Give the raceway size required for each of the following conductor combinations:
- Five, 10 AWG, Type THW in Schedule 80 PVC ___
- Three, 1/0 AWG, Type THHN in EMT ___
- Three, 1/0 AWG and one, 1 AWG, Type THW in EMT_______________
The Correct Answer and Explanation is :
To determine the correct raceway size for the given conductor combinations, we refer to NEC Chapter 9, Table 5 for conductor dimensions and Chapter 9, Table 4 for raceway fill limits. Here are the answers along with an explanation of the calculations:
1. Five, 10 AWG, Type THW in Schedule 80 PVC
Answer: 1/2 inch Schedule 80 PVC
- From NEC Chapter 9, Table 5, the cross-sectional area of a single 10 AWG THW conductor is 0.0211 in².
- Total conductor area = ( 5 \times 0.0211 \, \text{in}² = 0.1055 \, \text{in}² ).
- For Schedule 80 PVC, the maximum fill is 40% for more than 2 conductors.
- From NEC Chapter 9, Table 4, a 1/2-inch Schedule 80 PVC conduit has a total area of 0.122 in² with a 40% allowable fill of 0.0488 in².
- The calculated conductor area exceeds the 40% fill of a 1/2-inch Schedule 80 PVC. Thus, a 3/4-inch Schedule 80 PVC conduit is required, with a 40% fill of 0.164 in², which accommodates the conductors.
2. Three, 1/0 AWG, Type THHN in EMT
Answer: 1-inch EMT
- From Table 5, the area of one 1/0 AWG THHN conductor is 0.0837 in².
- Total conductor area = ( 3 \times 0.0837 \, \text{in}² = 0.2511 \, \text{in}² ).
- For EMT conduits with more than two conductors, the allowable fill is 40%.
- From Table 4, a 1-inch EMT has a total area of 0.346 in² and a 40% allowable fill of 0.1384 in².
- A 1-1/4-inch EMT conduit, with a 40% allowable fill of 0.581 in², is required to fit the total conductor area.
3. Three, 1/0 AWG and one, 1 AWG, Type THW in EMT
Answer: 1-1/4-inch EMT
- From Table 5, the area of one 1/0 AWG THW conductor is 0.1333 in², and one 1 AWG THW conductor is 0.1158 in².
- Total conductor area = ( 3 \times 0.1333 + 0.1158 = 0.5157 \, \text{in}² ).
- From Table 4, a 1-1/4-inch EMT has a total area of 0.785 in² with a 40% allowable fill of 0.314 in².
- A 1-1/2-inch EMT conduit, with a 40% allowable fill of 0.514 in², accommodates the calculated conductor area.
Explanation
The National Electrical Code (NEC) provides specific guidelines for raceway fill to prevent overheating and allow easy wire pulling. The fill percentage depends on the number of conductors:
- 53% fill for one conductor.
- 31% fill for two conductors.
- 40% fill for three or more conductors.
Correct sizing ensures compliance, safety, and functionality of the electrical system.