit will be protected by an inverse-time circuit breaker. According to NEC 430.52(C)(1), what is the MAXIMUM standard size circuit breaker permitted for short-circuit and ground-fault protection?
- 150A
- 175A (Correct answer)
- 200A
- 125A
Correct answer: 175A
NEC Table 430.52 specifies the maximum rating for a motor branch-circuit short-circuit and ground-fault protective device. For an inverse-time circuit breaker, this is 250% of the motor's FLC. Calculation: 65A (FLC) x 250% = 162.5A. According to 430.52(C)(1) Exception No. 1, if the calculated value does not correspond to a standard size listed in 240.6(A), the next higher standard size is permitted. The next standard size above 162.5A is 175A.
Question 2: An electrical installation in a commercial building is modified by the utility, resulting in a higher available fault current (AFC) at the service equipment. Per NEC 110.24(B), what is the primary action required by the master electrician?
- Notify the building owner in writing of the change.
- Replace all downstream circuit breakers with higher interrupting rated devices.
- Recalculate the AFC and adjust the field-installed label on the service equipment. (Correct answer)
- Request a variance from the AHJ to accept the existing equipment.
Correct answer: Recalculate the AFC and adjust the field-installed label on the service equipment.
NEC 110.24(B) requires that when modifications to the electrical installation affect the maximum available fault current at the service, the AFC must be verified or recalculated. The required field marking must then be adjusted to reflect the new AFC level to ensure the service equipment ratings are still sufficient.
Question 3: Which of the following wiring methods is permitted to be installed exposed in a space used for environmental air handling above a lay-in ceiling (plenum), without being installed in a raceway?
- Non-metallic sheathed cable (Type NM-B)
- Type MC cable with an overall nonmetallic covering
- Type AC (Armored Cable) (Correct answer)
- Service-entrance cable (Type SE)
Correct answer: Type AC (Armored Cable)
According to NEC 300.22(C), wiring in 'Other Spaces Used for Environmental Air (Plenums)' is restricted to specific methods to limit the spread of fire and smoke. Permitted methods without a raceway include Type AC cable, Type MC cable without an overall nonmetallic covering, and other specifically listed plenum-rated cables. Type NM-B and Type SE are not permitted in plenums.
Question 4: A 200A, 240V single-phase feeder supplies a panelboard located 250 feet from the source. To adhere to the NEC's recommendation to limit voltage drop to 3% on the feeder, what is the minimum size copper conductor required? (Use K=12.9 for copper)
- 2/0 AWG
- 3/0 AWG
- 4/0 AWG
- 250 kcmil (Correct answer)
Correct answer: 250 kcmil
First, calculate the allowable voltage drop: 240V * 3% = 7.2V. Next, use the voltage drop formula for circular mils (CM): CM = (2 * K * I * L) / VD. Plugging in the values: CM = (2 * 12.9 * 200A * 250 ft) / 7.2V = 179,167 CM. Referring to NEC Chapter 9, Table 8, the next standard conductor size with a circular mil area greater than 179,167 is 250 kcmil (250,000 CM). The NEC recommendation is found in 215.2(A)(1) Informational Note No. 2.
Question 5: For a system to be considered selectively coordinated, which of the following must occur during a fault condition?
- The overcurrent protective device nearest to the source must open first.
- All overcurrent protective devices in the fault path must open simultaneously.
- The overcurrent protective device immediately upstream of the fault must open, while all further upstream devices remain closed. (Correct answer)
- The main service disconnect and the branch circuit breaker must both trip.
Correct answer: The overcurrent protective device immediately upstream of the fault must open, while all further upstream devices remain closed.
Selective coordination is the localization of an overcurrent condition to restrict outages to the circuit or equipment affected. This is achieved by selecting and setting overcurrent protective devices so that the device closest to the fault on the line side (immediately upstream) opens first, leaving the rest of the system operational. NEC 700.32 mandates this for emergency systems.
Question 6: An electrician is installing two parallel sets of 4/0 AWG copper conductors in separate non-metallic conduits. Which of the following is NOT a requirement for this installation per NEC 310.10(G)?
- Both sets of conductors must be the same length.
- Each conductor within a phase must have the same insulation type.
- The equipment grounding conductor in each conduit must be full-size.
- The conduits must be grouped together to minimize inductive heating. (Correct answer)
Correct answer: The conduits must be grouped together to minimize inductive heating.
NEC 310.10(G) requires that all parallel conductors for a phase must be the same length, material, circular mil area, and insulation type. Per 250.122(F), the equipment grounding conductors in each parallel raceway must be sized based on the rating of the circuit's overcurrent device. The requirement to group conduits to minimize inductive heating applies to metallic raceways, not non-metallic ones.
CME System Design and Installation Questions and Answers 1
A 3-phase, 480V motor has a full-load current (FLC) of 65A per Table 430.250.
The branch circuit will be protected by an inverse-time circuit breaker.
According to NEC 430.52(C)(1), what is the MAXIMUM standard size circuit breaker permitted for short-circuit and ground-fault protection?