What conduit fill is, and what the percentage protects
Conduit fill is a ratio of areas: the summed cross-sectional area of every conductor you intend to pull, divided by the total internal area of the raceway. The National Electrical Code caps that ratio, and the cap is deliberately generous in absolute terms — a raceway filled to the 40% maximum still looks about half empty when you sight down it.
The reason for the cap is mechanical, not thermal. A bundle that occupies too much of the bore cannot be pulled without the conductors dragging hard against each other and against the inside radius of every bend. That drives sidewall pressure up, scores or strips insulation, and in the worst case jams the bundle solid partway through the run. Heat is handled separately: once you have more than three current-carrying conductors in the same raceway, the ampacity adjustment factors of NEC 310.15(C)(1) apply, and you size those with the ampacity derating calculator. Fill and derating are two independent checks, and a pull can pass one while failing the other.
The formula, and where each number comes from
You need three published numbers and one decision.
Conductor area. Chapter 9, Table 5 lists the approximate area in square inches of one insulated conductor, by size and by insulation type. Insulation matters more than most people expect: a 6 AWG conductor is 0.0507 in² in THHN but 0.0590 in² in XHHW and 0.0726 in² in THW, so changing insulation on a tight pull can cost you a trade size. You can check any of those areas yourself: Table 5 prints the approximate overall diameter beside the area, and the area is simply π ÷ 4 times the square of it — 6 AWG THHN is 0.254 in. across, and 0.7854 × 0.254² = 0.0507 in². Multiply each area by how many conductors of that size you are pulling and add the products. Count every conductor — ungrounded, grounded and equipment grounding conductors all take up bore.
Raceway area. Chapter 9, Table 4 gives the total internal area of each trade size of each raceway article, along with pre-computed 60%, 53%, 40% and 31% columns. The internal area is not the trade size: 3/4 in. EMT has an internal area of 0.533 in², which corresponds to an inside diameter of about 0.82 in., not 0.75 in.
The limit. Chapter 9, Table 1 sets the permitted fill by conductor count, and the count is every conductor in the raceway, not the number of circuits: 53% for one conductor, 31% for two, and 40% for three or more. The single-conductor figure is the highest because there is nothing for the conductor to bind against. The two-conductor figure is the lowest because two round conductors in a round bore tend to spiral around one another as they are pulled, which drives friction up faster than the area ratio suggests.
The decision. Note 4 to Chapter 9 permits 60% fill in a nipple — a length of conduit or tubing not exceeding 600 mm (24 in.) between boxes, cabinets or similar enclosures — and states that the 310.15(C)(1) adjustment factors need not be applied there. If your run is longer than 24 in., that allowance is off the table.
Note 7 to Chapter 9 covers the rounding. When you are working out how many conductors of one size fit and the arithmetic gives a decimal of 0.8 or larger, you may use the next whole number. That note is why Annex C lists twenty-two 14 AWG THHN in 3/4 in. EMT although 0.2132 ÷ 0.0097 gives only 21.98, and why it lists six 8 AWG in the same tubing where 0.2132 ÷ 0.0366 gives 5.83. It is a rounding allowance and nothing more: 16.03 for 12 AWG in the same raceway becomes sixteen because you drop the remainder, not because of Note 7.
Worked example: a mixed pull in 3/4 in. EMT
You are pulling one 30 A circuit and two 20 A circuits in a single EMT run, all THHN copper: three 10 AWG conductors, six 12 AWG conductors, and one 12 AWG equipment grounding conductor. That is ten conductors in total.
- Look up the conductor areas. Chapter 9, Table 5: 10 AWG THHN is 0.0211 in²; 12 AWG THHN is 0.0133 in².
- Sum the areas. Three 10 AWG × 0.0211 = 0.0633 in². Seven 12 AWG (six circuit conductors plus the ground) × 0.0133 = 0.0931 in². Total = 0.1564 in².
- Pick the limit. Ten conductors is more than two, so Table 1 gives 40%.
- Try 1/2 in. EMT. Table 4 internal area is 0.304 in², so the permitted area is 0.304 × 0.40 = 0.1216 in². Your 0.1564 in² exceeds that, so it fails; the actual fill would be 0.1564 ÷ 0.304 = 51.4%.
- Try 3/4 in. EMT. Table 4 internal area is 0.533 in², so the permitted area is 0.533 × 0.40 = 0.2132 in². Your 0.1564 in² fits with 0.0568 in² to spare.
- Report the fill. 0.1564 ÷ 0.533 × 100 = 29.3%. That is well inside the 40% cap, so 3/4 in. EMT is the smallest trade size that works.
How to read the percentage you get back
Anything at or below the Table 1 figure is code-compliant. There is no partial credit and no engineering judgement to apply: 40.1% fails and 40.0% passes. What varies is how comfortable the pull will be.
Read the number in three bands. Below roughly 30% you have a pull one person can usually make by hand on a short run with a couple of bends. From there up to the limit you are into pulling lubricant, a tugger on longer runs, and real attention to how many bends sit between pull points. At the limit itself you have no margin for the extra circuit the customer asks for on the last day of rough-in, which is the practical argument for buying one trade size up whenever the material cost difference is small.
