SizingKitRaceway
Conduit fill calculator
Enter the conductors — as many different sizes in one pull as you like — and it returns the smallest trade size of EMT, IMC, RMC, PVC, ENT or FMC that legally takes them, the percentage of the raceway they occupy, and the row of NEC Chapter 9 Table 1 the count put you on. It is free and needs no account. It also refuses to guess: an insulation type the site does not carry dimensions for produces no answer rather than a substituted one.
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- No signup
- 7 raceway types
- 10 trade sizes
- Mixed sizes in one pull
What is going in the pipe
One row per conductor size. Different sizes in one raceway are added on total area, which is what Chapter 9 asks for and what a per-size table cannot do.
NEC Article 358 · internal diameters from ANSI C80.3 · trade sizes 1/2 through 4
A nipple, or something Table 5 does not carryStandard fill · no untabulated conductors
For a multiconductor cable, a bare ground or an insulation this page does not carry. Chapter 9 Notes 3, 6 and 9 all count these at their actual dimensions; a cable counts as one conductor at the area of a circle of its major diameter.
It changes which Table 1 row applies, which is why it is asked separately from the area.
Smallest raceway that takes them
3/4 in EMT
Metric designator 21 · internal diameter 0.824 in (20.9 mm)
Occupied
26.8%
of the 0.533 in² inside it, against 40% allowed
9 conductors puts this pull on the more than two conductors row of Chapter 9 Table 1, which is 40%. Adding a third conductor to a two-conductor pull raises the allowance from 31% to 40% — the one place on this page where more wire means more room.
Where the area went
| 6 × 12 AWG THHN at 0.13 in over the insulation | 0.0798 in² |
|---|---|
| 3 × 10 AWG THHN at 0.164 in over the insulation | 0.0633 in² |
| Total conductor area | 0.1431 in² · 92 mm² |
| Allowed: 40% of 0.533 in² | 0.2130 in² · 137 mm² |
And the size above it
Going up to 1 in EMT drops the fill from 26.8% to 16.6%, leaving 0.203 in² spare. Worth taking when the run is long or has several bends: passing the percentage is necessary and not sufficient, because 300.17 separately requires the raceway to be large enough to pull the conductors in and back out without damaging the insulation, and no arithmetic on this page measures that.
One size, every trade size
The all-one-size case, which is what the hundred-odd tables of NEC Annex C answer. It is computed rather than looked up, so it covers combinations Annex C prints and combinations it does not.
| Trade size | 1/2 | 3/4 | 1 | 1-1/4 | 1-1/2 | 2 | 2-1/2 | 3 | 3-1/2 | 4 |
|---|---|---|---|---|---|---|---|---|---|---|
| 12 AWG THHN | 9 | 16 | 26 | 45 | 61 | 101 | 176 | 266 | 347 | 443 |
12 AWG THHN is 0.13 in over the insulation — 0.092 in of stranded conductor plus 0.019 in of wall each side — giving 0.0133 in². NEC (NFPA 70) 2023, Chapter 9, Note 7: when calculating the maximum number of conductors or cables, all of the same size, permitted in a conduit or tubing, the next higher whole number is used where the calculation results in a decimal of 0.8 or larger. The allowance applies only where every conductor is the same size — a mixed pull is sized on total area with no rounding allowance at all.
Raceway areas: NEC (NFPA 70) 2023, Chapter 9, Table 4 — Dimensions and Percent Area of Conduit and Tubing. Internal diameters are the manufactured dimension from each raceway's product standard (ANSI C80.3 for EMT, C80.6 for IMC, C80.1 for RMC, NEMA TC 2 / UL 651 for PVC, NEMA TC 13 for ENT, UL 1 for FMC); the total area is π/4 × d² and the four fill areas are that total times the Chapter 9 Table 1 percentages. Computing rather than transcribing reproduces every total-area figure Table 4 prints, at the three decimals it prints them, for all seven raceway types. Conductor areas: NEC (NFPA 70) 2023, Chapter 9, Table 5 — Dimensions of Insulated Conductors and Fixture Wires. Generated from the Class B stranded conductor diameters of ASTM B8, which Table 5 and Table 8 both print, plus the insulation wall schedules of UL 83 (THHN: PVC 15/20/30/40/50/60/70 mil with a 4/4/5/6/7/8/9 mil nylon jacket, by size band) and UL 44 (XHHW: cross-linked polyethylene 30/45/55/65/80 mil). Reproduces every printed diameter and every printed area of the 'THHN, THWN, THWN-2' and 'XHH, XHHW, XHHW-2, ZW' rows of Table 5 exactly.
Only two insulation types are carried. TW, THW, THHW, RHH, RHW, USE, SIS and the fixture wires are all in Table 5 with different and larger dimensions, and are not here — read them from the code book rather than substituting THHN, which is the thinnest common building-wire insulation and would under-state the fill. Sizes 700, 800 and 900 kcmil are likewise omitted so that the size names line up with the conductor table. Table 5 gives one dimension per size and it is the stranded one; solid 14, 12 and 10 AWG are physically slightly smaller and the code does not credit the difference. Compact stranded conductors are Table 5A, not this table.
