NEC Chapter 9 Table 4 Conduit Interior Areas

NEC Chapter 9 Table 4 of NFPA 70 (2023) is the lookup table for interior cross-sectional areas of every conduit and tubing type recognized by the Code. The table is structured per conduit type (EMT, ENT, FMC, IMC, LFNC, RMC, PVC) with rows for each trade size and columns for total area plus the 31%, 40%, 53%, and 60% pre-calculated fill values. Read it correctly and conduit sizing becomes pure arithmetic.

How Table 4 Is Organized

Each conduit type gets its own sub-table. Within each sub-table:

• Column 1: Trade size (1/2", 3/4", ..., 6")
• Column 2: Metric designator
• Column 3: Internal diameter (inches and mm)
• Column 4: Total area in square inches and mm^2
• Columns 5-8: Pre-calculated 60%, 53%, 40%, and 31% fill values

Interior Areas at 40% Fill (in^2)

The most-used number is "40% fill area" since most installations have 3 or more conductors. Selected values from NEC 2023 Table 4:

Values are in square inches. Note that EMT in trade sizes 2-1/2" and larger has significantly more interior area than RMC of the same trade size - a critical point when comparing conduit types for cost vs fill capacity. See our EMT vs RMC blog for the full comparison.

Why Interior Areas Vary by Conduit Type

Conduits share their trade size designation (a legacy of cast-iron pipe sizing) but differ in wall thickness:

• EMT has thin walls (cold-rolled steel tube), giving the largest interior area in small trade sizes
• RMC is heavy-wall rigid steel - thickest walls of the metal conduits
• IMC is intermediate between EMT and RMC
• PVC Schedule 40 is medium-wall plastic
• PVC Schedule 80 is heavy-wall plastic for impact protection
• FMC (flexible) has corrugated walls that consume interior space
• LFNC and ENT are nonmetallic flexible/corrugated types

This is why a 2" PVC Sch 80 (1.174 in^2 at 40%) holds fewer conductors than a 2" PVC Sch 40 (1.316 in^2 at 40%). See the deep-dives at what is PVC Sch 40 and what is PVC Sch 80.

Worked Example

Goal: Will 9 x 12 AWG THHN fit in 3/4" EMT?

• 12 AWG THHN area (Table 5) = 0.0133 in^2
• Total = 9 x 0.0133 = 0.1197 in^2
• 3/4" EMT at 40% fill (Table 4) = 0.213 in^2
• 0.1197 < 0.213 -> Yes, fits with margin

For 12 conductors:

• Total area = 12 x 0.0133 = 0.1596 in^2
• 0.1596 < 0.213 -> Still fits

For 17 conductors:

• Total area = 17 x 0.0133 = 0.2261 in^2
• 0.2261 > 0.213 -> No - bump up to 1" EMT (0.346 in^2 at 40%)

Common Pitfalls

• Reading the wrong column. The 31%, 40%, 53%, and 60% columns are stacked - it is easy to grab the wrong row when reading quickly. Annex C is the safer reference when you have identical conductors.
• Assuming EMT and RMC have the same area. They do not. EMT is consistently larger in smaller trade sizes.
• Forgetting that 1/2" PVC Sch 80 is dramatically smaller than 1/2" PVC Sch 40. The wall thickness penalty is most severe at small trade sizes.

Cross-References

• NEC Chapter 9 Table 1 - the fill percentages applied to Table 4 areas
• NEC Chapter 9 Table 5 - conductor dimensions; see Table 5 guide
• NEC Annex C - pre-calculated fill counts for identical conductors
• NEC 358 / 352 / 342 / 344 / 348 / 356 / 362 - conduit Article references

How WireFillChart Implements It

Our conduit fill calculator embeds the full Table 4 dataset for every conduit type and trade size. Selecting "EMT 3/4-inch" pulls 0.533 in^2 total area and applies the appropriate fill ceiling. See the published charts at EMT fill and PVC fill for the most-used types.

Related

• Conduit Fill Calculator
• EMT Conduit Fill Chart
• NEC Chapter 9 Table 5 Conductor Dimensions
• What is EMT Conduit
• What is PVC Schedule 40 Conduit
• Wire Fill Chart