NEC Conduit Fill Calculator
Calculate conduit fill per NEC Chapter 9 Tables 1, 4 & 5. Free to use, no account required.
Conduit & Wire Selection
PASS
Conduit fill is within NEC limits.
NEC Table 1 Fill Rules (Chapter 9)
- • 1 conductor: 53% maximum fill
- • 2 conductors: 31% maximum fill
- • 3 or more conductors: 40% maximum fill
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This calculator uses NEC Chapter 9 Tables 4 & 5 values. Always verify with your local AHJ and the current NEC edition adopted in your jurisdiction.
How the conduit fill calculation works
Conduit fill is a percentage of cross-sectional area, and the allowable percentage depends on how many conductors are in the raceway. Chapter 9 Table 1 sets 53 percent for one conductor, 31 percent for two conductors, and 40 percent for three or more conductors.
Table 4 gives the internal area of each raceway type and trade size, along with the pre-computed 60, 53, 31, and 40 percent columns so you rarely have to do the multiplication yourself. Raceway type matters: EMT, IMC, RMC, and the various PVC schedules all have different internal diameters at the same trade size.
Table 5 gives the approximate area of each insulated conductor by type and size. For conduit fill, use the applicable conductor area for the specific insulation type and size in Chapter 9 Table 5. Sum the Table 5 areas of every conductor, including equipment grounding conductors, and compare against the correct percentage column in Table 4.
Step by step
- 1
Count the conductors
All of them, including grounding conductors. This sets the allowable percentage from Chapter 9 Table 1.
- 2
Pick the percentage
1 conductor is 53 percent, 2 is 31 percent, 3 or more is 40 percent.
- 3
Total the conductor area
Chapter 9 Table 5, by insulation type and size. Different types in one raceway are simply added together.
- 4
Find the raceway area
Chapter 9 Table 4, correct raceway type and trade size, read the matching percentage column.
- 5
Compare
Total conductor area must not exceed the allowable area. If it does, step up a trade size or change to a thinner insulation.
- 6
Check the nipple exception
Chapter 9 Note 4: a raceway 24 inches or less is permitted to be filled to 60 percent.
Worked example
Nine 12 AWG THHN conductors plus one 12 AWG THHN equipment grounding conductor in 3/4 inch EMT. Does it fit?
- Ten conductors total, so Chapter 9 Table 1 gives 40 percent.
- Table 5: 12 AWG THHN is 0.0133 square inches each. 10 x 0.0133 = 0.1330 square inches.
- Table 4: 3/4 inch EMT, 40 percent column = 0.213 square inches.
- 0.1330 is less than 0.213.
Answer: Yes, it fits at about 25 percent fill. Note separately that ten current-carrying conductors would trigger a 50 percent adjustment factor under 310.15(C)(1), so fill passing does not mean the conductors are still adequately sized.
How this is tested on the exam
Chapter 9 Table 1, Table 4, and Table 5 provide the values needed for a conduit-fill calculation. Confirm relevance and permitted reference materials with the official exam information.
Frequently asked questions
Why is two conductors only 31 percent when three is 40 percent?
Chapter 9 Table 1 assigns a 31 percent allowable fill for exactly two conductors.
Do I count the equipment grounding conductor in conduit fill?
Yes. Every conductor in the raceway counts toward both the conductor count and the total area, including equipment grounding conductors. This differs from ampacity derating, where grounding conductors are excluded.
Can I mix conductor types in one raceway?
Yes, for fill purposes. Look up each conductor's area in Table 5 by its own type and size and add them together. Article 300.3(C) governs whether the different circuits are permitted to occupy the same raceway.
What is the rule for a short nipple?
Chapter 9 Note 4 permits a raceway or nipple not exceeding 24 inches in length to be filled to 60 percent of its total cross-sectional area. The ampacity adjustment factors of 310.15(C)(1) also do not apply at that length.
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