8.3 Conduit and Raceway Fill
Key Takeaways
- NEC Chapter 9 Table 1 fill percentages: 1 conductor = 53 percent, 2 conductors = 31 percent, 3 or more conductors = 40 percent (wire-type)
- Common THHN stranded copper areas from Table 5: #14 = 0.0097 in², #12 = 0.0133 in², #10 = 0.0211 in², #8 = 0.0366 in², #6 = 0.0507 in²
- Chapter 9 Note 2 requires rounding up to the next larger standard trade size whenever the calculated minimum falls between standard sizes
- For 3 or more conductors, compare total conductor area directly to the 40 percent fill column in Table 4 — the percentage is already applied
- Annex C lists pre-calculated maximum conductor counts for common raceway and conductor combinations and should match your manual Chapter 9 calculation
Why Conduit Fill Matters on the TX Journeyman Exam
Conduit fill calculations are a Calculations-part staple. The exam gives you a number and type of conductors plus a raceway type, then asks for the minimum trade size — or the maximum number of conductors that fit in a given raceway. The method is mechanical (look up areas, divide, compare), but you must know which NEC table to use, how to apply the fill percentages, and when to round.
NEC Chapter 9 — Tables for Raceway Fill
Chapter 9 of the NEC provides the tables used for raceway fill calculations. They are separate from the conduit body articles (342 through 362) and apply universally.
Table 1 — Percent of Cross Section
The fill percentage depends on the number of conductors, not the type of raceway:
| Number of Conductors | All Wire Types |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 | 40% |
| 4 or more | 40% |
The 40 percent rule for three or more conductors is the most common scenario on the exam. Use Table 5A for compact-strand conductors (slightly smaller areas).
Note 2 to Chapter 9 Table 1 — Rounding
Where the computation of the minimum size of the raceway results in a decimal less than the next larger standard size, the next larger standard size shall be used.
You always round up to the next standard trade size. You never round down. If your calculation requires 0.316 in² of total area and the next standard size offers 0.346 in² in the 40 percent column, that is the minimum.
Table 5 — Conductor Cross-Sectional Areas (Wire Types)
Table 5 gives the area of individual conductors in square inches. Common THHN/THWN-2 (stranded copper) values:
| Size | THHN/THWN-2 Area (in²) |
|---|---|
| 14 AWG | 0.0097 |
| 12 AWG | 0.0133 |
| 10 AWG | 0.0211 |
| 8 AWG | 0.0366 |
| 6 AWG | 0.0507 |
| 4 AWG | 0.0835 |
| 2 AWG | 0.1162 |
Table 4 — Raceway Cross-Sectional Areas
Table 4 gives the total internal area of each raceway type at 100 percent, plus the pre-calculated fill areas at 53 percent, 31 percent, and 40 percent. For EMT (Article 358), common values:
| Trade Size | Metric | Total Area (in²) | 40% Fill (in²) |
|---|---|---|---|
| 1/2 in. | 16 | 0.304 | 0.122 |
| 3/4 in. | 21 | 0.533 | 0.213 |
| 1 in. | 27 | 0.864 | 0.346 |
| 1-1/4 in. | 35 | 1.496 | 0.598 |
| 1-1/2 in. | 41 | 2.036 | 0.814 |
| 2 in. | 53 | 3.376 | 1.350 |
When calculating fill for three or more wires, compare the total conductor area directly to the 40 percent column — the NEC has already applied the percentage for you.
Annex C — Number of Conductors in Raceway
Annex C is a shortcut: for many common conductor and raceway combinations, it lists the maximum number of conductors that fit in each trade size without any calculation. Annex C is informational (not enforceable code text) but is fully derived from Chapter 9 Tables, so it should always match your manual calculation. Use it as a check on your work.
Worked Example — Minimum EMT Trade Size for 6 #10 THHN
Problem: Find the minimum EMT trade size for 6 THHN #10 copper conductors.
Step 1 — Conductor area:
From Table 5, #10 THHN = 0.0211 in².
6 × 0.0211 = 0.1266 in².
Step 2 — Fill percentage:
6 conductors = more than 2, so use the 40 percent fill (Table 1).
Step 3 — Compare to Table 4 40% column for EMT:
| Trade Size | 40% Fill (in²) | Fits? |
|---|---|---|
| 1/2 in. | 0.122 | 0.1266 > 0.122 → No |
| 3/4 in. | 0.213 | 0.1266 ≤ 0.213 → Yes |
Answer: The minimum EMT trade size is 3/4 in. (metric designator 21).
Alternative method: divide the conductor area by 0.40 and compare to the total area column. 0.1266 ÷ 0.40 = 0.3165 in² required total area. The 3/4 in. EMT total area is 0.533 in², and 0.3165 ≤ 0.533, so 3/4 in. fits. Both methods produce the same result — pick one and be consistent.
Minimum Trade Size and Pull Rules
Each raceway article sets a minimum trade size. For most metal and nonmetallic raceways, the minimum is 1/2 in. (metric 16). Conductors must be installed so the insulation is not damaged:
- 300.14 — At least 6 in. of free conductor at each box, with at least 3 in. outside the box opening.
- 342.22, 344.22, 358.22 (and similar for other raceways) — Conductors #4 AWG and larger require a bushing or listed fitting at the termination to protect from abrasion.
- 300.34 — Conductors must not be drawn into raceways in a way that damages the insulation.
Maximum Conductors in a Given Raceway
When the exam asks for the maximum number of a given conductor in a specific raceway, divide the 40 percent fill area by the conductor area and round down (you cannot exceed the fill). Example: maximum #10 THHN in 1 in. EMT:
1 in. EMT 40% fill = 0.346 in².
0.346 ÷ 0.0211 = 16.39 → round down → 16 conductors maximum.
You round down for maximum count (you cannot fit a partial conductor) but round up for minimum raceway size (Note 2).
What is the conduit fill percentage for 5 conductors (wire-type) in a single raceway, per NEC Chapter 9 Table 1?
When calculating the minimum raceway size and the result is a decimal value that falls between two standard trade sizes, what does Chapter 9 Note 2 require?
Using NEC Chapter 9 Table 5, what is the cross-sectional area of a single #12 THHN stranded copper conductor?