NEC Conduit Fill Calculator
Add up to three groups of common THHN/THWN-2 conductor sizes, compare their combined area with a selected common raceway’s internal area, and see the one-, two-, or over-two-conductor fill limit that controls the scenario.
Use the locally adopted code. This calculator uses a limited 2026 NEC Chapter 9 planning dataset. Raceways, cables, conductor types, installations, and jurisdictions can require different tables, notes, exceptions, or amendments.
Raceway fill result
Area ledger
3 × 12 AWG at 0.0133 in² = 0.0399 in² 3 × 10 AWG at 0.0211 in² = 0.0633 in² Total = 0.1032 in² ÷ 0.5330 in² = 19.36%
The conductor count changes the permitted percentage
The percentage applies to cross-sectional area, not a sum of conductor diameters. For every selected group, the calculator multiplies the Chapter 9 planning area for one conductor by the conductor count. It adds those occupied areas, selects the fill limit from the total number of conductors, and compares the sum with that percentage of the raceway’s internal area.
The one- and two-conductor percentages may seem unusual because two conductors have a lower percentage than one or three. They are intended to address the geometry and pullability of conductors in a circular raceway. Do not replace the code percentages with a simple visual judgment from a cross-section drawing.
Worked example: six conductors in 3/4-inch EMT
The default bundle contains three 12 AWG conductors and three 10 AWG conductors. Each 12 AWG THHN/THWN-2 planning area is 0.0133 square inch, producing 0.0399 square inch for that group. Each 10 AWG planning area is 0.0211 square inch, producing 0.0633 square inch. Together they occupy 0.1032 square inch.
The included 3/4-inch EMT internal area is 0.5330 square inch. Six conductors invoke the 40 percent fill limit, so allowed occupied area is 0.5330 × 0.40, or 0.2132 square inch. The bundle’s actual fill is 0.1032 ÷ 0.5330, or 19.36 percent. It uses 48.41 percent of the allowed area and leaves approximately 0.1100 square inch before the 40 percent boundary.
With the bundle already above two conductors, the calculator can add eight more of the selected group-1 size before the next identical conductor would exceed the modeled 40 percent area. That is a geometric result only. Ampacity adjustment, pulling tension, bend limits, conductor function, grounding, voltage drop, equipment terminals, raceway support, and future maintenance can justify a larger raceway.
Conduit-fill count is not the ampacity derating count
Fill counts physical occupants
Every insulated conductor or cable that occupies cross-sectional area must be handled under the applicable fill rules. Equipment grounding conductors occupy space even though they are not normally current carrying. Bare conductors also have physical area and require the correct table treatment.
Ampacity counts heat-producing conductors
Adjustment for current-carrying conductors asks a different question. Grounded conductors, neutrals with nonlinear loads, equipment grounding conductors, and circuit arrangements can be treated differently under ampacity rules. Never copy the fill count directly into a derating calculation without analysis.
Because these counts serve different purposes, the calculator reports only fill.
What the limited dataset covers
| Included item | Available choices | Boundary |
|---|---|---|
| Raceway types | EMT, RMC, IMC, and rigid PVC Schedule 40. | Only trade sizes 1/2 through 2 inches are included. Other raceways and larger sizes need the controlling Chapter 9 table. |
| Conductors | Common THHN/THWN-2 planning areas from 14 AWG through 4/0 AWG. | Compact, stranded, special insulation, multiconductor cable, oval cable, and manufacturer-specific dimensions may differ. |
| Group layout | Three size groups with independent whole-number counts. | The calculator does not decide conductor color, circuit function, neutral treatment, or conductor compatibility. |
| Fill limit | General 53%, 31%, and 40% Chapter 9 Table 1 percentages. | Nipples, lead-sheathed conductors, cable geometry, seals, boxes, fittings, and notes or exceptions require separate review. |
| Additional conductor estimate | More conductors matching group 1, recalculated under the applicable count percentage. | This is not reserved capacity and does not prove ampacity, pulling, or future-circuit compliance. |
A conduit can pass fill and still be a poor design
Fill is one geometric limit. Conductors must also be pulled without damage. Long runs, many bends, high sidewall pressure, large conductor stiffness, and unfavorable grouping can make a technically under-filled raceway difficult or unsafe to install. Pull boxes, bend radii, lubricant compatibility, conductor pulling tension, and raceway support need project-specific planning.
