Conduit fill calculator
Fill percentage, pass or fail and the maximum conductor count for a raceway, from the NEC Chapter 9 tables for each conduit type.
- If more than three of these carry current, apply the ampacity adjustment of NEC 310.15(C)(1). Equipment grounding conductors count toward fill but not toward that adjustment.
- Result
- Pass
- Most 12 AWG THHN, THWN, THWN-2 that fit
- 16
- Smallest EMT for 9 conductors
- 1/2 in (16)
- Conductor area, total
- 77.23 mm²
- Raceway internal area
- 343.9 mm²
| Trade size | Metric | Area (in²) | Allowed (in²) | Max conductors | Fill |
|---|---|---|---|---|---|
| 1/2 in | 16 | 0.304 | 0.122 | 9 | 39.4 % |
| 3/4 in | 21 | 0.533 | 0.213 | 16 | 22.5 % |
| 1 in | 27 | 0.864 | 0.346 | 26 | 13.9 % |
| 1-1/4 in | 35 | 1.496 | 0.598 | 45 | 8 % |
| 1-1/2 in | 41 | 2.036 | 0.814 | 61 | 5.88 % |
| 2 in | 53 | 3.356 | 1.34 | 101 | 3.57 % |
| 2-1/2 in | 63 | 5.858 | 2.34 | 176 | 2.04 % |
| 3 in | 78 | 8.846 | 3.54 | 266 | 1.35 % |
| 3-1/2 in | 91 | 11.54 | 4.62 | 347 | 1.04 % |
| 4 in | 103 | 14.75 | 5.9 | 443 | 0.811 % |
How this is calculated
Fill = n × A_wire / A_raceway
Limit: 53 % for 1 conductor, 31 % for 2, 40 % for more than 2 (Table 1)
60 % for a nipple of 24 in. (600 mm) or less (Note 4)
Max n = A_raceway × limit / A_wire, a decimal of 0.8 or more rounds up (Note 7)- Raceways
- Table 4 total internal areas for EMT (Art. 358), IMC (342), RMC (344), PVC Schedule 40 and 80 (352), FMC (348) and LFMC (350), 3/8 to 6 trade size where listed.
- Conductors
- Table 5 approximate areas including insulation: THHN, THWN, THWN-2 (14 AWG to 1000 kcmil); XHHW, XHHW-2, XHH and THW, THW-2, THHW, TW (14 AWG to 2000 kcmil). Values are for concentric stranding; compact conductors (Table 5A) are smaller.
- Count
- Include equipment grounding and bonding conductors (Note 3). Note 7 rounding applies only when all conductors are the same size, which is the case this calculator handles. It is applied at every count, including the case Note 7 names of one conductor at 0.8 or more, so a passing fill can sit slightly above the percentage; the result says when it does.
- Edition
- Table 1 and its notes are checked against the NFPA 2026 cycle First Draft, which reproduces the 2023 text with proposed edits, and against the 2017 edition. Table 4 and Table 5 values are checked against the 2017 edition, the latest full text that could be read; they are not known to have changed.
- NFPA 70, National Electrical Code: Chapter 9, Tables 1, 4 and 52023 edition
- NFPA 70 2026 cycle First Draft, Code-Making Panel 8 working draft (reproduces the 2023 Chapter 9 Table 1 and its notes)NFPA, January 2024
- NFPA 70, National Electrical Code, as published by Public.Resource.Org: Chapter 9 Tables 4 and 5, values checked2017 edition
- International Code Council, conduit fill and conductor area reference (sample pages)
Questions
16. Table 4 gives 3/4 in EMT 0.533 in² and Table 5 gives 12 AWG THHN 0.0133 in². At 40 % that is 0.2132 / 0.0133 = 16.03 conductors.
NEC Chapter 9 Table 1 sets 40 % for more than two conductors so they can be pulled without damage and can shed heat. One conductor may fill 53 % and two only 31 %, because two round conductors can jam across the diameter.
Yes. The wall is thicker, so 1 in Schedule 80 has 0.688 in² inside against 0.832 in² for Schedule 40, about 17 % less. At 40 % that is 20 against 25 conductors of 12 AWG THHN.
Yes. Chapter 9 Note 4 allows 60 % for a nipple of 24 in. (600 mm) or less between boxes or enclosures, and the 310.15(C)(1) adjustment factors need not apply to it.