Free TN-LLE Raceway and Box Fill Questions and Answers 1 — Questions and Answers
Question 1: What is the maximum number of 12 AWG THHN conductors permitted in a 4" x 4" x 1-1/2" square metal box?
- 7
- 8
- 9 (Correct answer)
- 10
Correct answer: 9
First, find the volume of the box in NEC Table 314.16(A), which is 21.0 cubic inches. Next, find the volume required per 12 AWG conductor in Table 314.16(B), which is 2.25 cubic inches. Divide the box volume by the conductor volume: 21.0 / 2.25 = 9.33. Since you cannot have a fraction of a conductor, the maximum number is 9.
Question 2: A metal device box contains one duplex receptacle, four 14 AWG current-carrying conductors, and one 14 AWG grounding conductor. What is the total volume allowance required for these components?
- 12.0 cu. in.
- 14.0 cu. in. (Correct answer)
- 16.0 cu. in.
- 10.0 cu. in.
Correct answer: 14.0 cu. in.
Per NEC 314.16(B), calculate the allowances: four 14 AWG conductors count as 4. All grounding conductors count as a single allowance (1). The duplex receptacle counts as a double allowance (2). The total is 4 + 1 + 2 = 7 allowances. The volume per 14 AWG conductor from Table 314.16(B) is 2.00 cu. in. Total volume is 7 x 2.00 = 14.0 cu. in.
Question 3: What is the maximum number of 10 AWG THHN conductors permitted in a 3/4" Electrical Metallic Tubing (EMT)?
- 10
- 12
- 14
- 16 (Correct answer)
Correct answer: 16
This is a direct lookup in the NEC Annex C tables. Go to Table C.1 for EMT. Find the row for THHN conductor type and 10 AWG size, then move across to the column for 3/4" trade size conduit. The table indicates a maximum of 16 conductors.
Question 4: What is the minimum trade size of Schedule 40 PVC conduit required to enclose three 6 AWG THWN conductors and two 8 AWG THWN conductors?
- 3/4"
- 1" (Correct answer)
- 1-1/4"
- 1-1/2"
Correct answer: 1"
First, find the area of each conductor from Chapter 9, Table 5: 6 AWG THWN = 0.0507 sq. in. and 8 AWG THWN = 0.0366 sq. in. Calculate total area: (3 x 0.0507) + (2 x 0.0366) = 0.1521 + 0.0732 = 0.2253 sq. in. Using Chapter 9, Table 4 for PVC Sch 40 and the 40% fill column (for over 2 wires), a 3/4" conduit allows 0.213 sq. in. (too small) and a 1" conduit allows 0.346 sq. in. Therefore, 1" is the minimum size.
Question 5: A junction box has straight pulls of 3" Rigid Metal Conduit (RMC) entering and leaving. What is the minimum required length of the box?
- 18 inches
- 24 inches (Correct answer)
- 30 inches
- 36 inches
Correct answer: 24 inches
NEC 314.28(A)(1) for straight pulls requires the length of the box to be a minimum of eight times the trade diameter of the largest raceway. The calculation is 8 x 3 inches = 24 inches. The box must be at least 24 inches long.
Question 6: A pull box has two 2" conduits entering one side and one 3" conduit leaving the opposite side for a straight pull. What is the minimum length of the box?
- 16 inches
- 21 inches
- 24 inches (Correct answer)
- 32 inches
Correct answer: 24 inches
According to NEC 314.28(A)(1), the calculation for straight pulls is based on the largest raceway entering the box. The length must be at least eight times the trade diameter of the largest raceway. The largest raceway is 3". The minimum length is 8 x 3" = 24 inches.
Free TN-LLE Raceway and Box Fill Questions and Answers 1
What is the maximum number of 12 AWG THHN conductors permitted in a 4" x 4" x 1-1/2" square metal box?