Conduit Fill Calculator
Add the conductors you're pulling — mixed sizes and insulation types (THHN, XHHW, THW, RHW, bare), including the equipment ground — pick a conduit type and trade size, and check the pull against the governing NEC Chapter 9 fill limit. If it doesn't fit, the tool recommends the smallest size that does, in your conduit type and across the others, and warns when a 3-wire pull lands in the jamming window.
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Per-conductor area (Table 5/8)
Compare minimum size across conduit types
NEC 300.17 & Ch. 9 — Table 1 (53% one / 31% two / 40% over two / 60% nipple ≤24"), Table 4 (raceway area), Table 5 (insulated) & Table 8 (bare) conductor areas. EGC counts toward fill (Note 3).
Conduit Fill Sheet
NEC Chapter 9 conduit fill — prepared with the NORDIX Conduit Fill Calculator (nordixhq.com/tools/conduit-fill-calculator)
| Raceway: EMT (electrical metallic tubing) — 3/4" | Fill limit: 40% (Ch. 9 Table 1) |
| Conductor | Qty | Area each (in²) | Area total (in²) |
|---|---|---|---|
| 12 AWG THHN / THWN / THWN-2 | 3 | 0.0133 | 0.0399 |
| Total conductor area (3 conductors) | 0.0399 in² |
| Allowable area @ 40% | 0.2132 in² |
| Fill used | 7.5% |
| Result | PASS — within the 40% limit |
| Jam ratio (ID ÷ wire OD) | 6.33 |
Method: NEC 300.17 and Chapter 9 — Table 1 fill limits (53% one conductor / 31% two / 40% over two / 60% for nipples ≤24″ between enclosures, Note 4), Table 4 raceway areas, Table 5 (insulated) and Table 8 (bare) conductor areas. The equipment grounding conductor counts toward fill (Note 3). Jam-ratio window per Table 1, Informational Note 2. More than three current-carrying conductors may require an ampacity adjustment per 310.15(C)(1) — verify separately. The AHJ governs.
How conduit fill is calculated
NEC 300.17 requires that the number and size of conductors in any raceway not exceed what its tables allow, and every conduit article (358.22 for EMT, 342.22 for IMC, 344.22 for RMC, 352.22 for PVC, 362.22 for ENT, 348.22 for FMC, 350.22 for LFMC, 356.22 for LFNC) defers to the same rule: Chapter 9, Table 1. The check is three steps:
- Sum the cross-sectional area of every conductor in the raceway using Table 5 (insulated conductors, by insulation type) or Table 8 (bare conductors). The equipment grounding conductor counts like any other (Note 3).
- Take the raceway's total internal area from Table 4 for the specific conduit type — a 2" PVC Schedule 80 (2.874 in²) holds noticeably less than a 2" EMT (3.356 in²).
- Compare against the governing Table 1 percentage of that area: 53% for one conductor, 31% for two, 40% for more than two — or 60% for a nipple of 24" or less (Note 4).
| Conductors in raceway | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| Over 2 | 40% |
| Nipple ≤ 24" between enclosures | 60% (Note 4) |
Two conductors get a lower percentage than three because a pair packs a round bore least efficiently. When counting how many same-size conductors fit, Chapter 9 Note 7 lets a result ending in .8 or higher round up to the next whole conductor.
Worked example
Pull four 8 AWG THHN circuit conductors plus a 10 AWG THHN equipment ground. Will ½" EMT take it?
4 × 8 AWG THHN = 4 × 0.0366 = 0.1464 in² (Table 5)
1 × 10 AWG THHN = 1 × 0.0211 = 0.0211 in² (EGC counts — Note 3)
Total conductor area = 0.1675 in², 5 conductors → 40% limit
½" EMT: 0.304 in² × 0.40 = 0.1216 in² < 0.1675 in² → ✗ overfill
¾" EMT: 0.533 in² × 0.40 = 0.2132 in² ≥ 0.1675 in² → ✓ fits (31.4% of the pipe)
The answer: ½" EMT fails; ¾" EMT is the minimum. Enter the same conductors above and the calculator reproduces each line — and because five current-carrying conductors may share the raceway, it also points you to the ampacity adjustment check.
