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Conduit Fill Calculator for Multiple Wire Sizes

Published fill tables assume a uniform conductor size, so mixed-size raceways fall outside them. This tool calculates fill from Chapter 9 dimensions for any combination of sizes and insulation types, applies the limit for the conductor count, and returns the minimum compliant trade size with a scaled cross-section.
Code basis
NEC 2023, Ch. 9
Tables used
1, 4, 5, 5A
Raceway types
8
Insulation types
6
Conductor sizes
14 AWG – 1000 kcmil
Stranding
Concentric, compact
Fill limits
53 / 31 / 40 / 60%

Raceway

Available trade sizes vary by type; the size list updates accordingly.

Match the edition your jurisdiction enforces.

Conductors

One group per size and insulation type. Any combination is supported.

  • Group 1

  • Group 2

Fill

Pass

26.8%

40 percent permitted for 9 conductors in 3/4 in EMT.

Conductor area
0.1431 in²of 0.213 in² permitted
Area to spare
0.0699 in²
Smallest that fits
3/4 in26.8% fill
  • 6 × 12 AWG THHN/THWN-2
  • 3 × 10 AWG THHN/THWN-2
Drawn to scale. The arrangement is illustrative: the fill limit compares cross-sectional areas and does not address conductor position or pulling feasibility.

Area by group

GroupEachTotalShare
6 x 12 AWG THHN/THWN-20.01330.079855.8%
3 x 10 AWG THHN/THWN-20.02110.063344.2%
Cross-sectional area contributed by each conductor group.

Raceway internal area is 0.533 in2, of which 0.213 in2 may be occupied.

Calculated against the NEC 2023 edition using Chapter 9, Table 1, Chapter 9, Table 4, Chapter 9, Table 5, Chapter 9, Table 5A. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Compact stranded conductors in THW, THHW and the RHH/RHW/USE group, which this tool does not offer as insulation types.
  • Cable assemblies such as UF, NM and MC, which are treated as a single conductor using the manufacturer's actual dimensions.
  • Conduit bodies, boxes and wireways, which are sized by different rules.
  • Bending radius, pulling tension and lubrication, none of which the fill limit addresses.

Data last verified

Calculation Method

Four steps, equivalent to a manual Chapter 9 calculation.

  1. Step 1

    Conductor area

    Cross-sectional area per conductor from Table 5, or Table 5A where the group is compact stranded, multiplied by quantity and summed across all groups.

  2. Step 2

    Raceway area

    Internal cross-sectional area for the selected type and trade size from Table 4.

  3. Step 3

    Permitted fill

    Determined by conductor count per Table 1. The permitted area is read from the published column rather than recomputed, so marginal results agree with the printed table.

  4. Step 4

    Comparison

    Total conductor area against permitted area, reported as a percentage of the full internal area with remaining headroom in square inches.

All conductors count toward fill, including the neutral and the equipment grounding conductor. The derating count under Article 310 is different: it includes current-carrying conductors only.

Permitted Fill by Conductor Count

Fill limits are set by conductor quantity, not by raceway type or material.

In the racewayPermitted fillBasis
1 conductor53%The full remaining bore is available for the pull.
2 conductors31%Most restrictive case. Two round conductors pack inefficiently and leave minimal pulling clearance.
3 or more40%Applies to the majority of installations.
Nipple, 24 in or less60%A short length between enclosures presents negligible pulling length.
Maximum permitted fill by number of conductors in the raceway.

Conductor Areas

Approximate area in square inches per insulated conductor, for verifying a result by hand.

SizeTHHN / THWN-2XHHW / XHHW-2TW / THW
14 AWG0.00970.01390.0139
12 AWG0.01330.01810.0181
10 AWG0.02110.02430.0243
8 AWG0.03660.04370.0437
6 AWG0.05070.0590.0726
4 AWG0.08240.08140.0973
2 AWG0.11580.11460.1333
1/0 AWG0.18550.18250.2223
2/0 AWG0.22230.2190.2624
4/0 AWG0.32370.31970.3718
250 kcmil0.3970.39040.4596
350 kcmil0.52420.51660.5958
500 kcmil0.70730.69840.7901
Approximate area in square inches per insulated conductor. Derived and reorganised from the NEC conductor dimensions table; consult the published code for the authoritative values.

EMT Capacity by Trade Size

Internal dimensions and the area permitted for three or more conductors. Other raceway types are available in the calculator.

