IEC Wiring Methods and Materials 5 — Questions and Answers
Question 1: Which conductor insulation type is rated for use in both dry and wet locations at 90°C?
- THHN
- TW
- THW
- XHHW-2 (Correct answer)
Correct answer: XHHW-2
XHHW-2 is rated 90°C in both dry and wet locations, while THHN is 90°C dry/75°C wet and THW is 75°C in both.
Question 2: When installing conductors in parallel (per NEC 310.10(H)), what is the minimum size conductor permitted to be paralleled?
- No. 2 AWG
- No. 1/0 AWG (Correct answer)
- No. 2/0 AWG
- No. 4/0 AWG
Correct answer: No. 1/0 AWG
NEC 310.10(H)(1) requires that conductors installed in parallel must be 1/0 AWG or larger.
Question 3: Armored cable (Type AC) differs from metal-clad cable (Type MC) primarily in that AC cable:
- Uses a plastic outer jacket over the armor
- Contains an internal bonding strip that serves as the EGC (Correct answer)
- Is rated for outdoor direct-burial use
- Uses aluminum armor while MC uses steel armor
Correct answer: Contains an internal bonding strip that serves as the EGC
Type AC cable contains an internal aluminum bonding strip that, combined with the armor, provides the equipment grounding path per NEC 320.108.
Question 4: What is the purpose of an expansion fitting (expansion joint) installed in a PVC conduit run?
- To allow the conduit to be pulled apart for wire pulling
- To accommodate thermal expansion and contraction of the conduit (Correct answer)
- To reduce the number of support hangers required
- To provide a watertight seal at conduit penetrations
Correct answer: To accommodate thermal expansion and contraction of the conduit
PVC conduit expands and contracts significantly with temperature changes, and expansion fittings allow this movement without stressing the system.
Question 5: When must conductors be derated according to NEC 310.15(C) (adjustment factors)?
- When more than three current-carrying conductors are in a raceway or cable (Correct answer)
- Whenever conductors are longer than 100 feet
- When conductors are installed in conduit exposed to direct sunlight
- When conductors are aluminum instead of copper
Correct answer: When more than three current-carrying conductors are in a raceway or cable
NEC 310.15(C) requires ampacity adjustment when more than three current-carrying conductors are bundled or installed in a raceway, because heat dissipation is reduced.
Question 6: What type of conduit fitting is required when terminating EMT into a weatherproof (rainproof) enclosure?
- A standard set-screw connector is acceptable
- A raintight or weatherproof connector listed for the application (Correct answer)
- A locknut and bushing combination only
- Any listed EMT connector with a plastic bushing
Correct answer: A raintight or weatherproof connector listed for the application
NEC 358.42 requires fittings for EMT used in wet locations to be listed for wet locations (raintight or weatherproof), maintaining the enclosure's environmental rating.
Question 7: Service entrance cable (Type SE) installed exposed on the exterior of a building must be supported at intervals not exceeding:
- 18 inches
- 24 inches
- 30 inches
- 48 inches (Correct answer)
Correct answer: 48 inches
NEC 338.30(A) requires SE cable installed as a service-entrance conductor to be supported at intervals not exceeding 30 inches along the building exterior—but when used as branch-circuit or feeder wiring, it follows NEC 334.30 at 4.5-foot intervals; for service runs the standard is every 30 inches.
IEC Wiring Methods and Materials 5
Which conductor insulation type is rated for use in both dry and wet locations at 90°C?