Conduit Fill Calculator — EMT, PVC, RMC & IMC
Conduit fill is an area problem. Add up the cross-sectional area of every conductor going in (Chapter 9, Table 5), compare it to the raceway's internal area (Table 4), and check the result against the limit in Table 1 — 53% for one conductor, 31% for two, 40% for three or more. Enter up to four different conductor sizes below for a mixed fill.
Fill your raceway
Conductors going in
Count every conductor including neutrals and grounds — fill is about space, not current.
Raceway fill
Within the 40% limit for 4 conductors.
Area used
0.0532 in²
Area allowed
0.1216 in²
See the breakdown
Experienced installers target 30–35% for a pullable raceway. A 24-inch nipple may go to 60% (Chapter 9, Note 4). More than three current-carrying conductors also triggers ampacity derating.
The formula, explained in plain English
Three tables, one comparison. Every conductor's area against the pipe's area, judged by a percentage that depends on how many conductors there are.
Everything counts
Hots, neutrals, and grounds all occupy space, so all of them count toward fill. Only ampacity derating gets to ignore the grounding conductor.
Two is the awkward number
At 31%, two conductors sometimes need a bigger pipe than three of the same conductor would at 40%. If you're at two and failing, check whether the run actually needs a third.
Legal is not pullable
40% passes inspection. It also fights you the whole way through the pipe, especially with bends. Most installers design to 30–35% and thank themselves later.
PVC Schedule 80 is smaller
Its thicker wall means less internal area than Schedule 40 at the same trade size — 0.217 in² versus 0.285 in² at 1/2 inch. Switching to Sch 80 for physical protection can cost you a conductor.
Worked examples
A comfortable branch circuit, a legal-but-miserable pull, and the two-conductor trap.
One 20 A circuit — 4 × #12 THHN in 1/2" EMT
Two hots, a neutral, and a ground. The defaults above.
1/2" EMT internal area = 0.304 in² · limit for 3+ = 40% → 0.1216 in²
fill = 0.0532 ÷ 0.304 = 17.5%
→ PASS · room for 5 more #12 (9 total)
Result: comfortable at 17.5%, and an easy pull. This is why 1/2-inch EMT handles a single branch circuit without a second thought.
Maxed out — 9 × #12 THHN in 1/2" EMT
Four 20 A circuits — 4 hots, 4 neutrals, and one shared equipment ground.
allowable at 40% = 0.1216 in² → 0.1197 ≤ 0.1216 ✓
fill = 0.1197 ÷ 0.304 = 39.4%
→ PASS, barely — and 8 current-carrying conductors triggers a 0.70 ampacity derate
Result: legal on fill, but those #12 THHN conductors are derated from 30 A to 21 A (30 × 1.00 × 0.70), so the 20 A breakers still just work. Add one more circuit and both rules fail at once — fill goes to 48.1% and the bundling factor drops to 0.50, leaving 15 A on a 20 A breaker.
The 31% trap — 2 × 4/0 THHN in 1-1/2" EMT
A two-wire 240 V feeder, no neutral in this raceway.
1-1/2" EMT internal = 2.036 in² · limit for 2 conductors = 31% → 0.6312 in²
0.6474 > 0.6312 → FAIL at 31.8%
at 40% (3+ conductors) it would pass easily
Result: a raceway only 31.8% full fails, because two conductors get the stricter limit. Go to 2-inch EMT, or note that adding a third conductor moves you to the 40% tier and makes the 1-1/2 inch legal.
Conduit fill chart — THHN in EMT
Maximum number of identical THHN/THWN-2 conductors by trade size, computed by the Chapter 9 area method with the correct fill tier applied at each count. Where the maximum is one or two conductors, the stricter 53% and 31% limits are what govern.
| Conductor | Area | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" |
|---|---|---|---|---|---|---|---|
| 14 AWG | 0.0097 in² | 12 | 21 | 35 | 61 | 83 | 138 |
| 12 AWG | 0.0133 in² | 9 | 16 | 25 | 44 | 61 | 100 |
| 10 AWG | 0.0211 in² | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG | 0.0366 in² | 3 | 5 | 9 | 16 | 22 | 36 |
| 6 AWG | 0.0507 in² | 1 | 4 | 6 | 11 | 16 | 26 |
| 4 AWG | 0.0824 in² | 1 | 2 | 4 | 7 | 9 | 16 |
| 2 AWG | 0.1158 in² | 1 | 1 | 2 | 5 | 7 | 11 |
Sources & standards: NEC (NFPA 70) 2023 — Chapter 9 Table 1 percent-fill limits, Table 4 raceway dimensions and percent areas, Table 5 conductor dimensions, Chapter 9 Note 4 for nipples, Annex C pre-computed conductor counts, 310.15(C)(1) bundling adjustment. Values computed by the Chapter 9 area method may differ from Annex C by one conductor at the exact boundary. Local amendments override the model code.
Frequently asked questions
Common questions about conduit fill, the 40% rule, and what counts.
What is the 40% conduit fill rule?
NEC Chapter 9, Table 1 sets the maximum percentage of a raceway's internal cross-section that conductors may occupy. For three or more conductors that limit is 40%. The empty 60% is not waste — it is the room heat needs to escape and the clearance a pull needs to be physically possible.
Why is two conductors 31% but three is 40%?
Geometry. Two round conductors in a round pipe leave awkward crescent-shaped voids and jam against each other during a pull, so Table 1 allows only 31%. Three or more pack more efficiently and get 40%. A single conductor gets the most generous allowance at 53% because there is nothing for it to bind against. This is why a two-conductor pull sometimes needs a larger pipe than three of the same conductor.
Does the ground wire count in conduit fill?
Yes. Every conductor in the raceway counts toward fill, including equipment grounding conductors and neutrals — fill is about physical space, so anything occupying space counts. This differs from ampacity derating, where NEC 310.15(E)(3) excludes grounding conductors from the conductor count. Same raceway, two different counts, and mixing them up is a common error.
How many #12 THHN fit in 3/4 inch EMT?
By the Chapter 9 area method, 16. The 40% allowable area for 3/4-inch EMT is 0.213 in², and 12 AWG THHN is 0.0133 in² each — so 16 conductors use 0.213 in² and land right at the limit. In practice most electricians stop around 9 or 10 in a 3/4-inch run, because 16 conductors also triggers a 0.50 bundling derate and makes the pull very hard.
Do I use Table 4 or Annex C?
Either, and they occasionally disagree by one conductor. Annex C gives pre-computed maximum counts for a raceway full of one identical conductor type — fast, but only valid for a uniform fill. Chapter 9 Tables 1, 4, and 5 are the general method and the only option for a mixed fill. This calculator uses the Chapter 9 area method, so a result may differ from Annex C by one conductor at the exact boundary because of rounding in the published areas.
Does a short nipple get 60% fill?
Yes. Chapter 9, Note 4 permits 60% fill in a nipple — a raceway not more than 24 inches long between boxes, cabinets, or similar enclosures. Note 4 also exempts nipples from ampacity adjustment. This calculator uses the standard Table 1 limits; for a nipple you can compare the computed area against 60% of the Table 4 total shown in the breakdown.
Does derating apply once I pass three current-carrying conductors?
Yes, and it is easy to forget. Fill and derating are separate rules that trigger at different counts. Once more than three current-carrying conductors share a raceway for over 24 inches, NEC 310.15(C)(1) reduces ampacity — 0.80 for 4–6 conductors, 0.70 for 7–9, 0.50 for 10–20. A raceway can be legally filled and still contain conductors that are now too small for their breakers. Check with the Ampacity & Derating Calculator.
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