Electrical
Conduit fill is one calculation: add up the cross-sectional area of every conductor going in the pipe, then check it against a percentage of the pipe's interior area. The percentage is40% for three or more conductors, 53% for one conductor alone, and 31% for exactly two. That middle case is where almost every chart on the internet, including NEC's own Annex C, quietly gets it wrong.
Maximum number of same-size THHN/THWN-2 conductors in EMT at the 40% limit. Every cell is derived from NEC Chapter 9 Table 4 (conduit area) and Table 5 (conductor area), so you can reproduce any number here with a calculator and the sourced values at the bottom of this page.
| AWG | 1/2″ | 3/4″ | 1″ | 1-1/4″ | 1-1/2″ | 2″ |
|---|---|---|---|---|---|---|
| 14 | 12 | 21 | 35 | 61 | 83 | 138 |
| 12 | 9 | 16 | 25 | 44 | 61 | 100 |
| 10 | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 | 3 | 5 | 9 | 16 | 22 | 36 |
| 6 | 2 | 4 | 6 | 11 | 16 | 26 |
| 4 | 1 | 2 | 4 | 7 | 9 | 16 |
| 3 | 1 | 2 | 3 | 6 | 8 | 13 |
| 2 | 1 | 1 | 2 | 5 | 7 | 11 |
| 1 | — | 1 | 2 | 3 | 5 | 8 |
| 1/0 | — | 1 | 1 | 3 | 4 | 7 |
| 2/0 | — | — | 1 | 2 | 3 | 6 |
| 3/0 | — | — | 1 | 2 | 3 | 5 |
| 4/0 | — | — | 1 | 1 | 2 | 4 |
A dash means not even one conductor of that gauge fits that trade size at this limit. For the full run out to 4″, for PVC and RMC, and for conductor mixes, use the conduit fill calculator.
Take two 6 AWG THHN conductors and a length of ½″ EMT. Search for a conduit fill chart and you will be told this is fine. NEC Annex C says 2. The chart above, drawn at 40%, says 2. Both are wrong for this pull, and the arithmetic is not close.
Two 6 AWG THHN come to 0.1014 square inches. Half-inch EMT has an interior area of 0.304 square inches. At the 40% three-or-more limit the allowance would be 0.1216 square inches, and 0.1014 fits inside that comfortably. But this is a two-conductor pull, and Table 1 holds exactly two conductors to 31% — 0.0942 square inches. The conductors are 0.1014. They do not fit, by about 8%.
The correct answer is 3/4″ EMT, where the same two conductors land at 19.0% fill against the same 31% ceiling. One conductor of 6 AWG in ½″ is fine. Three or more of a smaller gauge is fine. Exactly two 6 AWG is the case that fails, and it fails because of a rule most people have never read.
This ordering looks like a typo the first time you see it. More conductors should mean less room, not more. The reason is jamming. Three or more round conductors of similar size arrange themselves in a bundle that pulls through a bend predictably. Exactly two can wedge — they lie side by side, and if the ratio of conduit diameter to conductor diameter lands near 3.0, they can lock against each other and the pipe wall partway through a pull and stop dead.
So the 31% figure is not about heat or about physical volume. It is a pulling-mechanics allowance, which is why it sits below the 40% you get for three. Chapter 9 carries a related note about jam ratio for exactly three same-size conductors, and the calculator flags that case as a warning rather than a failure, because it depends on the bends in your actual run rather than on area alone.
Annex C computes its entire table at 40%. That is correct for the great majority of its cells and wrong for the band where the real limit is 31%. Comparing a 40% chart against the count-aware Table 1 rule across all four conduit types and all thirteen gauges, 21 cells permit one conductor more than Chapter 9 allows. Every one of them is a two-conductor cell. In EMT specifically:
| EMT size | Gauge | Charts say | Table 1 |
|---|---|---|---|
| 1/2″ | 6 AWG | 2 | 1 |
| 3/4″ | 3 AWG | 2 | 1 |
| 1″ | 1 AWG | 2 | 1 |
| 1-1/4″ | 3/0 AWG | 2 | 1 |
| 1-1/2″ | 4/0 AWG | 2 | 1 |
None of these will burn a building down. Fill limits exist so conductors can be pulled without damaging their insulation and so heat can escape, not because 31% is a cliff edge. But an inspector working from Chapter 9 rather than Annex C can fail a two-conductor run that a chart told you was fine, and re-pulling a run because you trusted the wrong column is an expensive afternoon.
Nine 12 AWG THHN in ½″ EMT is the number most electricians carry in their head, and it is correct — but it is closer to the line than the round number suggests. Nine conductors at 0.0133 square inches each come to 0.1197 square inches. The 40% allowance in ½″ EMT is 0.1216. That is 98.4% of the permitted fill.
