Conduit Fill Calculation, Step by Step

Conduit fill questions look simple and still cost people points, because the method has four separate table lookups and a counting rule that is easy to skip. This guide breaks the calculation into steps you can repeat on any problem, then works through three examples with real table values.

The tables you need

All of the conduit fill data lives in NEC Chapter 9. You will flip between these:

• Chapter 9, Table 1: the maximum percent of the raceway area you are allowed to fill, based on how many conductors are inside.
• Chapter 9, Table 4: the internal area of each raceway type and trade size (EMT, rigid, PVC, flexible metal conduit and others), with columns already worked out for the allowed fill percentages.
• Chapter 9, Table 5: the area of insulated conductors by insulation type and size (THHN, THWN-2, XHHW and so on). Table 5A covers compact aluminum conductors.
• Chapter 9, Table 8: the area of bare conductors, which you need when a bare equipment grounding conductor is in the raceway.
• Informative Annex C: ready-made tables for the maximum number of conductors when all of them are the same size and insulation type.

The fill percentages from Table 1 are worth memorizing because they show up everywhere:

The two-conductor limit is lower than the one-conductor limit on purpose: two round conductors side by side can jam when pulled around bends.

The method in four steps

1. Count every conductor, including the equipment grounding conductor. The notes to Table 1 say grounding and bonding conductors are counted when you calculate fill. This is the step people skip.
2. Look up the area of each conductor in Table 5 for its exact insulation type and size. THHN and THW of the same size do not have the same area.
3. Add the areas to get the total conductor area in square inches.
4. Go to Table 4 for the raceway type in the question, use the column for the fill percentage that applies (usually 40%), and pick the smallest trade size whose allowed area is equal to or larger than your total.

Values used in the examples

These are the values commonly listed for THHN/THWN in Chapter 9, Table 5 and for EMT in Table 4. Confirm them in your own code book before you rely on them, and always use the book you are allowed to bring into the exam.

Worked example 1: mixed sizes in EMT

A run of EMT will carry three 8 AWG THHN conductors and one 10 AWG THHN equipment grounding conductor. What is the minimum trade size?

If you had forgotten the grounding conductor, you would have calculated 0.1098 sq in and picked 1/2 in EMT. That is exactly the trap the question is built around.

Worked example 2: all the same size

How many 12 AWG THHN conductors are allowed in 3/4 in EMT?

When all conductors are the same size and insulation, you can also go straight to Annex C, which lists the answer for each raceway type. Using Annex C is faster, but you should know how to get the number yourself in case the question mixes sizes or the conductor type is not in the table.

The 0.8 rounding rule

When every conductor is the same size, the notes to Chapter 9, Table 1 let you round up to the next whole number if the division leaves a decimal of 0.8 or more. Below 0.8, you round down.

This rule only applies to the same-size calculation. For mixed sizes you compare total area to allowed area, and the total must not exceed the allowed area.

Worked example 3: four 6 AWG plus a ground

A feeder uses four 6 AWG THHN conductors and one 8 AWG THHN equipment grounding conductor in EMT.

Special cases to know

• Short nipples. The notes to Table 1 allow 60% fill in a nipple not over 24 in long between boxes or enclosures. Table 4 has a 60% column for this.
• Bare conductors. Use the area from Chapter 9, Table 8 for a bare grounding conductor, not Table 5.
• Cables in raceways. A multiconductor cable is treated as a single conductor for fill, using its overall area. Elliptical cables have their own rule in the notes.
• Raceway type matters. 3/4 in EMT, 3/4 in rigid metal conduit and 3/4 in PVC Schedule 80 do not have the same internal area. Always use the Table 4 section for the raceway named in the question.

Fill is not the only limit on how many conductors you put in a raceway. More than three current-carrying conductors also triggers the ampacity adjustment factors in 310.15(C)(1). That is covered in ampacity and derating.

Exam-day routine

1. Write down every conductor with its size and insulation type, ground included.
2. Count them and decide the fill percentage: 53, 31 or 40.
3. Look up and add the areas. Write the total with four decimal places.
4. Find the raceway type in Table 4 and step up through the trade sizes until the allowed area covers your total.
5. For same-size problems, check Annex C to confirm your answer.

Conduit fill and voltage drop often show up in the same job: a long run that needs bigger conductors may also need a bigger raceway. You can check the voltage drop side with the free calculator.