Maximum THHN/THWN conductors in EMT
Computed from Tables 1, 4, and 5 with the Note 7 round-up — the same math as NEC Informative Annex C, Table C.1. Use the calculator's "How many conductors fit?" mode for other conduit types and insulations.
| Wire size | 1/2″ EMT | 3/4″ EMT | 1″ EMT | 1-1/4″ EMT | 1-1/2″ EMT | 2″ EMT |
|---|---|---|---|---|---|---|
| 14 AWG | 12 | 22 | 35 | 61 | 84 | 138 |
| 12 AWG | 9 | 16 | 26 | 45 | 61 | 101 |
| 10 AWG | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG | 3 | 6 | 9 | 16 | 22 | 36 |
| 6 AWG | 2 | 4 | 7 | 12 | 16 | 26 |
| 4 AWG | 1 | 2 | 4 | 7 | 10 | 16 |
| 3 AWG | 1 | 1 | 3 | 6 | 8 | 13 |
| 2 AWG | 1 | 1 | 3 | 5 | 7 | 11 |
| 1 AWG | 1 | 1 | 1 | 4 | 5 | 8 |
| 1/0 AWG | 1 | 1 | 1 | 3 | 4 | 7 |
| 2/0 AWG | 0 | 1 | 1 | 2 | 3 | 6 |
| 3/0 AWG | 0 | 1 | 1 | 1 | 3 | 5 |
| 4/0 AWG | 0 | 1 | 1 | 1 | 2 | 4 |
Conduit internal areas (NEC Chapter 9, Table 4)
Total internal cross-section in square inches per trade size — the "100%" column the fill percentages apply to. Blank cells are trade sizes the NEC does not list for that raceway type.
| Trade size | EMT | IMC | RMC | PVC-40 | PVC-80 | ENT | FMC | LFMC | LFNC-B |
|---|---|---|---|---|---|---|---|---|---|
| 3/8″ | 0.116 | 0.192 | 0.192 | ||||||
| 1/2″ | 0.304 | 0.342 | 0.314 | 0.285 | 0.217 | 0.285 | 0.317 | 0.314 | 0.314 |
| 3/4″ | 0.533 | 0.586 | 0.549 | 0.508 | 0.409 | 0.508 | 0.533 | 0.541 | 0.541 |
| 1″ | 0.864 | 0.959 | 0.887 | 0.832 | 0.688 | 0.832 | 0.817 | 0.873 | 0.873 |
| 1-1/4″ | 1.496 | 1.647 | 1.526 | 1.453 | 1.237 | 1.453 | 1.277 | 1.528 | 1.528 |
| 1-1/2″ | 2.036 | 2.225 | 2.071 | 1.986 | 1.711 | 1.986 | 1.858 | 1.981 | 1.981 |
| 2″ | 3.356 | 3.630 | 3.408 | 3.291 | 2.874 | 3.291 | 3.269 | 3.246 | 3.246 |
| 2-1/2″ | 5.858 | 5.135 | 4.866 | 4.695 | 4.119 | 4.909 | 4.881 | ||
| 3″ | 8.846 | 7.922 | 7.499 | 7.268 | 6.442 | 7.069 | 7.475 | ||
| 3-1/2″ | 11.545 | 10.584 | 10.010 | 9.737 | 8.688 | 9.621 | 9.731 | ||
| 4″ | 14.753 | 13.631 | 12.882 | 12.554 | 11.258 | 12.566 | 12.692 | ||
| 5″ | 20.212 | 19.761 | 17.855 | ||||||
| 6″ | 29.158 | 28.567 | 25.598 |
PVC-40/PVC-80 are rigid PVC Schedule 40 and 80 (Article 352). The 2023 NEC permits EMT through 6" trade size (358.20), but its 5" and 6" Table 4 rows are omitted here pending verification. 3/8" FMC has special conductor rules when fittings are inside the conduit (348.22).
