TN LLE Conductors and Cables 3 — Questions and Answers
Question 1: Per NEC 310.15(B)(7), what ampacity method applies to 120/240V, single-phase dwelling service conductors?
- Standard Table 310.16 values apply
- A special table with higher ampacities for service entrance conductors (Correct answer)
- 83% of the standard ampacity
- Conductors may be sized one trade size smaller
Correct answer: A special table with higher ampacities for service entrance conductors
NEC 310.15(B)(7) provides a special table with higher ampacities specifically for dwelling unit service and feeder conductors because of the diversity of residential loads.
NEC 310.15(B)(7) recognizes that dwelling unit loads have a natural diversity — not all circuits operate at full load simultaneously. The special ampacity table allows smaller conductors for dwelling services and feeders. For example, a 100A service can use 4 AWG copper instead of the 3 AWG that Table 310.16 would require. This provision applies only to 120/240V, single-phase dwelling service entrance conductors and main power feeder conductors.
Question 2: What is the minimum size copper grounding electrode conductor for a 200-ampere service per NEC Table 250.66?
- 6 AWG
- 4 AWG (Correct answer)
- 2 AWG
- 1/0 AWG
Correct answer: 4 AWG
Per NEC Table 250.66, a 200A service with copper service-entrance conductors of 2/0 AWG requires a minimum 4 AWG copper grounding electrode conductor.
NEC Table 250.66 sizes the grounding electrode conductor based on the largest service-entrance conductor or equivalent area. For a 200A residential service typically using 2/0 AWG copper or 4/0 AWG aluminum service conductors, the minimum copper grounding electrode conductor is 4 AWG. If the grounding electrode is a ground rod (only), NEC 250.66(A) allows the GEC to be no larger than 6 AWG copper regardless of service size.
Question 3: When pulling conductors through a conduit, what is the maximum recommended pulling tension for copper conductors to avoid damage?
- 0.006 x cmil per conductor
- 0.008 x cmil per conductor (Correct answer)
- 75 lbs per conductor
- No limit if a pulling lubricant is used
Correct answer: 0.008 x cmil per conductor
The industry standard maximum pulling tension for copper conductors is 0.008 pounds per circular mil of conductor area.
The standard maximum pulling tension for copper conductors is 0.008 lbs per circular mil (cmil) of conductor cross-sectional area. For example, a 500 kcmil copper conductor has a maximum pulling tension of 500,000 x 0.008 = 4,000 lbs. For aluminum conductors, the limit is lower at 0.006 lbs per cmil. These values prevent permanent stretching or insulation damage. NEC 300.31 requires that conductors not be damaged during installation.
Question 4: Per NEC 300.17, what percentage of a conduit's internal cross-sectional area can be filled when installing 3 or more conductors?
- 31%
- 40% (Correct answer)
- 53%
- 60%
Correct answer: 40%
NEC Chapter 9, Table 1 limits conduit fill to 40% of the cross-sectional area when 3 or more conductors are installed.
NEC Chapter 9, Table 1 establishes conduit fill limits: 1 conductor = 53%, 2 conductors = 31%, and 3 or more conductors = 40% of the conduit's internal cross-sectional area. The 40% limit for 3+ conductors balances the need for adequate space to pull conductors without damage while preventing excessive heat buildup. These percentages apply to all raceway types including EMT, IMC, RMC, and PVC.
Question 5: What conductor type is required for wet locations per NEC 310.10(C)?
- THHN only
- Any conductor rated for wet locations such as THWN, XHHW, or USE (Correct answer)
- Only underground-rated conductors
- Conductors with lead sheath only
Correct answer: Any conductor rated for wet locations such as THWN, XHHW, or USE
NEC 310.10(C) requires conductors in wet locations to be moisture-resistant types listed for the purpose, such as THWN, THWN-2, XHHW, or USE.
NEC 310.10(C) requires that conductors installed in wet locations be of a type listed for wet locations. Common wet-rated conductor types include THWN (75 degrees C wet), THWN-2 (90 degrees C wet), XHHW (75 degrees C wet/90 degrees C dry), XHHW-2 (90 degrees C wet and dry), and USE (underground service entrance). The 'W' in the type designation indicates wet-location suitability. Many modern conductors like THHN/THWN-2 carry dual ratings for both dry and wet locations.
Question 6: Per NEC 240.4(D), what is the maximum overcurrent protection permitted for 14 AWG copper conductors?
- 15 amperes (Correct answer)
- 20 amperes
- 25 amperes
- 30 amperes
Correct answer: 15 amperes
NEC 240.4(D) limits overcurrent protection for 14 AWG copper to 15 amperes maximum.
NEC 240.4(D) establishes fixed overcurrent protection limits for small conductors: 14 AWG = 15A, 12 AWG = 20A, and 10 AWG = 30A. These limits cannot be exceeded regardless of the conductor's actual ampacity from Table 310.16. For example, 14 AWG THHN has a 25A ampacity at 90 degrees C, but the overcurrent device must still be limited to 15A. This provision protects the weakest point in the circuit — typically the connections and terminations.
TN LLE Conductors and Cables 3
Per NEC 310.15(B)(7), what ampacity method applies to 120/240V, single-phase dwelling service conductors?