Bends matter as much as fill. Every raceway article caps the total bend between pull points at 360 degrees — 358.26 for EMT, 344.26 for RMC, and the corresponding section in each other article. Four quarter bends and you must install a pull point, no matter how empty the conduit is. A run at 38% fill with three 90s and an offset will fight you far harder than a dead straight run at the same fill.
Conductor areas and how many fit in EMT (THHN, 40% basis)
| Size | Area (in²) | 1/2 in. | 3/4 in. | 1 in. | 1-1/4 in. | 1-1/2 in. | 2 in. |
|---|---|---|---|---|---|---|---|
| 14 AWG | 0.0097 | 12 | 22 | 35 | 61 | 84 | 138 |
| 12 AWG | 0.0133 | 9 | 16 | 26 | 45 | 61 | 101 |
| 10 AWG | 0.0211 | 5 | 10 | 16 | 28 | 38 | 63 |
| 8 AWG | 0.0366 | 3 | 6 | 9 | 16 | 22 | 36 |
| 6 AWG | 0.0507 | 1 | 4 | 7 | 12 | 16 | 26 |
| 4 AWG | 0.0824 | 1 | 2 | 4 | 7 | 10 | 16 |
| 2 AWG | 0.1158 | 1 | 1 | 3 | 5 | 7 | 11 |
| 1/0 AWG | 0.1855 | 0 | 1 | 1 | 3 | 4 | 7 |
| 4/0 AWG | 0.3237 | 0 | 0 | 1 | 1 | 1 | 4 |
EMT internal areas used: 1/2 in. 0.304, 3/4 in. 0.533, 1 in. 0.864, 1-1/4 in. 1.496, 1-1/2 in. 2.036, 2 in. 3.356 in². Where only one or two conductors fit, the 53% and 31% columns govern instead of 40%, which is why several cells stop at 1.
Which edition this follows
The areas and limits used here are those of NFPA 70, National Electrical Code, 2023 edition, Chapter 9 Tables 1, 4 and 5, with Notes 4 and 7. Chapter 9 has been stable across recent cycles, but your jurisdiction may still enforce an earlier edition and some states amend Chapter 9. Confirm the internal area of an unusual raceway against the printed table before you commit a large material order, and confirm any local amendment with the authority having jurisdiction.
Mistakes that make a fill calculation wrong
- Forgetting the equipment grounding conductor. It takes up bore like every other conductor. So does an isolated ground, and so does a spare pulled ‘for later’.
- Using the trade size as the diameter. Trade size is a name, not a dimension. Always take the internal area from Chapter 9, Table 4.
- Applying 40% to a two-conductor run. Two conductors is the 31% row. It catches people on two-wire control drops and switch legs.
- Applying the 60% nipple allowance to a real run. Note 4 stops at 600 mm (24 in.) between enclosures. A 30 in. piece is an ordinary raceway.
- Treating fill as the only limit. Ampacity adjustment under 310.15(C)(1), the 360-degree bend rule, and box fill under 314.16 all bite independently.
What this calculator does not decide for you
It answers the fill question and nothing else. It does not size the conductor for the load, it does not apply the ampacity adjustment factors, and it does not check the boxes at either end — that is a separate calculation under 314.16, which you can run with the box fill calculator.
It also assumes ordinary building wire with the published Table 5 dimensions. Multiconductor cable, tray cable, fibre and communications cable are handled by Notes 5 and 9 instead: for a cable with a known outside diameter you compute the area from that diameter and use it in the same ratio, treating the whole cable as one conductor. Manufacturers publish actual diameters, and for a large single-conductor cable those can differ noticeably from the Table 5 approximations.
Annex C, the Table 4 columns, and when to use each
There are three legitimate routes to the same answer, and knowing which to reach for saves time.
Annex C is fastest when every conductor in the raceway is the same size and insulation. Table C1 covers EMT, C4 covers RMC, C8 covers IMC and C10 covers PVC Schedule 40; you read the count straight out. Annex C is derived from exactly the arithmetic on this page, including the Note 7 rounding, which is why the counts in the reference table above match it.
The pre-computed columns in Table 4 are fastest when your sizes are mixed. Sum your conductor areas from Table 5, then find the first raceway row whose 40% column exceeds your sum. That is what this calculator does, and what an inspector will do if they check your work.
Once the raceway is settled, the next two questions on any real job are the conductor size and the run length. Take the load through the ampacity calculator, then check the run with the voltage drop calculator: on long circuits, voltage drop upsizes the conductor and sends you straight back here for a bigger raceway. If your drawings are metric, the AWG to mm² converter moves you between the two systems, and grounding conductors are sized separately with the equipment grounding conductor calculator.
Key terms
- Trade size
- The nominal designation of a raceway, such as 3/4 in. or metric designator 21. It is a name rather than a measurement; the internal area comes from Chapter 9, Table 4.
- Nipple
- A length of conduit or tubing not exceeding 600 mm (24 in.) installed between boxes, cabinets and similar enclosures. Chapter 9, Note 4 permits 60% fill in a nipple.
- Current-carrying conductor
- A conductor that carries load current for the purposes of 310.15(C)(1). Equipment grounding conductors never count; a neutral counts or does not depending on 310.15(E).