An electrician needs to determine the maximum number of 12 AWG THHN conductors that can be installed in a 1-inch EMT conduit, assuming the conductors are all current-carrying and the conduit is less than 24 inches long (no derating for length).
- A32 conductors
- B8 conductors
- C26 conductors
- D15 conductors
Show answer & explanationAnswer & explanation
Correct answer: C. 26 conductors
According to NEC Chapter 9, Table 4 for EMT, a 1-inch EMT conduit has a 40% fill area of 0.347 square inches. From NEC Chapter 9, Table 5, a 12 AWG THHN conductor has an approximate area of 0.0172 square inches. The number of conductors = 0.347 / 0.0172 ≈ 20.17. However, the question implies using the specific tables for conductor fill. NEC Chapter 9, Table C.1 for EMT, specifically lists 26 for 12 AWG THHN in a 1-inch EMT.
Why the other options are wrong
- A. Too many conductors; this would exceed the permissible fill capacity.
- B. Too few conductors; this might be for a smaller conduit size or different wire type.
- D. Incorrect count; likely from a miscalculation or incorrect table reference.
Conduit Fill Calculation
Conduit fill calculations determine the maximum number of conductors allowed in a conduit to prevent overheating and allow for easy installation/removal, based on conductor size, insulation type, and conduit size, as specified in NEC Chapter 9.
- NEC Chapter 9, Table 1 specifies percentage fill limits (e.g., 40% for over 2 wires).
- NEC Chapter 9, Table 4 provides conduit dimensions and fill areas.
- NEC Chapter 9, Table 5 provides conductor dimensions.
- Alternatively, NEC Chapter 9, Annex C tables provide pre-calculated numbers for common conductor/conduit types.
Memory trick: Tables C for Count, Tables 4 & 5 for Area, Table 1 for Percent share.