Table 1 applies to complete raceway systems, not to a sleeve used only to protect exposed wiring from physical damage (Note 2). Equipment grounding and bonding conductors, insulated or bare, count toward the fill, at their actual dimensions (Note 3). A multiconductor cable counts as one conductor at the area of a circle of its major diameter (Note 9), and any conductor or cable not tabulated in Chapter 9 is counted at its actual dimensions (Note 6). Passing the fill percentage is necessary but not sufficient: 300.17 independently requires the raceway to be large enough to install and withdraw the conductors without damaging their insulation, which on long runs with many bends means going a size up regardless of what the arithmetic permits.
Everything above is cross-sectional area and nothing above is a permit. Whether the pull can be made, whether the bends are inside the radius the raceway article allows, and whether the assembly is what was designed are all judgments made on site and signed off by somebody qualified.
How to size a raceway for a mixed pull
Loaded with six 12 AWG and three 10 AWG THHN, which is a small enough pull that the trade-size answer is not obvious by eye.
List the conductors, one row per size
Count, size and insulation on each row. Add rows for every different size going in — the calculation sums cross-sectional areas, which is the only correct way to handle a pull that is not all one size, and it is why a per-size lookup table cannot answer this question.
Pick the raceway and check the exceptions
The seven types carry different internal diameters at the same trade size: 1/2 in EMT has 0.622 in inside it and 1/2 in PVC Schedule 80 has 0.526 in. Open the exceptions panel if the length between enclosures is 24 in or less, or if something in the pull is not in Chapter 9 Table 5 and has to be entered at its measured area.
Read the Table 1 row you landed on, not just the answer
The allowable percentage depends on the total conductor count: 53% for one, 31% for exactly two, 40% for more than two, 60% for a nipple. Adding one conductor to a two-conductor pull raises the allowance rather than lowering it, and that is the one place on this page where more wire buys more room.
Technical specifications
| Raceway types and sizes | EMT, IMC, RMC, PVC Schedule 40, PVC Schedule 80, ENT and FMC across 10 trade sizes from 1/2 in to 4 in, metric designators 16 through 103. ENT stops at 2 in here, as it does in the table |
|---|---|
| Where the areas come from | π/4 × d² on the internal diameter in each raceway's product standard: ANSI C80.3 for EMT, C80.6 for IMC, C80.1 for RMC, NEMA TC 2 / UL 651 for PVC, NEMA TC 13 for ENT and UL 1 for FMC. Computing rather than transcribing reproduces every printed total-area figure of Chapter 9 Table 4 |
| Fill percentages | 53% for one conductor, 31% for exactly two, 40% for more than two — all from Chapter 9 Table 1 — and 60% for a nipple not over 24 in, which is Chapter 9 Note 4 rather than Table 1 |
| Conductor dimensions | NEC Chapter 9 Table 5 for the THHN/THWN/THWN-2 row and the XHH/XHHW/XHHW-2/ZW row, 21 sizes each, generated from the ASTM B8 Class B stranded diameters plus the UL 83 and UL 44 insulation wall schedules |
| Insulations deliberately absent | TW, THW, THHW, RHH, RHW, USE and SIS are all in Table 5 with larger dimensions and are not reproduced here. THHN is the thinnest common building-wire wall, so substituting it for any of them would understate the fill |
| Other omissions | Trade sizes 5 and 6, the 3/8 in size Table 4 lists for FMC only, liquidtight conduit (LFMC, LFNC-A, LFNC-B) and conductors of 700, 800 and 900 kcmil. A size picker that stops where the table stops is the point |
| Same-size count strip | The maximum number of one size in every trade size — the computation behind the hundred-odd tables of NEC Annex C — with the Chapter 9 Note 7 rounding at a decimal of 0.8 applied only to the over-two case |
| Where it computes | Entirely in the browser. A conductor schedule describes a specific job, and this one is never uploaded or retained |
Frequently asked questions
Why do two conductors get less room than three?
Because two circles inside a circle jam rather than nest, and the code accounts for it: Chapter 9 Table 1 gives exactly two conductors 31% while more than two get 40%. It is the row most calculators quietly drop, and dropping it permits a raceway that is both a violation and a genuinely miserable pull. Geometrically the reason is that two equal circles inside a larger one are forced into a diametral line and leave two crescent voids that nothing can occupy, whereas three or more start to pack. Note that it is exactly two — three conductors go back up to 40%.
Does the equipment grounding conductor count toward the fill?
