Electrical Planning
Conduit Fill Calculator
Check NEC 2026 conduit fill with mixed wire sizes, brand OD seed data, custom cable diameter, and Cat6 planning rows.
Rows
40% fill chart
Maximum same-size cable counts by conduit size.
| Cable | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" |
|---|---|---|---|---|---|---|---|---|
| 14 AWG THHN | 12 | 21 | 35 | 61 | 83 | 138 | 197 | 304 |
| 12 AWG THHN | 9 | 16 | 25 | 44 | 61 | 100 | 143 | 222 |
| 10 AWG THHN | 5 | 10 | 16 | 28 | 38 | 63 | 90 | 140 |
| 8 AWG THHN | 3 | 5 | 9 | 16 | 22 | 36 | 52 | 80 |
| 6 AWG THHN | 2 | 4 | 6 | 11 | 16 | 26 | 37 | 58 |
| 4 AWG THHN | 1 | 2 | 4 | 7 | 9 | 16 | 23 | 35 |
| 2 AWG THHN | 1 | 1 | 2 | 5 | 7 | 11 | 16 | 25 |
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About this calculator
Method, formulas, and limits.What this does
Checks conduit fill compliance against NEC 2026 percentage limits for one, two, or three or more conductors. Supports mixed wire sizes and types, includes Southwire and Belden seed outside-diameter data for common products, custom cable diameters, Cat6/Cat6A data cable rows with 40% planning fill, and NM-B/Romex flagging with NEC Article 334 reminders.
Who it is for
Electricians, electrical engineers, contractors, and DIYers planning conduit runs for power, control, or data cabling. Useful for verifying fill compliance before pulling wire, sizing conduit for mixed cable types, and generating material lists or Excel exports for the job site.
How it works
Each wire or cable row contributes its actual cross-sectional area (computed from the outside diameter) to the total fill. The calculator compares the sum against the conduit's internal area multiplied by the NEC allowable fill percentage (53% for one wire, 31% for two, 40% for three or more). Brand seed records supply typical ODs, and custom rows accept any diameter. The result shows fill percentage, pass/fail status, and a visual fill gauge.
Limitations
The calculator uses NEC 2026 planning values; local jurisdictions may enforce other code editions. Seed ODs are typical values and should be verified against the manufacturer's current datasheet. Does not model pull tension, jam ratio, conduit friction, or derating for ambient temperature or more than three current-carrying conductors.
Key calculations
- Wire Cross-Sectional Area
- area = π × (OD/2)², where OD is the overall outside diameter of the wire or cable including insulation. Each conductor contributes its full area to the total fill.
- NEC Fill Percentage
- fillPercent = totalWireArea / conduitInternalArea × 100. The allowable limit is 53% for one conductor, 31% for two conductors, and 40% for three or more conductors.
- Conduit Internal Area
- The internal area depends on the conduit type and trade size. EMT, PVC, RMC, and ENT each have different internal diameters for the same nominal trade size, and the calculator uses the correct values per NEC tables.
- Total Fill for Mixed Sizes
- The sum of all conductor areas Σ (π × (ODᵢ/2)² × quantityᵢ) is computed across every row, regardless of wire type or gauge. The total is then compared to the conduit's capacity at the applicable fill limit.
Reference ranges
- NEC Fill Limits
- One conductor: 53% fill. Two conductors: 31% fill. Three or more: 40% fill. These limits prevent overheating and allow safe wire pulling.
- Common Conduit Sizes
- 1/2 in EMT (internal area ~0.29 sq in) and 3/4 in EMT (~0.53 sq in) are most common for branch circuits. 1 in and larger are used for feeder runs or multiple cables.
- Typical Fill Percentages
- A well-designed run typically fills 25-35% of the conduit, leaving room for pulling and future additions. Fill above 38% for three or more wires starts getting tight for pulling.
- THHN vs Data Cable Density
- A single Cat6 cable (OD ~0.25 in) takes roughly the same area as three 12 AWG THHN conductors. Mixing data and power requires careful fill accounting and separation considerations.
How to use it
- 1.Select conduit type and sizeChoose the conduit material (EMT, PVC, RMC, ENT) and trade size. The calculator loads the correct internal diameter and area for that conduit type.
- 2.Add wire or cable rowsFor each wire type, select the source (Southwire, Belden, or Custom), the gauge or catalog number, and the quantity of conductors. Brand seed rows auto-fill the outside diameter.
- 3.Mix sizes and typesAdd multiple rows with different sizes, sources, and quantities. The calculator sums area across all rows, so you can mix power and data cables in the same conduit.
- 4.Enable data mode for Cat6Toggle data mode to see Belden Cat5e, Cat6, Cat6A, and Cat7 rows with a 40% planning fill limit and a bend-radius reminder for data cables.
- 5.Review and exportCheck the fill percentage, pass/fail status, and visual gauge for each conduit. Use the chart view or export to Excel for job site documentation.
Yes, the conduit fill limits are based on NEC 2026 planning values for the selected conduit type and conductor count. However, local jurisdictions may still enforce an earlier edition — always confirm the adopted code year before relying on the results for permit or inspection purposes.
Yes, each row has its own size, source, and quantity settings. The calculator sums the actual cross-sectional area across all rows before applying the appropriate fill limit based on total conductor count. This enables mixed power and data cable runs in a single conduit.
Brand rows provide seed outside-diameter records for common Southwire building wire and Belden data cable products. These seed values are typical catalog dimensions and are useful for quick planning, but you should always verify against the exact manufacturer datasheet before ordering materials or committing to a design.
Yes, data mode includes Belden Cat5e, Cat6, Cat6A, and Cat7 seed rows and applies a 40% planning fill limit with a bend-radius reminder. Data cables have larger ODs than individual THHN conductors, so they consume fill quickly — a single Cat6 cable can use as much area as several power conductors.
NM-B (Romex) can run in conduit in certain situations, but NEC Article 334 restricts its use in wet locations and in raceways longer than necessary for physical protection. The calculator flags NM-B rows so you can verify the installation complies with local code and the specific conduit application.
The calculator supports EMT (Electrical Metallic Tubing), PVC Schedule 40/80, RMC (Rigid Metal Conduit), and ENT (Electrical Nonmetallic Tubing). Each type has different internal diameters for the same trade size, and the calculator uses the correct NEC table values for each material.
Yes, the calculator includes a chart view of fill percentages across conduit runs and an Excel export feature for printing job site documentation or submitting fill calculations with permit applications.
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