Thermal performance remains separate. A raceway below 40 percent fill can still contain enough current-carrying conductors to require ampacity adjustment. Ambient temperature, rooftops, insulation, underground conditions, conductor temperature ratings, and terminal limits also affect ampacity. A larger raceway does not automatically remove every derating requirement.
Boxes, conduit bodies, fittings, and seals can become the controlling physical bottleneck. Conductor bending space and box-fill rules are not the same as raceway fill. Equipment grounding and bonding must also be designed, including raceway continuity and grounding-conductor sizing where required.
Finally, confirm the code edition enforced by the state or local authority. NFPA makes the 2026 NEC available, but local adoption can lag or include amendments. A correct 2026 screening result is not automatically the permitted result in a jurisdiction enforcing another edition.
A field-ready conduit planning workflow
- Identify every occupant. List phase, grounded, control, grounding, spare, communication, and cable assemblies that will share the raceway.
- Confirm exact construction. Obtain conductor or cable type, size, stranding, insulation, and listed dimensions.
- Select the raceway and method. Verify material, schedule, location, corrosion exposure, support, grounding role, and permitted use.
- Calculate fill with the controlling tables. Apply notes and exceptions for the actual installation, not merely the general percentages.
- Calculate ampacity separately. Count current-carrying conductors correctly and apply all temperature and terminal limits.
- Check installation mechanics. Review bends, pull length, sidewall pressure, pulling tension, boxes, conduit bodies, and access.
- Document local approval. Record the adopted NEC edition, amendments, permits, and inspector requirements.
Frequently asked questions
Why is the limit 31% for two conductors but 40% for three?
The general percentages account for conductor geometry and pulling behavior in a circular raceway. Two round conductors can bind in ways that justify the lower area percentage. Use the applicable code rule rather than assuming the percentage must rise with every added conductor.
Do equipment grounding conductors count for conduit fill?
They occupy physical space and must be included under the applicable fill calculation. That is different from deciding which conductors count as current carrying for ampacity adjustment. The two analyses should be documented separately.
Can I mix conductor sizes in one calculation?
Yes. The three groups can use different sizes. Enter zero for an unused group. The calculator multiplies each group’s unit area by its count and adds all groups before applying the limit selected by total conductor count.
Does a 40% result mean the conduit is full?
For more than two conductors, 40 percent is the general maximum occupied cross-sectional area, not the physical interior being completely packed. Remaining open area supports pulling and accommodates geometry. Notes, exceptions, installation mechanics, or design policy may require less fill.
Can I use conductor diameter instead of listed area?
Use the code table or actual cable method that governs the product. This calculator stores planning areas directly. Squaring an approximate nominal diameter can introduce error and may ignore the insulation or cable shape that determines occupied area.
Does the maximum-additional result reserve space for future circuits?
No. It is a geometric estimate for more group-1 conductors under the applicable count percentage. Future circuits also require ampacity, conductor compatibility, grounding, protection, pulling, box, and local-code review. Intentional spare capacity should be established by project policy.
If the chosen raceway exceeds the permitted fill, use the conduit size calculator to compare the next practical trade size.
References
- National Fire Protection Association, NFPA LiNK available publications: 2026 NFPA 70, National Electrical Code
- National Fire Protection Association, NFPA 70 code-development materials addressing Chapter 9 raceway and conductor tables
- Occupational Safety and Health Administration, 29 CFR 1926.405, Wiring Methods, Components, and Equipment for General Use
Educational screening only. Use the complete locally adopted NEC, actual listed product dimensions, equipment instructions, permits, and qualified electrical professionals.