Yes. Chapter 9 Note 3 requires equipment grounding and bonding conductors, insulated or bare, to be included in the fill calculation at their actual dimensions. Bare conductors are the awkward case because they are not in Table 5 at all — a bare 12 AWG is 0.0808 in in diameter, which is 0.0051 in², and that goes in the extra-area field on this page. What grounding conductors do not do is count as current-carrying conductors for the ampacity adjustment of 310.15(C)(1), which is a different count for a different purpose.
Can I use the THHN dimensions for THW or TW?
No, and this page will not let you. THHN is PVC insulation plus a thin nylon jacket, which is the thinnest wall in common building wire; THW and TW have substantially thicker walls and therefore larger diameters and areas in Chapter 9 Table 5. Substituting one for the other understates the fill, which is the direction that produces an undersized raceway. Those rows exist in the code book and are not reproduced here, so the honest instruction is to read them from Table 5 directly and enter the total as an area.
What counts as a nipple, and does 60% really apply?
A nipple is a length of conduit or tubing not exceeding 600 mm — 24 in — between boxes, cabinets or similar enclosures, and Chapter 9 Note 4 does permit it to be filled to 60% of its total cross-sectional area at any conductor count. The reason is that there is no length over which the conductors can bind, so the pull is trivially achievable. The same 24 in threshold appears in 310.15(C)(1), which exempts a short bundle from the conductor-count ampacity adjustment. It is a genuine allowance and not a loophole, but measure it: 25 in is not a nipple.
My pull passes at 39% and the apprentice still cannot get it in. What is missing?
NEC 300.17, which is a separate requirement from the fill percentage and is not arithmetic. It requires the raceway to be large enough to permit the conductors to be installed and withdrawn without damaging their insulation, and on a long run with several bends that can mean a trade size above whatever Table 1 permits. Nothing on this page measures pulling tension, jam ratio or sidewall pressure. The rule of thumb worth knowing is the jam ratio: three conductors of the same size in a raceway whose internal diameter is between about 2.8 and 3.2 times the conductor diameter can wedge, which is a geometry problem that a percentage cannot detect.
How do I enter a multiconductor cable, like MC or a control cable?
As one conductor at the area of a circle of its major diameter, using the extra-area field. Chapter 9 Note 9 says exactly that for cables of two or more conductors, and Note 6 says anything not tabulated in Chapter 9 is counted at its actual dimensions. Take the outside diameter from the manufacturer's data sheet, square it, and multiply by 0.7854. For an elliptical cable the code uses the major diameter, which is the conservative choice.
Why does this pick a bigger conduit than the Annex C table says?
Almost always because your pull is not all one size. Chapter 9 Note 7 permits rounding up when the calculation lands on a decimal of 0.8 or more, and that allowance applies only when every conductor is the same size — which is what Annex C tabulates. A mixed pull is sized on total area with no rounding allowance at all. The strip at the bottom of this page is the all-one-size case with Note 7 applied, so you can see both answers and know which rule produced each.
About Chapter 9, and why fill is not one percentage
Nearly everyone in the trade can recite 40%, and nearly everyone stops there. Chapter 9 Table 1 has three rows and a note attached, and the count decides which one applies: 53% for a single conductor, 31% for exactly two, 40% for more than two, and 60% for a nipple under Note 4. The 31% row is the interesting one because it runs against intuition — two conductors get a tighter limit than three — and because a calculator that takes the percentage as an input instead of deriving it from the count will miss it every time. That is why this page never asks which percentage to use.
The other half of the calculation is Table 5, and it is where the errors that matter live. A conductor's fill area is the area over its insulation, not the area of the metal: 12 AWG THHN is 0.0808 in of copper and 0.111 in over the nylon, and squaring the wrong one is a 90% error. Insulation type changes it again, which is why THHN and XHHW are offered separately and why the thicker-walled types are not offered at all rather than approximated. If the size itself is what you are still deciding, that happens on the wire size calculator and the conductor geometry behind both pages is on the wire gauge chart. Filling a raceway also has a consequence the fill calculation does not mention: more than three current-carrying conductors in one raceway triggers the ampacity adjustment of 310.15(C)(1), so a pull that passes on area can still force a larger conductor, which then takes more area again.
Worth keeping separate in your head: fill is a legal test and pullability is a physical one, and passing the first says nothing about the second. NEC 300.17 requires the raceway to be large enough to install and withdraw the conductors without damaging the insulation, and it is enforced by an inspector's judgment rather than by a percentage. Long runs, multiple 90° bends and jam-prone three-conductor geometries all argue for the next trade size up, and the headroom figure under the answer is there so that decision is made with a number rather than a feeling. Upstream of all of this sits the question of whether the service can carry what you are adding, which is an Article 220 calculation on the electrical load calculator, and if the raceway is feeding a transformer the current on each side comes from the kVA to amps calculator. This page reports what the tables say; the installation is signed off by somebody licensed to sign it.
What happens to your conductor schedule
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
Rows you add, counts you type and the raceway you pick exist only as React state in this tab. There is no save button because there is nowhere to save to, and reloading gives you the two example rows back.