Trade sizeInternal dia.Total area40% area
1/2 in0.6220.3040.122
3/4 in0.8240.5330.213
1 in1.0490.8640.346
1-1/4 in1.381.4960.598
1-1/2 in1.612.0360.814
2 in2.0673.3561.342
2-1/2 in2.7315.8582.343
3 in3.3568.8463.538
3-1/2 in3.83411.5454.618
4 in4.33414.7535.901
EMT internal dimensions and the area permitted for three or more conductors. Derived and reorganised from the NEC conduit dimensions table.

Common Errors

Four errors responsible for most fill-related inspection failures.

  • Omitting the equipment grounding conductor

    All conductors count toward fill, including the equipment grounding conductor and the neutral. The derating count excludes the EGC, so a count carried over from a derating calculation is one conductor short.

  • Treating 31 percent as the lenient limit

    The two-conductor limit is the most restrictive of the three. Adding a third conductor raises the permitted area from 31 to 40 percent, so a failing two-wire pull can pass once the grounding conductor is included.

  • Itemising a cable as separate conductors

    A multiconductor cable counts as one conductor, using the major diameter of its cross-section. A bundle of single conductors without an overall covering is not a cable and is calculated individually. UF, NM and MC are excluded here because they require manufacturer dimensions.

  • Equating compliant fill with a practical pull

    Fill compliance is necessary but not sufficient. A raceway at 39 percent with four 90-degree bends and no lubricant may be impractical to pull. Verify jam ratio, bend count and pulling tension separately, and note that derating may require larger conductors regardless.

FAQs

Can different wire sizes be combined in one calculation?
Yes. Add one group per size and insulation type; each contributes its own area to the total. The Annex C tables cannot represent a mixed pull because each is precalculated for a uniform conductor size, which is why Chapter 9 provides the dimension tables for calculation.
Why is the limit 40 percent?
Permitted fill is set by conductor count, not raceway type: 53 percent for one conductor, 31 percent for exactly two, 40 percent for three or more, and 60 percent for a nipple not exceeding 24 inches between enclosures. Two conductors carry the most restrictive limit because they pack inefficiently and leave minimal pulling clearance.
Does the equipment grounding conductor count toward conduit fill?
Yes. All conductors in the raceway count, including the EGC and the neutral. The derating provisions of Article 310 use a different count that includes current-carrying conductors only and excludes the EGC. Applying one count to both calculations is a frequent source of error.
Can bare conductor dimensions be used for a grounding conductor?
Where a bare conductor is permitted, the bare dimensions from the conductor properties table may be used. This calculator uses insulated dimensions only, so entering a bare grounding conductor as insulated yields a conservative result that overstates fill rather than understating it.
What is the jam ratio?
When three conductors of equal size are pulled into a raceway whose internal diameter is near three times the conductor diameter, they can wedge side by side at a bend. The commonly cited caution band is a ratio of 2.8 to 3.2. It is not a code requirement, so it is reported as a pulling caution and never affects the pass or fail verdict. Increasing one trade size eliminates the risk.
Does the cross-section drawing confirm the pull is achievable?
No. The fill limit compares total conductor area against a percentage of raceway area and does not address conductor position, so cross-sectional area is not equivalent to achievable packing. The drawing renders the raceway and each conductor at true relative scale to show proportion; the arrangement is illustrative. Bend radius, bend count, pulling tension and lubrication determine whether a compliant pull is practical.
Why does this differ from the Annex C tables?
Annex C is precalculated for a uniform conductor type and size and rounds the conductor count down to a whole number, making it slightly more conservative than calculating from the dimension tables. Chapter 9 permits either method. An inspector working from Annex C may reach a stricter conclusion on a marginal result, so retain margin rather than designing to 39.9 percent.
Does aluminum change the calculation?
The method is identical; the dimensions differ. Most aluminum building wire in THWN/THHN and XHHW is compact stranded, which is tabulated separately in Table 5A with smaller diameters than concentric-lay-stranded conductors of the same size. Set Stranding to Compact for those conductors. It frequently saves a trade size: four 2/0 XHHW compact fit 1-1/2 in EMT where the concentric figures require 2 in.
When should Stranding be set to Compact?
When the conductor is compact stranded, which covers most aluminum building wire and some copper. Compact is not made below 8 AWG, and compact THHN starts at 6 AWG, so the option is unavailable for smaller sizes. Using the concentric figures for a compact conductor overstates fill and can send you to a larger raceway than required; using compact figures for a concentric conductor understates it, which is the unsafe direction. Check the conductor marking or the manufacturer's data sheet rather than assuming.