CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
Six 10 AWG THWN copper conductors must be installed in a single EMT raceway. Using NEC Chapter 9, Annex C, Table C.1 (EMT conductor fill for over two conductors), what is the minimum trade size EMT required?
- A1-1/4 inch
- B1/2 inch
- C1 inch
- D3/4 inch
Show answer & explanationAnswer & explanation
Correct answer: D. 3/4 inch
NEC Annex C Table C.1 lists the maximum number of conductors permitted in EMT trade sizes. For 10 AWG THWN, 1/2 inch EMT permits only 4 conductors, while 3/4 inch EMT permits 8 conductors. Since 6 conductors exceed the 1/2 inch capacity but fit within 3/4 inch, the minimum trade size is 3/4 inch.
Why the other options are wrong
- A. 1-1/4 inch EMT far exceeds what is needed for this conductor count.
- B. 1/2 inch EMT holds only 4 10 AWG THWN conductors, insufficient for 6.
- C. 1 inch EMT is larger than necessary for only 6 conductors.
Annex C Conduit Fill Tables
NEC Chapter 9, Annex C provides pre-calculated maximum conductor fill counts for specific raceway types and conductor types, eliminating manual area calculations.
- Tables apply only when all conductors are the same size and type.
- Table C.1 is specific to EMT.
- Using Annex C tables directly is faster than manual 40%/31% fill calculations.
Memory trick: 'Same size, same table' — Annex C works only for uniform conductors.