Checked against primary sources 2026-08-24

Conduit fill is one comparison, and four notes decide what goes into it

Three lookups and one division. The notes underneath Table 1 are where the answer actually gets decided, and most study material never mentions them.

What the comparison is

Total conductor area against permitted raceway area. Everything else is looking up the two sides.

A fill item hands you a count, a wire size, an insulation type and a raceway type, then asks either whether it fits or what size raceway it needs. Both directions run through the same three tables in the same order.

What separates a fast candidate from a slow one is not arithmetic. It is knowing which of the notes under Chapter 9, Table 1 applies before opening any table at all.

The percentage, and what counts as one conductor

Chapter 9, Table 1 sets the permitted percentage and it depends only on the number of conductors. Over two conductors is 40 percent, which covers nearly every item you will see.

One conductor and two conductors each get their own figure. The two-conductor figure is the lowest of the three, which is not intuitive, and the reason is mechanical rather than electrical: two round conductors in a round pipe jam against each other in a way that three or more do not. Read both figures in your own code book and know which is which before test day.

Now the part that decides which row you are in. Note 9 to Table 1 treats a multiconductor cable of two or more conductors as a SINGLE conductor for fill purposes, using the overall cable, and where the cross section is elliptical it is taken on the major diameter of the ellipse.

So two cables pulled into a conduit is the two-conductor case at the lowest of the three percentages, not the six-conductor case at 40 percent. Getting that backwards moves the answer by two trade sizes, and an item that names a cable type rather than a wire type is asking exactly this.

Note 3 pushes the other way. The equipment grounding conductor or bonding conductor is included in the fill, at its own actual dimensions, insulated or bare. It carries no current in normal service and it occupies exactly as much room as anything else.

The nipple allowance, which almost nobody has read

Note 4 to Table 1 permits a conduit or tubing nipple no longer than 24 inches, installed between boxes, cabinets and similar enclosures, to be filled to 60 percent of its total cross-sectional area. The same note says the conductor count adjustment factors need not apply to that condition.

Two allowances out of one sentence, and they come from one physical fact. A run that short cannot build heat, so neither the fill limit nor the derating penalty is doing the job it was written for.

Table 4 already carries a 60 percent column beside the others, so the nipple case is a column change rather than a calculation. Find that column in your book now and note where it sits, because under time pressure people compute 60 percent of the total area by hand while the answer sits one column over.

The trigger is length and location together. Over 24 inches it is an ordinary raceway. Not between enclosures, it is an ordinary raceway.

Conductor area, by type before size

Chapter 9, Table 5 gives the approximate area of insulated conductors by insulation type and size.

Type matters as much as size, because the insulation thickness differs. THHN and XHHW of the same wire size are not the same area, and the gap is wide enough to change the raceway size on a full pull. Find the insulation type first, then the size inside it.

Where an item mixes types or sizes, total each group separately and add the totals. There is no shortcut for a mixed pull and the tables do not offer one.

Raceway area, organized by article

Chapter 9, Table 4 holds the raceway areas and it is arranged by article rather than alphabetically. Electrical metallic tubing sits under Article 358, rigid metal conduit under Article 344, and each raceway type gets its own block with its own numbers.

That arrangement is the reason people lose time here. Two raceway types of the same trade size have different internal areas, and flipping to the wrong block gives a number that looks entirely reasonable.

The table has already done the percentage arithmetic. It prints a column for each fill case alongside the total area, so once you know which case you are in you read that column directly instead of multiplying.

The rounding rule that reverses the obvious answer

On a how-many-fit item you divide the permitted area by the area of one conductor and get a decimal. The obvious move is to throw the decimal away, and most of the time that is right.

Note 7 to Table 1 carves out the case that is not. Where every conductor in the raceway is the same size, the next higher whole number is used when the calculation results in a decimal of 0.8 or larger.

So a division landing on something point nine is not eleven conductors. It is twelve, and Annex C agrees, because Annex C was built by running this same note.

The sequence

1. Decide what counts as a conductor. Multiconductor cables count as one each under Note 9. Grounding and bonding conductors count under Note 3.
2. Check for a nipple. Under 24 inches between enclosures, you are in the 60 percent column and the derating factors are off.
3. Otherwise pick the fill case from Chapter 9, Table 1. Over two conductors is 40 percent.
4. Look up each conductor area in Chapter 9, Table 5 by insulation type, then by size, and total them.
5. Go to Chapter 9, Table 4, find the raceway type by its article, and read down the column for your fill case.
6. Take the first trade size whose column value equals or exceeds your total.

Running it backwards, from a given raceway to a conductor count, uses the same tables in the same order. Read the permitted area from Table 4, divide by the single conductor area from Table 5, then apply Note 7 to the decimal.

Annex C, and when it closes

Annex C carries finished conductor counts for raceways filled entirely with one size and one type. It is faster than the long method, it is legitimate, and using it is not cutting a corner.

It closes the moment the conductors stop being identical. A pull with a smaller grounding conductor in it, or two sizes of phase conductor, is outside Annex C and back on the long method. Items mix sizes precisely to shut that door, so check for uniformity before reaching for the annex.

The same uniformity test governs Note 7, which is not a coincidence. Both exist for the all-one-size case and both stop at the same boundary.

Why none of it is memorization

Every value lives in the book, which is deliberate on the exam's part. This subject is not testing whether you memorized areas. It is testing whether you can move between three tables and four notes under a clock without losing your place.

Drill the sequence rather than the numbers. The page turns are what you are being timed on.

What this page cites

• NEC Chapter 9, Table 1 Percent of cross section of conduit and tubing available for conductors, and the notes that govern the whole method. Note 3 includes equipment grounding and bonding conductors at their actual dimensions. Note 4 is the 24 inch nipple at 60 percent with the adjustment factors switched off. Note 7 is the rounding rule at a decimal of 0.8 or larger, for conductors all of the same size. Note 9 treats a multiconductor cable as a single conductor and takes an elliptical section on its major diameter. Note 1 is the pointer to Informative Annex C, which carries finished counts for raceways filled with a single size and type and was built by applying these same notes. source
• NEC Chapter 9, Table 4 Dimensions and percent area of conduit and tubing, arranged by article. Carries a precomputed column for each fill case, including the 60 percent nipple column.
• NEC Chapter 9, Table 5 Dimensions of insulated conductors and fixture wires, by insulation type and size.
• NEC Chapter 9, Table 8 Conductor properties, which is where the dimensions of a bare conductor are taken from when a bare grounding conductor is included in the fill.
• NEC 310.15(C) The conductor count adjustment factors that Note 4 to Chapter 9, Table 1 says need not apply in a nipple.