NFPA 731 Section 4.6.3.12(2) clearly states that whenever raceways, conduits, or tubing are used (even for security systems), they must be sized in accordance with the NEC®.
Improperly sized conduits can lead to:
- Damaged insulation (abrasion) during pulling.
- Overheating due to lack of air space.
- Impossible future upgrades.
Which Table Do I Use?
The NEC provides two methods for calculating conduit fill. Knowing which one to use saves time.
1. The “Easy” Method: NEC Annex C
Use this when all conductors are the same size and insulation type (e.g., all 12 AWG THHN).
- You simply look up the conduit size and wire size to find the max count.
- Example: Table C.1 is for EMT, Table C.3 is for Flex (FMC).
2. The “Math” Method: NEC Chapter 9
Use this when you have mixed conductor sizes (e.g., three 12 AWG and two 6 AWG).
- You must calculate the total cross-sectional area of all wires and ensure it doesn’t exceed 40% of the conduit’s internal area.
Reference: NEC Table C.1 (EMT)
Maximum Number of Conductors (THHN, THWN, THWN-2)
The table below shows the maximum number of same-size conductors allowed in Electrical Metallic Tubing (EMT).
| Wire Size (AWG) | ½” | ¾” | 1″ | 1 ¼” | 1 ½” | 2″ |
| 14 | 12 | 22 | 35 | 61 | 84 | 138 |
| 12 | 9 | 16 | 26 | 45 | 61 | 101 |
| 10 | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 | 3 | 6 | 9 | 16 | 22 | 36 |
| 6 | 2 | 4 | 7 | 12 | 16 | 26 |
| 4 | 1 | 2 | 4 | 7 | 10 | 16 |
| 3 | 1 | 1 | 3 | 6 | 8 | 13 |
| 2 | 1 | 1 | 3 | 5 | 7 | 11 |
| 1 | 1 | 1 | 1 | 4 | 5 | 8 |
Note: This table applies only when all wires are the same size. If mixing sizes, you must perform a Chapter 9 calculation.