The jam ratio — a pull that passes fill can still stick
When exactly three conductors are pulled and the conduit's inside diameter is 2.8 to 3.2 times the conductor OD, two conductors can lie side by side across the bore and wedge the third at a bend — Table 1, Informational Note No. 2. Example: three 6 AWG THHN (0.254" OD) in ¾" Schedule 40 PVC (0.804" ID) pass the 40% fill check at 30% but sit at a jam ratio of 3.16, squarely in the window. This calculator computes the ratio on every 3-wire pull; the usual fix is one trade size up or down, which moves the ratio out of the window entirely.
Conduit fill FAQ
What percentage fill does the NEC allow in conduit?
NEC Chapter 9, Table 1: 53% of the conduit’s internal area for one conductor, 31% for two conductors, and 40% for more than two. A conduit nipple no longer than 24 inches between enclosures may be filled to 60% (Chapter 9, Note 4).
Does the ground wire count toward conduit fill?
Yes. Chapter 9, Note 3 counts equipment grounding and bonding conductors at their actual cross-sectional area — Table 5 for insulated grounds, Table 8 for bare ones. This calculator has a dedicated EGC row that is included in both the area and the conductor count.
How many 12 AWG THHN wires can go in 3/4-inch EMT?
16. A 3/4″ EMT has 0.533 in² of internal area; 40% of that is 0.2132 in², and each 12 AWG THHN occupies 0.0133 in² (Table 5), giving 16.03 — truncated to 16 conductors, which matches NEC Informative Annex C, Table C.1.
What is the 60% conduit nipple rule?
Chapter 9, Note 4: a conduit or tubing nipple not exceeding 24 inches installed between boxes, cabinets, or similar enclosures may be filled to 60% of its cross-sectional area, and the ampacity adjustment factors of 310.15(C)(1) do not apply to conductors in it.
What is the conduit jam ratio?
When pulling exactly three conductors, jamming can occur where the ratio of conduit inside diameter to conductor outside diameter falls between 2.8 and 3.2 (Table 1, Informational Note No. 2) — two wires can lie side by side and wedge the third at a bend. The fix is usually one trade size up or down; this calculator flags the condition automatically.
Is conduit fill the same as conductor derating?
No. Conduit fill (Chapter 9, Table 1) limits the physical space conductors occupy. Ampacity adjustment (310.15(C)(1)) reduces how much current each conductor may carry when more than three current-carrying conductors share a raceway. A pull can pass fill and still require derating — use a wire ampacity calculator for that check.
Which NEC edition does this calculator use?
The Table 1, 4, 5, and 8 values used here are identical across the 2017, 2020, and 2023 editions of the NEC; section numbering follows the 2023 edition. Always confirm against the edition and amendments adopted by your jurisdiction.
Conduit fill is one of three checks on most feeder and branch-circuit runs — pair it with the Voltage Drop Calculator for run length and the Wire Ampacity & Derating Calculator for the 310.15(C)(1) adjustment when more than three current-carrying conductors share the raceway.
Prefer to eyeball it? The conduit size & fill chart lists the usable fill area for every trade size of EMT, IMC, RMC, and PVC.
Method: conductor areas from NEC Chapter 9, Table 5 (insulated, by insulation group) and Table 8 (bare), summed and compared to the governing Table 1 maximum — 53% for one conductor, 31% for two, 40% for more than two, 60% for a nipple ≤24" (Note 4) — of the raceway area from Table 4. Values are identical across NEC 2017/2020/2023; section numbering follows 2023. This is a design aid, not a substitute for the code — verify against the edition and amendments adopted in your jurisdiction.