TN LLE Conductors and Cables 2 — Questions and Answers
Question 1: Per NEC Table 310.16, what is the ampacity of a 12 AWG THHN copper conductor at 30 degrees C ambient?
- 20 amperes
- 25 amperes
- 30 amperes (Correct answer)
- 35 amperes
Correct answer: 30 amperes
NEC Table 310.16 lists 12 AWG THHN copper at 30 amperes in the 90 degrees C column.
THHN insulation is rated for 90 degrees C, so when using NEC Table 310.16, the 90 degrees C column applies. A 12 AWG copper conductor with THHN insulation has an ampacity of 30 amperes at 30 degrees C ambient temperature. However, per NEC 240.4(D), the overcurrent protection for 12 AWG copper must not exceed 20 amperes for most applications, meaning the conductor capacity exceeds the maximum protection allowed.
Question 2: What is the maximum number of 12 AWG THHN conductors permitted in a 3/4-inch EMT conduit per NEC Chapter 9?
- 7
- 9
- 12 (Correct answer)
- 16
Correct answer: 12
Per NEC Chapter 9, Annex C, Table C1, a 3/4-inch EMT can hold up to 12 conductors of 12 AWG THHN.
NEC Annex C, Table C1 provides conduit fill tables for EMT. For 3/4-inch (trade size 21) EMT with 12 AWG THHN conductors, the maximum fill is 12 conductors. This is based on the 40% fill limit for three or more conductors per NEC Table 1, Chapter 9. When using more than 9 current-carrying conductors, ampacity adjustment factors from NEC 310.15(C)(1) must also be applied.
Question 3: Per NEC 310.15(C)(1), what ampacity adjustment factor applies when 7-9 current-carrying conductors are in a single conduit?
- 80%
- 70% (Correct answer)
- 50%
- 60%
Correct answer: 70%
NEC Table 310.15(C)(1) requires a 70% ampacity adjustment when 7-9 current-carrying conductors share a raceway.
NEC Table 310.15(C)(1) provides ampacity adjustment factors based on the number of current-carrying conductors in a raceway: 4-6 conductors = 80%, 7-9 conductors = 70%, 10-20 conductors = 50%, 21-30 conductors = 45%, 31-40 conductors = 40%, and 41+ = 35%. For 7-9 conductors, the 70% factor accounts for reduced heat dissipation. Equipment grounding conductors and neutral conductors carrying only unbalanced current are not counted.
Question 4: What is the minimum burial depth for UF cable installed under a residential driveway per NEC Table 300.5?
- 6 inches
- 12 inches
- 18 inches
- 24 inches (Correct answer)
Correct answer: 24 inches
Per NEC Table 300.5, direct-buried UF cable under a one- and two-family dwelling driveway requires a minimum cover of 24 inches.
NEC Table 300.5 specifies minimum cover requirements for underground installations. UF cable (direct burial without raceway) requires 24 inches of cover. Under a residential driveway, the same 24-inch depth applies. If the cable were installed in rigid metal conduit (RMC), the required depth reduces to 6 inches. In non-vehicular areas, UF cable still requires 24 inches unless GFCI protection is provided, which can reduce it to 12 inches.
Question 5: Per NEC 334.80, what limitation applies to NM cable when installed in thermal insulation?
- NM cable cannot be installed in insulation
- Ampacity must be based on the 60 degrees C conductor rating (Correct answer)
- A derating factor of 50% applies
- NM cable must be derated to the 75 degrees C column
Correct answer: Ampacity must be based on the 60 degrees C conductor rating
NEC 334.80 requires that the ampacity of NM cable installed in thermal insulation be based on the 60 degrees C conductor temperature rating.
When NM (Romex) cable is installed where it passes through or is in contact with thermal insulation, NEC 334.80 requires the ampacity to be determined using the 60 degrees C conductor temperature rating from NEC Table 310.16. For 14 AWG this means 15 amperes, for 12 AWG it means 20 amperes, and for 10 AWG it means 30 amperes. This conservative rating accounts for the reduced heat dissipation caused by surrounding insulation.
Question 6: What is the maximum voltage rating for Type NM cable per NEC 334.116?
- 150 volts
- 300 volts
- 600 volts (Correct answer)
- 1000 volts
Correct answer: 600 volts
NEC 334.116 states that NM cable shall be rated at 600 volts.
Per NEC 334.116, Types NM, NMC, and NMS cable are rated for a maximum of 600 volts. Despite this voltage rating, NM cable is primarily used in residential and light commercial applications at 120/240V. The 600V rating provides a safety margin and is consistent with other common cable types like UF and SE cable. Tennessee LLEs should note that NM cable usage in commercial applications is restricted by NEC 334.10.
TN LLE Conductors and Cables 2
Per NEC Table 310.16, what is the ampacity of a 12 AWG THHN copper conductor at 30 degrees C ambient?