A tenth #12 needs 0.1330 square inches and pushes you to ¾″. There is no rounding-up available and no engineering judgment to apply — it is arithmetic, and it is the single most common place a residential branch-circuit pull outgrows its pipe.
The fill percentages never change with conduit type. The interior area does, and by more than people expect, because trade size names a nominal dimension rather than a real one. Twelve 12 AWG THHN conductors need a different trade size depending on what the pipe is made of:
| Type | ½″ area | ¾″ area | Size for 12× #12 |
|---|---|---|---|
| EMT | 0.304 | 0.533 | 3/4″ |
| PVC Sch 40 | 0.285 | 0.508 | 3/4″ |
| PVC Sch 80 | 0.217 | 0.409 | 3/4″ |
| RMC | 0.314 | 0.549 | 3/4″ |
PVC Schedule 80 is the one that catches people. It shares a trade size name with Schedule 40 but carries a thicker wall for physical protection, so the interior is meaningfully smaller. If you sized a run on a Schedule 40 chart and installed Schedule 80 because the run was exposed to damage, re-check it.
Every chart on this page, and every chart anywhere else, assumes all your conductors are the same size. Real pulls frequently are not — a circuit with an undersized equipment grounding conductor breaks the assumption immediately, and so does any feeder sharing a raceway.
For a mix, there is no row to look up. You add the areas individually. Three 6 AWG plus one 10 AWG THHN comes to 0.1732 square inches across 4 conductors, which at the 40% limit needs 3/4″ EMT and lands at 32.5% fill. The conduit fill calculator handles mixed pulls and shows the intermediate areas, which is the part you want when an inspector asks how you arrived at the size.
Passing a fill check means the conductors physically fit and can be pulled without damage. It says nothing about whether those conductors are big enough for the load. Those are separate questions with separate tables, and fill is the one people do first because it is easier.
Ampacity comes from NEC Table 310.16, and it drops once you put more than three current-carrying conductors in the same raceway — the adjustment factors in 310.15(C)(1) can take a conductor well below its table rating precisely because you filled the pipe efficiently. Long runs add a third constraint again, since voltage drop can demand a larger conductor than either fill or ampacity would. Size the conductor first with the wire gauge calculator, then find the pipe that holds it. Doing it the other way round means redoing both.
Grouped by manufacturer or publisher, then by document. Expand a group for the cited document, its verified-on date, and every value taken from it — all of it stays in the HTML either way.
NFPA 70 (NEC), Chapter 9, Table 1 — Percent of Cross Section of Conduit and Tubing for Conductors and Cables. Confirmed against two independent reproductions (2020 NEC). · verified 2026-08-01
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NFPA 70 (NEC), Chapter 9, Table 1. The 2-conductor case is the strictest, to avoid jamming. · verified 2026-08-01
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NFPA 70 (NEC), Chapter 9, Table 1. The most commonly applied figure in real installations. · verified 2026-08-01
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In ½″ EMT with THHN/THWN-2, at the 40% limit that applies to three or more conductors: 12 of 14 AWG, 9 of 12 AWG, 5 of 10 AWG, or 3 of 8 AWG. Six AWG is the exception worth knowing: published charts list 2, but exactly two conductors is capped at 31%, and two 6 AWG THHN need 3/4″.
¾″ EMT holds 21 of 14 AWG THHN, 16 of 12 AWG, 10 of 10 AWG, 5 of 8 AWG or 4 of 6 AWG, again at the 40% three-or-more limit. Its interior area is 0.533 sq in against ½″ EMT's 0.304 — roughly 1.75× the room for one trade size up.
NEC Chapter 9, Table 1 caps conductor fill at 40% of a conduit's interior cross-section when three or more conductors are in the raceway. It is not a universal figure. One conductor alone may fill 53%, and exactly two conductors are held to 31% — stricter than three, because two round conductors can jam against each other and the pipe wall during a pull.
Annex C computes its whole table at 40%, so its one- and two-conductor cells do not reflect the 53% and 31% limits Table 1 assigns to those counts. In 21 cells across the four conduit types, that makes Annex C one conductor more permissive than Chapter 9 arithmetic. The clearest is two 6 AWG THHN in ½″ EMT: 0.1014 sq in against an allowable 0.0942.
Yes. The percentages are identical, but the interior area is not, so the same conductors can need a different trade size. ½″ EMT is 0.304 sq in, PVC Schedule 80 is smaller because of its thicker wall, and RMC differs again. A chart drawn for EMT is not valid for PVC — check the type you are actually installing.
A same-size chart cannot answer that, and this is where most people misuse one. Add up the individual areas from NEC Chapter 9, Table 5 and compare the total against the percentage for your total conductor count. Three 6 AWG plus one 10 AWG THHN comes to 0.1732 sq in, which needs 3/4″ EMT — not something any single row of a chart would tell you.
